STM32F10xxx Cortex M3 Programming Manual Stm32

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October 2009 Doc ID 15491 Rev 2 1/137
PM0056
Programming manual
STM32F10xxx Cortex-M3 programming manual
This programming manual provides information for application and system-level software
developers. It gives a full description of the STM32F10xxx Cortex™-M3 processor
programming model, instruction set and core peripherals.
The STM32F10xxx Cortex™-M3 processor is a high performance 32-bit processor designed
for the microcontroller market. It offers significant benefits to developers, including:
●Outstanding processing performance combined with fast interrupt handling
●Enhanced system debug with extensive breakpoint and trace capabilities
●Efficient processor core, system and memories
●Ultra-low power consumption with integrated sleep modes
●Platform security
www.st.com
Contents PM0056
2/137 Doc ID 15491 Rev 2
Contents
1 About this document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.1 Typographical conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.2 List of abbreviations for registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 About the STM32 Cortex-M3 processor and core peripherals . . . . . . . . . . 8
1.3.1 System level interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3.2 Integrated configurable debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.3.3 Cortex-M3 processor features and benefits summary . . . . . . . . . . . . . . 10
1.3.4 Cortex-M3 core peripherals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 The Cortex-M3 processor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1 Programmers model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.1 Processor mode and privilege levels for software execution . . . . . . . . . 12
2.1.2 Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.3 Core registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.4 Exceptions and interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.1.5 Data types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.1.6 The Cortex microcontroller software interface standard (CMSIS) . . . . . 22
2.2 Memory model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.1 Memory regions, types and attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.2 Memory system ordering of memory accesses . . . . . . . . . . . . . . . . . . . 25
2.2.3 Behavior of memory accesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2.4 Software ordering of memory accesses . . . . . . . . . . . . . . . . . . . . . . . . 26
2.2.5 Bit-banding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2.6 Memory endianness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2.7 Synchronization primitives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.2.8 Programming hints for the synchronization primitives . . . . . . . . . . . . . . 31
2.3 Exception model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.1 Exception states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.2 Exception types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.3 Exception handlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.3.4 Vector table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.3.5 Exception priorities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.3.6 Interrupt priority grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.3.7 Exception entry and return . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
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2.4 Fault handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.1 Fault types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.2 Fault escalation and hard faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.4.3 Fault status registers and fault address registers . . . . . . . . . . . . . . . . . 41
2.4.4 Lockup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.5 Power management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.5.1 Entering sleep mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.5.2 Wakeup from sleep mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.5.3 The external event input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.5.4 Power management programming hints . . . . . . . . . . . . . . . . . . . . . . . . 43
3 The Cortex-M3 instruction set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.1 Instruction set summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.2 Intrinsic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.3 About the instruction descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.3.1 Operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.3.2 Restrictions when using PC or SP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3.3 Flexible second operand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.3.4 Shift operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.3.5 Address alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.6 PC-relative expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.7 Conditional execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3.8 Instruction width selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.4 Memory access instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.4.1 ADR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.4.2 LDR and STR, immediate offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.4.3 LDR and STR, register offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.4.4 LDR and STR, unprivileged . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.4.5 LDR, PC-relative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.6 LDM and STM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.4.7 PUSH and POP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.4.8 LDREX and STREX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.4.9 CLREX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.5 General data processing instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.5.1 ADD, ADC, SUB, SBC, and RSB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.5.2 AND, ORR, EOR, BIC, and ORN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
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3.5.3 ASR, LSL, LSR, ROR, and RRX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.5.4 CLZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.5.5 CMP and CMN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.5.6 MOV and MVN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
3.5.7 MOVT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.5.8 REV, REV16, REVSH, and RBIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
3.5.9 TST and TEQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
3.6 Multiply and divide instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.6.1 MUL, MLA, and MLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.6.2 UMULL, UMLAL, SMULL, and SMLAL . . . . . . . . . . . . . . . . . . . . . . . . . 84
3.6.3 SDIV and UDIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.7 Saturating instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.7.1 SSAT and USAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.8 Bitfield instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.8.1 BFC and BFI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
3.8.2 SBFX and UBFX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
3.8.3 SXT and UXT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
3.8.4 Branch and control instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
3.8.5 B, BL, BX, and BLX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.8.6 CBZ and CBNZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.8.7 IT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.8.8 TBB and TBH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
3.9 Miscellaneous instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
3.9.1 BKPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
3.9.2 CPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
3.9.3 DMB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.9.4 DSB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.9.5 ISB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
3.9.6 MRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
3.9.7 MSR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
3.9.8 NOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.9.9 SEV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.9.10 SVC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
3.9.11 WFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
3.9.12 WFI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
4 Core peripherals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
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4.1 About the STM32 core peripherals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.2 Nested vectored interrupt controller (NVIC) . . . . . . . . . . . . . . . . . . . . . . 104
4.2.1 The CMSIS mapping of the Cortex-M3 NVIC registers . . . . . . . . . . . . 105
4.2.2 Interrupt set-enable registers (NVIC_ISERx) . . . . . . . . . . . . . . . . . . . . 106
4.2.3 Interrupt clear-enable registers (NVIC_ICERx) . . . . . . . . . . . . . . . . . . 107
4.2.4 Interrupt set-pending registers (NVIC_ISPRx) . . . . . . . . . . . . . . . . . . . 108
4.2.5 Interrupt clear-pending registers (NVIC_ICPRx) . . . . . . . . . . . . . . . . . 109
4.2.6 Interrupt active bit registers (NVIC_IABRx) . . . . . . . . . . . . . . . . . . . . . 110
4.2.7 Interrupt priority registers (NVIC_IPRx) . . . . . . . . . . . . . . . . . . . . . . . . 111
4.2.8 Software trigger interrupt register (NVIC_STIR) . . . . . . . . . . . . . . . . . 112
4.2.9 Level-sensitive and pulse interrupts . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.2.10 NVIC design hints and tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
4.2.11 NVIC register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.3 System control block (SCB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.3.1 CPUID base register (SCB_CPUID) . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.3.2 Interrupt control and state register (SCB_ICSR) . . . . . . . . . . . . . . . . . 116
4.3.3 Vector table offset register (SCB_VTOR) . . . . . . . . . . . . . . . . . . . . . . . 118
4.3.4 Application interrupt and reset control register (SCB_AIRCR) . . . . . . 119
4.3.5 System control register (SCB_SCR) . . . . . . . . . . . . . . . . . . . . . . . . . . 120
4.3.6 Configuration and control register (SCB_CCR) . . . . . . . . . . . . . . . . . . 121
4.3.7 System handler priority registers (SHPRx) . . . . . . . . . . . . . . . . . . . . . 122
4.3.8 System handler control and state register (SCB_SHCSR) . . . . . . . . . 124
4.3.9 Configurable fault status register (SCB_CFSR) . . . . . . . . . . . . . . . . . . 126
4.3.10 Hard fault status register (SCB_HFSR) . . . . . . . . . . . . . . . . . . . . . . . . 129
4.3.11 Memory management fault address register (SCB_MMFAR) . . . . . . . 130
4.3.12 Bus fault address register (SCB_BFAR) . . . . . . . . . . . . . . . . . . . . . . . 130
4.3.13 System control block design hints and tips . . . . . . . . . . . . . . . . . . . . . 131
4.3.14 SCB register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
4.4 SysTick timer (STK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.4.1 SysTick control and status register (STK_CTRL) . . . . . . . . . . . . . . . . 132
4.4.2 SysTick reload value register (STK_LOAD) . . . . . . . . . . . . . . . . . . . . . 133
4.4.3 SysTick current value register (STK_VAL) . . . . . . . . . . . . . . . . . . . . . . 134
4.4.4 SysTick calibration value register (STK_CALIB) . . . . . . . . . . . . . . . . . 134
4.4.5 SysTick design hints and tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
4.4.6 SysTick register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
List of tables PM0056
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List of tables
Table 1. Summary of processor mode, execution privilege level, and stack use options. . . . . . . . . 13
Table 2. Core register set summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 3. PSR register combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Table 4. APSR bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Table 5. IPSR bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Table 6. EPSR bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Table 7. PRIMASK register bit definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 8. FAULTMASK register bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Table 9. BASEPRI register bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 10. CONTROL register bit definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 11. Ordering of memory accesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Table 12. Memory access behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Table 13. SRAM memory bit-banding regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Table 14. Peripheral memory bit-banding regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Table 15. C compiler intrinsic functions for exclusive access instructions . . . . . . . . . . . . . . . . . . . . . 31
Table 16. Properties of the different exception types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Table 17. Exception return behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Table 18. Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Table 19. Fault status and fault address registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Table 20. Cortex-M3 instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Table 21. CMSIS intrinsic functions to generate some Cortex-M3 instructions . . . . . . . . . . . . . . . . . 49
Table 22. CMSIS intrinsic functions to access the special registers. . . . . . . . . . . . . . . . . . . . . . . . . . 50
Table 23. Condition code suffixes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Table 24. Memory access instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Table 25. Immediate, pre-indexed and post-indexed offset ranges . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Table 26. label-PC offset ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
Table 27. Data processing instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Table 28. Multiply and divide instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Table 29. Packing and unpacking instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Table 30. Branch and control instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Table 31. Branch ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Table 32. Miscellaneous instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Table 33. STM32 core peripheral register regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Table 34. Mapping of interrupts to the interrupt variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Table 35. IPR bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Table 36. CMSIS functions for NVIC control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Table 37. NVIC register map and reset values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Table 38. Priority grouping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Table 39. System fault handler priority fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
Table 40. SCB register map and reset values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Table 41. SysTick register map and reset values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Table 42. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
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List of figures
Figure 1. STM32 Cortex-M3 implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 2. Processor core registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 3. APSR, IPSR and EPSR bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 4. PSR bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 5. PRIMASK bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 6. FAULTMASK bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 7. BASEPRI bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 8. CONTROL bit assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 9. Memory map. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 10. Bit-band mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Figure 11. Little-endian example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Figure 12. Vector table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Figure 13. ASR#3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Figure 14. LSR#3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Figure 15. LSL#3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Figure 16. ROR #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Figure 17. RRX #3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Figure 18. NVIC__IPRx register mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Figure 19. CFSR subregisters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
About this document PM0056
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1 About this document
This document provides the information required for application and system-level software
development. It does not provide information on debug components, features, or operation.
This material is for microcontroller software and hardware engineers, including those who
have no experience of ARM products.
1.1 Typographical conventions
The typographical conventions used in this document are:
1.2 List of abbreviations for registers
The following abbreviations are used in register descriptions:
1.3 About the STM32 Cortex-M3 processor and core peripherals
The Cortex-M3 processor is built on a high-performance processor core, with a 3-stage
pipeline Harvard architecture, making it ideal for demanding embedded applications. The
processor delivers exceptional power efficiency through an efficient instruction set and
extensively optimized design, providing high-end processing hardware including single-
cycle 32x32 multiplication and dedicated hardware division.
italic Highlights important notes, introduces special terminology, denotes
internal cross-references, and citations.
< and > Enclose replaceable terms for assembler syntax where they appear
in code or code fragments. For example:
LDRSB<cond> <Rt>, [<Rn>, #<offset>]
read/write (rw) Software can read and write to these bits.
read-only (r) Software can only read these bits.
write-only (w) Software can only write to this bit. Reading the bit returns the reset
value.
read/clear (rc_w1) Software can read as well as clear this bit by writing 1. Writing ‘0’ has
no effect on the bit value.
read/clear (rc_w0) Software can read as well as clear this bit by writing 0. Writing ‘1’ has
no effect on the bit value.
toggle (t) Software can only toggle this bit by writing ‘1’. Writing ‘0’ has no effect.
Reserved (Res.) Reserved bit, must be kept at reset value.
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Figure 1. STM32 Cortex-M3 implementation
To facilitate the design of cost-sensitive devices, the Cortex-M3 processor implements
tightly-coupled system components that reduce processor area while significantly improving
interrupt handling and system debug capabilities. The Cortex-M3 processor implements a
version of the Thumb® instruction set, ensuring high code density and reduced program
memory requirements. The Cortex-M3 instruction set provides the exceptional performance
expected of a modern 32-bit architecture, with the high code density of 8-bit and 16-bit
microcontrollers.
The Cortex-M3 processor closely integrates a configurable nested interrupt controller
(NVIC), to deliver industry-leading interrupt performance. The NVIC includes a non-
maskable interrupt (NMI), and provides up to 256 interrupt priority levels. The tight
integration of the processor core and NVIC provides fast execution of interrupt service
routines (ISRs), dramatically reducing the interrupt latency. This is achieved through the
hardware stacking of registers, and the ability to suspend load-multiple and store-multiple
operations. Interrupt handlers do not require any assembler stubs, removing any code
overhead from the ISRs. Tail-chaining optimization also significantly reduces the overhead
when switching from one ISR to another.
To optimize low-power designs, the NVIC integrates with the sleep modes, that include a
deep sleep function that enables the STM32 to enter STOP or STDBY mode.
1.3.1 System level interface
The Cortex-M3 processor provides multiple interfaces using AMBA® technology to provide
high speed, low latency memory accesses. It supports unaligned data accesses and
implements atomic bit manipulation that enables faster peripheral controls, system spinlocks
and thread-safe Boolean data handling.
1.3.2 Integrated configurable debug
The Cortex-M3 processor implements a complete hardware debug solution. This provides
high system visibility of the processor and memory through either a traditional JTAG port or
a 2-pin Serial Wire Debug (SWD) port that is ideal for small package devices.
Processor
core
Embedded
Trace Macrocell
NVIC
Debug
access
port
Serial
wire
viewer
Bus matrix
Code
interface
SRAM and
peripheral interface
Data
watchpoints
Flash
patch
STM32 Cortex-M3
processor
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For system trace the processor integrates an Instrumentation Trace Macrocell (ITM)
alongside data watchpoints and a profiling unit. To enable simple and cost-effective profiling
of the system events these generate, a Serial Wire Viewer (SWV) can export a stream of
software-generated messages, data trace, and profiling information through a single pin.
The optional Embedded Trace Macrocell™ (ETM) delivers unrivalled instruction trace capture
in an area far smaller than traditional trace units, enabling many low cost MCUs to
implement full instruction trace for the first time.
1.3.3 Cortex-M3 processor features and benefits summary
●Tight integration of system peripherals reduces area and development costs
●Thumb instruction set combines high code density with 32-bit performance
●Code-patch ability for ROM system updates
●Power control optimization of system components
●Integrated sleep modes for low power consumption
●Fast code execution permits slower processor clock or increases sleep mode time
●Hardware division and fast multiplier
●Deterministic, high-performance interrupt handling for time-critical applications
●Extensive debug and trace capabilities:
– Serial Wire Debug and Serial Wire Trace reduce the number of pins required for
debugging and tracing.
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1.3.4 Cortex-M3 core peripherals
These are:
Nested vectored interrupt controller
The nested vectored interrupt controller (NVIC) is an embedded interrupt controller that
supports low latency interrupt processing.
System control block
The system control block (SCB) is the programmers model interface to the processor. It
provides system implementation information and system control, including
configuration, control, and reporting of system exceptions.
System timer
The system timer, SysTick, is a 24-bit count-down timer. Use this as a Real Time
Operating System (RTOS) tick timer or as a simple counter.
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2 The Cortex-M3 processor
2.1 Programmers model
This section describes the Cortex-M3 programmers model. In addition to the individual core
register descriptions, it contains information about the processor modes and privilege levels
for software execution and stacks.
2.1.1 Processor mode and privilege levels for software execution
The processor modes are:
The privilege levels for software execution are:
In Thread mode, the CONTROL register controls whether software execution is privileged or
unprivileged, see CONTROL register on page 20. In Handler mode, software execution is
always privileged.
Only privileged software can write to the CONTROL register to change the privilege level for
software execution in Thread mode. Unprivileged software can use the SVC instruction to
make a supervisor call to transfer control to privileged software.
2.1.2 Stacks
The processor uses a full descending stack. This means the stack pointer indicates the last
stacked item on the stack memory. When the processor pushes a new item onto the stack, it
decrements the stack pointer and then writes the item to the new memory location. The
processor implements two stacks, the main stack and the process stack, with independent
copies of the stack pointer, see Stack pointer on page 14.
Thread mode Used to execute application software. The processor enters Thread
mode when it comes out of reset.
Handler mode Used to handle exceptions. The processor returns to Thread mode
when it has finished exception processing.
Unprivileged The software:
●Has limited access to the MSR and MRS instructions, and cannot
use the CPS instruction
●Cannot access the system timer, NVIC, or system control block
●Might have restricted access to memory or peripherals.
Unprivileged software executes at the unprivileged level.
Privileged The software can use all the instructions and has access to all
resources.
Privileged software executes at the privileged level.
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In Thread mode, the CONTROL register controls whether the processor uses the main
stack or the process stack, see CONTROL register on page 20. In Handler mode, the
processor always uses the main stack. The options for processor operations are:
2.1.3 Core registers
Figure 2. Processor core registers
Table 1. Summary of processor mode, execution privilege level, and stack use
options
Processor
mode
Used to
execute
Privilege level for
software execution Stack used
Thread Applications Privileged or unprivileged(1)
1. See CONTROL register on page 20.
Main stack or process stack(1)
Handler Exception handlers Always privileged Main stack
Table 2. Core register set summary
Name Type(1) Required
privilege(2)
Reset
value Description
R0-R12 read-write Either Unknown General-purpose registers on page 14
MSP read-write Privileged See description Stack pointer on page 14
PSP read-write Either Unknown Stack pointer on page 14
SP (R13)
LR (R14)
PC (R15)
R5
R6
R7
R0
R1
R3
R4
R2
R10
R11
R12
R8
R9
Low registers
High registers
MSP‡
PSP‡
PSR
PRIMASK
FAULTMASK
BASEPRI
CONTROL
General-purpose registers
Stack Pointer
Link Register
Program Counter
Program status register
Exception mask registers
CONTROL register
Special registers
‡Banked version of SP
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General-purpose registers
R0-R12 are 32-bit general-purpose registers for data operations.
Stack pointer
The Stack Pointer (SP) is register R13. In Thread mode, bit[1] of the CONTROL register
indicates the stack pointer to use:
●0 = Main Stack Pointer (MSP). This is the reset value.
●1 = Process Stack Pointer (PSP).
On reset, the processor loads the MSP with the value from address 0x00000000.
Link register
The Link Register (LR) is register R14. It stores the return information for subroutines,
function calls, and exceptions. On reset, the processor loads the LR value 0xFFFFFFFF.
Program counter
The Program Counter (PC) is register R15. It contains the current program address. Bit[0] is
always 0 because instruction fetches must be halfword aligned. On reset, the processor
loads the PC with the value of the reset vector, which is at address 0x00000004.
LR read-write Either 0xFFFFFFFF Link register on page 14
PC read-write Either See description Program counter on page 14
PSR read-write Privileged 0x01000000 Program status register on page 15
ASPR read-write Either 0x00000000 Application program status register on
page 16
IPSR read-only Privileged 0x00000000 Interrupt program status register on
page 17
EPSR read-only Privileged 0x01000000 Execution program status register on
page 18
PRIMASK read-write Privileged 0x00000000 Priority mask register on page 19
FAULTMASK read-write Privileged 0x00000000 Fault mask register on page 19
BASEPRI read-write Privileged 0x00000000 Base priority mask register on page 20
CONTROL read-write Privileged 0x00000000 CONTROL register on page 20
1. Describes access type during program execution in thread mode and Handler mode. Debug access can
differ.
2. An entry of Either means privileged and unprivileged software can access the register.
Table 2. Core register set summary (continued)
Name Type(1) Required
privilege(2)
Reset
value Description
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Program status register
The Program Status Register (PSR) combines:
●Application Program Status Register (APSR)
●Interrupt Program Status Register (IPSR)
●Execution Program Status Register (EPSR)
These registers are mutually exclusive bitfields in the 32-bit PSR. The bit assignments are
as shown in Figure 3 and Figure 4.
Figure 3. APSR, IPSR and EPSR bit assignments
Figure 4. PSR bit assignments
Access these registers individually or as a combination of any two or all three registers,
using the register name as an argument to the MSR or MRS instructions. For example:
●Read all of the registers using PSR with the MRS instruction
●Write to the APSR using APSR with the MSR instruction.
The PSR combinations and attributes are:
See the instruction descriptions MRS on page 99 and MSR on page 100 for more
information about how to access the program status registers.
Table 3. PSR register combinations
Register Type Combination
PSR read-write(1), (2)
1. The processor ignores writes to the IPSR bits.
2. Reads of the EPSR bits return zero, and the processor ignores writes to the these bits
APSR, EPSR, and IPSR
IEPSR read-only EPSR and IPSR
IAPSR read-write(1) APSR and IPSR
EAPSR read-write(2) APSR and EPSR
25 24 23
Reserved ISR_NUMBER
31 30 29 28 27
NZCV
0
Reserved
APSR
IPSR
EPSR Reserved Reserved
26 16 15 10 9
ReservedICI/IT ICI/ITT
Q
8
N
31 30 29 28 27 26 25 24 23 16 15 10 9 8 0
Z C V Q ICI/IT T Reserved ICI/IT ISR_NUMBER
Reserved
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Application program status register
The APSR contains the current state of the condition flags from previous instruction
executions. See the register summary in Table 2 on page 13 for its attributes. The bit
assignments are:
Table 4. APSR bit definitions
Bits Description
Bit 31 N: Negative or less than flag:
0: Operation result was positive, zero, greater than, or equal
1: Operation result was negative or less than.
Bit 30 Z: Zero flag:
0: Operation result was not zero
1: Operation result was zero.
Bit 29 C: Carry or borrow flag:
0: Add operation did not result in a carry bit or subtract operation resulted in a
borrow bit
1: Add operation resulted in a carry bit or subtract operation did not result in a
borrow bit.
Bit 28 V: Overflow flag:
0: Operation did not result in an overflow
1: Operation resulted in an overflow.
Bit 27 Q: Sticky saturation flag:
0: Indicates that saturation has not occurred since reset or since the bit was last
cleared to zero
1: Indicates when an SSAT or USAT instruction results in saturation.
This bit is cleared to zero by software using an MRS instruction.
Bits 26:0 Reserved.
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Interrupt program status register
The IPSR contains the exception type number of the current Interrupt Service Routine
(ISR). See the register summary in Table 2 on page 13 for its attributes. The bit assignments
are:
Table 5. IPSR bit definitions
Bits Description
Bits 31:9 Reserved
Bits 8:0 ISR_NUMBER:
This is the number of the current exception:
0: Thread mode
1: Reserved
2: NMI
3: Hard fault
4: Memory management fault
5: Bus fault
6: Usage fault
7: Reserved
....
10: Reserved
11: SVCall
12: Reserved for Debug
13: Reserved
14: PendSV
15: SysTick
16: IRQ0(1)
....
....
83: IRQ67(1)
see Exception types on page 32 for more information.
1. See STM32 product reference manual/datasheet for more information on interrupt mapping
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Execution program status register
The EPSR contains the Thumb state bit, and the execution state bits for either the:
●If-Then (IT) instruction
●Interruptible-Continuable Instruction (ICI) field for an interrupted load multiple or store
multiple instruction.
See the register summary in Table 2 on page 13 for the EPSR attributes. The bit
assignments are:
Attempts to read the EPSR directly through application software using the MSR instruction
always return zero. Attempts to write the EPSR using the MSR instruction in application
software are ignored. Fault handlers can examine EPSR value in the stacked PSR to
indicate the operation that is at fault. See Section 2.3.7: Exception entry and return on
page 36
Interruptible-continuable instructions
When an interrupt occurs during the execution of an LDM or STM instruction, the processor:
●Stops the load multiple or store multiple instruction operation temporarily
●Stores the next register operand in the multiple operation to EPSR bits[15:12].
After servicing the interrupt, the processor:
●Returns to the register pointed to by bits[15:12]
●Resumes execution of the multiple load or store instruction.
When the EPSR holds ICI execution state, bits[26:25,11:10] are zero.
If-Then block
The If-Then block contains up to four instructions following a 16-bit IT instruction. Each
instruction in the block is conditional. The conditions for the instructions are either all the
same, or some can be the inverse of others. See IT on page 93 for more information.
Exception mask registers
The exception mask registers disable the handling of exceptions by the processor. Disable
exceptions where they might impact on timing critical tasks.
Table 6. EPSR bit definitions
Bits Description
Bits 31:27 Reserved.
Bits 26:25, 15:10 ICI: Interruptible-continuable instruction bits
See Interruptible-continuable instructions on page 18.
Bits 26:25, 15:10 IT: Indicates the execution state bits of the IT instruction, see IT on page 93.
Bit 24 Always set to 1.
Bits 23:16 Reserved.
Bits 9:0] Reserved.
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To access the exception mask registers use the MSR and MRS instructions, or the CPS
instruction to change the value of PRIMASK or FAULTMASK. See MRS on page 99, MSR
on page 100, and CPS on page 97 for more information.
Priority mask register
The PRIMASK register prevents activation of all exceptions with configurable priority. See
the register summary in Table 2 on page 13 for its attributes. Figure 5 shows the bit
assignments.
Figure 5. PRIMASK bit assignments
Fault mask register
The FAULTMASK register prevents activation of all exceptions except for Non-Maskable
Interrupt (NMI). See the register summary in Table 2 on page 13 for its attributes. Figure 6
shows the bit assignments.
Figure 6. FAULTMASK bit assignments
The processor clears the FAULTMASK bit to 0 on exit from any exception handler except the
NMI handler.
Table 7. PRIMASK register bit definitions
Bits Description
Bits 31:1 Reserved
Bit 0
PRIMASK:
0: No effect
1: Prevents the activation of all exceptions with configurable priority.
Table 8. FAULTMASK register bit definitions
Bits Function
Bits 31:1 Reserved
Bit 0 FAULTMASK:
0: No effect
1: Prevents the activation of all exceptions except for NMI.
31
Reserved
10
PRIMASK
Reserved
0
131
FAULTMASK
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Base priority mask register
The BASEPRI register defines the minimum priority for exception processing. When
BASEPRI is set to a nonzero value, it prevents the activation of all exceptions with same or
lower priority level as the BASEPRI value. See the register summary in Table 2 on page 13
for its attributes. Figure 7 shows the bit assignments.
Figure 7. BASEPRI bit assignments
CONTROL register
The CONTROL register controls the stack used and the privilege level for software
execution when the processor is in Thread mode. See the register summary in Ta bl e 2 o n
page 13 for its attributes. Figure 8 shows the bit assignments.
Figure 8. CONTROL bit assignments
Table 9. BASEPRI register bit assignments
Bits Function
Bits 31:8 Reserved
Bits 7:4 BASEPRI[7:4] Priority mask bits(1)
0x00: no effect
Nonzero: defines the base priority for exception processing.
The processor does not process any exception with a priority value greater than or
equal to BASEPRI.
1. This field is similar to the priority fields in the interrupt priority registers. See Interrupt priority registers
(NVIC_IPRx) on page 111 for more information. Remember that higher priority field values correspond to
lower exception priorities.
Bits 3:0 Reserved
Table 10. CONTROL register bit definitions
Bits Function
Bits 31:2 Reserved
Bit 1 ASPSEL: Active stack pointer selection
Selects the current stack:
0: MSP is the current stack pointer
1: PSP is the current stack pointer.
In Handler mode this bit reads as zero and ignores writes.
BASEPRIReserved
31 078
31 210
Reserved
Active stack pointer
Thread mode privilege level
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Handler mode always uses the MSP, so the processor ignores explicit writes to the active
stack pointer bit of the CONTROL register when in Handler mode. The exception entry and
return mechanisms update the CONTROL register.
In an OS environment, it is recommended that threads running in Thread mode use the
process stack and the kernel and exception handlers use the main stack.
By default, Thread mode uses the MSP. To switch the stack pointer used in Thread mode to
the PSP, use the MSR instruction to set the Active stack pointer bit to 1, see MSR on
page 100.
When changing the stack pointer, software must use an ISB instruction immediately after
the MSR instruction. This ensures that instructions after the ISB execute using the new
stack pointer. See ISB on page 99
2.1.4 Exceptions and interrupts
The Cortex-M3 processor supports interrupts and system exceptions. The processor and
the Nested Vectored Interrupt Controller (NVIC) prioritize and handle all exceptions. An
exception changes the normal flow of software control. The processor uses handler mode to
handle all exceptions except for reset. See Exception entry on page 37 and Exception return
on page 38 for more information.
The NVIC registers control interrupt handling. See Nested vectored interrupt controller
(NVIC) on page 104 for more information.
2.1.5 Data types
The processor:
●Supports the following data types:
– 32-bit words
– 16-bit halfwords
– 8-bit bytes
●supports 64-bit data transfer instructions.
●manages all memory accesses (data memory, instruction memory and Private
Peripheral Bus (PPB)) as little-endian. See Memory regions, types and attributes on
page 24 for more information.
Bit 0 TPL: Thread mode privilege level
Defines the Thread mode privilege level.
0: Privileged
1: Unprivileged.
Table 10. CONTROL register bit definitions (continued)
Bits Function
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2.1.6 The Cortex microcontroller software interface standard (CMSIS)
For a Cortex-M3 microcontroller system, the Cortex Microcontroller Software Interface
Standard (CMSIS) defines:
●A common way to:
– Access peripheral registers
– Define exception vectors
●The names of:
– The registers of the core peripherals
– The core exception vectors
●A device-independent interface for RTOS kernels, including a debug channel
The CMSIS includes address definitions and data structures for the core peripherals in the
Cortex-M3 processor. It also includes optional interfaces for middleware components
comprising a TCP/IP stack and a Flash file system.
CMSIS simplifies software development by enabling the reuse of template code and the
combination of CMSIS-compliant software components from various middleware vendors.
Software vendors can expand the CMSIS to include their peripheral definitions and access
functions for those peripherals.
This document includes the register names defined by the CMSIS, and gives short
descriptions of the CMSIS functions that address the processor core and the core
peripherals.
This document uses the register short names defined by the CMSIS. In a few cases these
differ from the architectural short names that might be used in other documents.
The following sections give more information about the CMSIS:
●Section 2.5.4: Power management programming hints on page 43
●Intrinsic functions on page 49
●The CMSIS mapping of the Cortex-M3 NVIC registers on page 105
●NVIC programming hints on page 113
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2.2 Memory model
This section describes the processor memory map, the behavior of memory accesses, and
the bit-banding features. The processor has a fixed memory map that provides up to 4 GB of
addressable memory.
Figure 9. Memory map
The regions for SRAM and peripherals include bit-band regions. Bit-banding provides
atomic operations to bit data, see Section 2.2.5: Bit-banding on page 27.
The processor reserves regions of the Private peripheral bus (PPB) address range for core
peripheral registers, see Section 4.1: About the STM32 core peripherals on page 104.
Vendor-specific
memory
External device
External RAM
Peripheral
SRAM
Code
0xFFFFFFFF
Private peripheral
bus
0xE0100000
0xE00FFFFF
0x9FFFFFFF
0xA0000000
0x5FFFFFFF
0x60000000
0x3FFFFFFF
0x40000000
0x1FFFFFFF
0x20000000
0x00000000
0x40000000 Bit band region
Bit band alias
32MB
1MB
0x400FFFFF
0x42000000
0x43FFFFFF
Bit band region
Bit band alias
32MB
1MB
0x20000000
0x200FFFFF
0x22000000
0x23FFFFFF
1.0GB
1.0GB
0.5GB
0.5GB
0.5GB
0xDFFFFFFF
0xE0000000
1.0MB
511MB
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2.2.1 Memory regions, types and attributes
The memory map splits the memory map into regions. Each region has a defined memory
type, and some regions have additional memory attributes. The memory type and attributes
determine the behavior of accesses to the region.
The memory types are:
The different ordering requirements for Device and Strongly-ordered memory mean that the
memory system can buffer a write to Device memory, but must not buffer a write to Strongly-
ordered memory.
The additional memory attributes include:
Normal The processor can re-order transactions for efficiency, or
perform speculative reads.
Device The processor preserves transaction order relative to other
transactions to Device or Strongly-ordered memory.
Strongly-ordered The processor preserves transaction order relative to all other
transactions.
Execute Never (XN) Means the processor prevents instruction accesses. Any
attempt to fetch an instruction from an XN region causes a
memory management fault exception.
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2.2.2 Memory system ordering of memory accesses
For most memory accesses caused by explicit memory access instructions, the memory
system does not guarantee that the order in which the accesses complete matches the
program order of the instructions, providing this does not affect the behavior of the
instruction sequence. Normally, if correct program execution depends on two memory
accesses completing in program order, software must insert a memory barrier instruction
between the memory access instructions, see Section 2.2.4: Software ordering of memory
accesses on page 26.
However, the memory system does guarantee some ordering of accesses to Device and
Strongly-ordered memory. For two memory access instructions A1 and A2, if A1 occurs
before A2 in program order, the ordering of the memory accesses caused by two
instructions is:
2.2.3 Behavior of memory accesses
The behavior of accesses to each region in the memory map is:
Table 11. Ordering of memory accesses(1)
1. - means that the memory system does not guarantee the ordering of the accesses.
< means that accesses are observed in program order, that is, A1 is always observed before A2.
A1
A2
Normal access
Device access Strongly ordered
access
Non-shareable Shareable
Normal access - - - -
Device access, non-
shareable -<- <
Device access, shareable - - < <
Strongly ordered access - < < <
Table 12. Memory access behavior
Address
range
Memory
region
Memory
type XN Description
0x00000000- 0x1FFFFFFF Code Normal(1) -Executable region for program code.
You can also put data here.
0x20000000- 0x3FFFFFFF SRAM Normal(1) -
Executable region for data. You can
also put code here.
This region includes bit band and bit
band alias areas, see Table 13 on
page 27.
0x40000000- 0x5FFFFFFF Peripheral Device(1) XN(1)
This region includes bit band and bit
band alias areas, see Table 14 on
page 27.
0x60000000- 0x9FFFFFFF External
RAM Normal(1) - Executable region for data.
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The Code, SRAM, and external RAM regions can hold programs. However, it is
recommended that programs always use the Code region. This is because the processor
has separate buses that enable instruction fetches and data accesses to occur
simultaneously.
2.2.4 Software ordering of memory accesses
The order of instructions in the program flow does not always guarantee the order of the
corresponding memory transactions. This is because:
●The processor can reorder some memory accesses to improve efficiency, providing this
does not affect the behavior of the instruction sequence.
●The processor has multiple bus interfaces
●Memory or devices in the memory map have different wait states
●Some memory accesses are buffered or speculative.
Section 2.2.2: Memory system ordering of memory accesses on page 25 describes the
cases where the memory system guarantees the order of memory accesses. Otherwise, if
the order of memory accesses is critical, software must include memory barrier instructions
to force that ordering. The processor provides the following memory barrier instructions:
Use memory barrier instructions in, for example:
●Vector table. If the program changes an entry in the vector table, and then enables the
corresponding exception, use a DMB instruction between the operations. This ensures
that if the exception is taken immediately after being enabled the processor uses the
new exception vector.
0xA0000000- 0xDFFFFFFF External
device Device(1) XN(1) External Device memory
0xE0000000- 0xE00FFFFF
Private
Peripheral
Bus
Strongly-
ordered(1) XN(1)
This region includes the NVIC,
System timer, and system control
block.
0xE0100000- 0xFFFFFFFF
Memory
mapped
peripherals
Device(1) XN(1) This region includes all the STM32
standard peripherals.
1. See Memory regions, types and attributes on page 24 for more information.
Table 12. Memory access behavior (continued)
Address
range
Memory
region
Memory
type XN Description
DMB The Data Memory Barrier (DMB) instruction ensures that outstanding
memory transactions complete before subsequent memory transactions.
See DMB on page 98.
DSB The Data Synchronization Barrier (DSB) instruction ensures that
outstanding memory transactions complete before subsequent
instructions execute. See DSB on page 98.
ISB The Instruction Synchronization Barrier (ISB) ensures that the effect of all
completed memory transactions is recognizable by subsequent
instructions. See ISB on page 99.
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●Self-modifying code. If a program contains self-modifying code, use an ISB
instruction immediately after the code modification in the program. This ensures
subsequent instruction execution uses the updated program.
●Memory map switching. If the system contains a memory map switching mechanism,
use a DSB instruction after switching the memory map in the program. This ensures
subsequent instruction execution uses the updated memory map.
●Dynamic exception priority change. When an exception priority has to change when
the exception is pending or active, use DSB instructions after the change. This ensures
the change takes effect on completion of the DSB instruction.
●Using a semaphore in multi-master system. If the system contains more than one
bus master, for example, if another processor is present in the system, each processor
must use a DMB instruction after any semaphore instructions, to ensure other bus
masters see the memory transactions in the order in which they were executed.
Memory accesses to Strongly-ordered memory, such as the system control block, do not
require the use of DMB instructions.
2.2.5 Bit-banding
A bit-band region maps each word in a bit-band alias region to a single bit in the bit-band
region. The bit-band regions occupy the lowest 1 MB of the SRAM and peripheral memory
regions.
The memory map has two 32 MB alias regions that map to two 1 MB bit-band regions:
●Accesses to the 32 MB SRAM alias region map to the 1 MB SRAM bit-band region, as
shown in Ta bl e 1 3
●Accesses to the 32 MB peripheral alias region map to the 1 MB peripheral bit-band
region, as shown in Ta bl e 1 4 .
Table 13. SRAM memory bit-banding regions
Address
range
Memory
region Instruction and data accesses
0x20000000-
0x200FFFFF SRAM bit-band region Direct accesses to this memory range behave as SRAM memory
accesses, but this region is also bit addressable through bit-band alias.
0x22000000-
0x23FFFFFF SRAM bit-band alias
Data accesses to this region are remapped to bit band region. A write
operation is performed as read-modify-write. Instruction accesses are not
remapped.
Table 14. Peripheral memory bit-banding regions
Address
range
Memory
region Instruction and data accesses
0x40000000-
0x400FFFFF Peripheral bit-band alias
Direct accesses to this memory range behave as peripheral memory
accesses, but this region is also bit addressable through bit-band
alias.
0x42000000-
0x43FFFFFF
Peripheral bit-band
region
Data accesses to this region are remapped to bit band region. A write
operation is performed as read-modify-write. Instruction accesses are
not permitted.
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A word access to the SRAM or peripheral bit-band alias regions map to a single bit in the
SRAM or peripheral bit-band region.
The following formula shows how the alias region maps onto the bit-band region:
bit_word_offset = (byte_offset x 32) + (bit_number x 4)
bit_word_addr = bit_band_base + bit_word_offset
Where:
●Bit_word_offset is the position of the target bit in the bit-band memory region.
●Bit_word_addr is the address of the word in the alias memory region that maps to the
targeted bit.
●Bit_band_base is the starting address of the alias region.
●Byte_offset is the number of the byte in the bit-band region that contains the targeted
bit.
●Bit_number is the bit position, 0-7, of the targeted bit.
Figure 10 on page 28 shows examples of bit-band mapping between the SRAM bit-band
alias region and the SRAM bit-band region:
●The alias word at 0x23FFFFE0 maps to bit[0] of the bit-band byte at
0x200FFFFF: 0x23FFFFE0 = 0x22000000 + (0xFFFFF*32) + (0*4).
●The alias word at 0x23FFFFFC maps to bit[7] of the bit-band byte at
0x200FFFFF: 0x23FFFFFC = 0x22000000 + (0xFFFFF*32) + (7*4).
●The alias word at 0x22000000 maps to bit[0] of the bit-band byte at
0x20000000: 0x22000000 = 0x22000000 + (0*32) + (0 *4).
●The alias word at 0x2200001C maps to bit[7] of the bit-band byte at
0x20000000: 0x2200001C = 0x22000000+ (0*32) + (7*4).
Figure 10. Bit-band mapping
0x23FFFFE4
0x22000004
0x23FFFFE00x23FFFFE80x23FFFFEC0x23FFFFF00x23FFFFF40x23FFFFF80x23FFFFFC
0x220000000x220000140x220000180x2200001C 0x220000080x22000010 0x2200000C
32MB alias region
0
7 0
07
0x200000000x200000010x200000020x20000003
6 5 4 3 2 1 07 6 5 4 3 2 1 7 6 5 4 3 2 1 07 6 5 4 3 2 1
07 6 5 4 3 2 1 6 5 4 3 2 107 6 5 4 3 2 1 07 6 5 4 3 2 1
0x200FFFFC0x200FFFFD0x200FFFFE0x200FFFFF
1MB SRAM bit-band region
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Directly accessing an alias region
Writing to a word in the alias region updates a single bit in the bit-band region.
Bit[0] of the value written to a word in the alias region determines the value written to the
targeted bit in the bit-band region. Writing a value with bit[0] set to 1 writes a 1 to the bit-
band bit, and writing a value with bit[0] set to 0 writes a 0 to the bit-band bit.
Bits[31:1] of the alias word have no effect on the bit-band bit. Writing 0x01 has the same
effect as writing 0xFF. Writing 0x00 has the same effect as writing 0x0E.
Reading a word in the alias region:
●0x00000000 indicates that the targeted bit in the bit-band region is set to zero
●0x00000001 indicates that the targeted bit in the bit-band region is set to 1
Directly accessing a bit-band region
Behavior of memory accesses on page 25 describes the behavior of direct byte, halfword, or
word accesses to the bit-band regions.
2.2.6 Memory endianness
The processor views memory as a linear collection of bytes numbered in ascending order
from zero. For example, bytes 0-3 hold the first stored word, and bytes 4-7 hold the second
stored word.
Little-endian format
In little-endian format, the processor stores the least significant byte of a word at the lowest-
numbered byte, and the most significant byte at the highest-numbered byte. See Figure 11
for an example.
Figure 11. Little-endian example
Memory Register
Address A
A+1
lsbyte
msbyte
A+2
A+3
07
B0B1B3 B2
31 24 23 16 15 8 7 0
B0
B1
B2
B3
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2.2.7 Synchronization primitives
The Cortex-M3 instruction set includes pairs of synchronization primitives. These provide a
non-blocking mechanism that a thread or process can use to obtain exclusive access to a
memory location. Software can use them to perform a guaranteed read-modify-write
memory update sequence, or for a semaphore mechanism.
A pair of synchronization primitives comprises:
The pairs of Load-Exclusive and Store-Exclusive instructions are:
●The word instructions LDREX and STREX
●The halfword instructions LDREXH and STREXH
●The byte instructions LDREXB and STREXB.
Software must use a Load-Exclusive instruction with the corresponding Store-Exclusive
instruction.
To perform a guaranteed read-modify-write of a memory location, software must:
1. Use a Load-Exclusive instruction to read the value of the location.
2. Update the value, as required.
3. Use a Store-Exclusive instruction to attempt to write the new value back to the memory
location, and tests the returned status bit. If this bit is:
0: The read-modify-write completed successfully,
1: No write was performed. This indicates that the value returned at step 1 might
be out of date. The software must retry the read-modify-write sequence,
A Load-Exclusive instruction Used to read the value of a memory location, requesting
exclusive access to that location.
A Store-Exclusive instruction Used to attempt to write to the same memory location,
returning a status bit to a register. If this bit is:
0: it indicates that the thread or process gained
exclusive access to the memory, and the write
succeeds
1: it indicates that the thread or process did not gain
exclusive access to the memory, and no write is
performed
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Software can use the synchronization primitives to implement a semaphores as follows:
1. Use a Load-Exclusive instruction to read from the semaphore address to check
whether the semaphore is free.
2. If the semaphore is free, use a Store-Exclusive to write the claim value to the
semaphore address.
3. If the returned status bit from step 2 indicates that the Store-Exclusive succeeded then
the software has claimed the semaphore. However, if the Store-Exclusive failed,
another process might have claimed the semaphore after the software performed step
1.
The Cortex-M3 includes an exclusive access monitor, that tags the fact that the processor
has executed a Load-Exclusive instruction.
The processor removes its exclusive access tag if:
●It executes a CLREX instruction
●It executes a Store-Exclusive instruction, regardless of whether the write succeeds.
●An exception occurs. This means the processor can resolve semaphore conflicts
between different threads.
For more information about the synchronization primitive instructions, see LDREX and
STREX on page 69 and CLREX on page 70.
2.2.8 Programming hints for the synchronization primitives
ANSI C cannot directly generate the exclusive access instructions. Some C compilers
provide intrinsic functions for generation of these instructions:
The actual exclusive access instruction generated depends on the data type of the pointer
passed to the intrinsic function. For example, the following C code generates the require
LDREXB operation:
__ldrex((volatile char *) 0xFF);
Table 15. C compiler intrinsic functions for exclusive access instructions
Instruction Intrinsic function
LDREX, LDREXH, or LDREXB unsigned int __ldrex(volatile void *ptr)
STREX, STREXH, or STREXB int __strex(unsigned int val, volatile void *ptr)
CLREX void __clrex(void)
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2.3 Exception model
This section describes the exception model.
2.3.1 Exception states
Each exception is in one of the following states:
2.3.2 Exception types
The exception types are:
Inactive The exception is not active and not pending.
Pending The exception is waiting to be serviced by the processor. An
interrupt request from a peripheral or from software can change
the state of the corresponding interrupt to pending.
Active An exception that is being serviced by the processor but has not
completed.
Note: An exception handler can interrupt the execution of another
exception handler. In this case both exceptions are in the active
state.
Active and pending The exception is being serviced by the processor and there is a
pending exception from the same source.
Reset Reset is invoked on power up or a warm reset. The exception model
treats reset as a special form of exception. When reset is asserted,
the operation of the processor stops, potentially at any point in an
instruction. When reset is deasserted, execution restarts from the
address provided by the reset entry in the vector table. Execution
restarts as privileged execution in Thread mode.
NMI A NonMaskable Interrupt (NMI) can be signalled by a peripheral or
triggered by software. This is the highest priority exception other
than reset. It is permanently enabled and has a fixed priority of -2.
NMIs cannot be:
●Masked or prevented from activation by any other exception
●Preempted by any exception other than Reset.
Hard fault A hard fault is an exception that occurs because of an error during
exception processing, or because an exception cannot be managed
by any other exception mechanism. Hard faults have a fixed priority
of -1, meaning they have higher priority than any exception with
configurable priority.
Memory management
fault
A memory management fault is an exception that occurs because of
a memory protection related fault. The fixed memory protection
constraints determines this fault, for both instruction and data
memory transactions. This fault is used to abort instruction accesses
to Execute Never (XN) memory regions.
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Bus fault A bus fault is an exception that occurs because of a memory related
fault for an instruction or data memory transaction. This might be
from an error detected on a bus in the memory system.
Usage fault A usage fault is an exception that occurs because of a fault related to
instruction execution. This includes:
●An undefined instruction
●An illegal unaligned access
●Invalid state on instruction execution
●An error on exception return.
The following can cause a usage fault when the core is configured to
report them:
●An unaligned address on word and halfword memory access
●Division by zero
SVCall A supervisor call (SVC) is an exception that is triggered by the SVC
instruction. In an OS environment, applications can use SVC
instructions to access OS kernel functions and device drivers.
PendSV PendSV is an interrupt-driven request for system-level service. In an
OS environment, use PendSV for context switching when no other
exception is active.
SysTick A SysTick exception is an exception the system timer generates
when it reaches zero. Software can also generate a SysTick
exception. In an OS environment, the processor can use this
exception as system tick.
Interrupt (IRQ) A interrupt, or IRQ, is an exception signalled by a peripheral, or
generated by a software request. All interrupts are asynchronous to
instruction execution. In the system, peripherals use interrupts to
communicate with the processor.
Table 16. Properties of the different exception types
Exception
number(1)
IRQ
number(1)
Exception
type Priority Vector address
or offset(2) Activation
1 - Reset -3, the highest 0x00000004 Asynchronous
2 -14 NMI -2 0x00000008 Asynchronous
3 -13 Hard fault -1 0x0000000C -
4 -12 Memory
management fault Configurable(3) 0x00000010 Synchronous
5 -11 Bus fault Configurable(3) 0x00000014 Synchronous when precise,
asynchronous when imprecise
6 -10 Usage fault Configurable(3) 0x00000018 Synchronous
7-10 - - - Reserved -
11 -5 SVCall Configurable(3) 0x0000002C Synchronous
12-13 - - - Reserved -
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For an asynchronous exception, other than reset, the processor can execute another
instruction between when the exception is triggered and when the processor enters the
exception handler.
Privileged software can disable the exceptions that Table 16 on page 33 shows as having
configurable priority, see:
●System handler control and state register (SCB_SHCSR) on page 124
●Interrupt clear-enable registers (NVIC_ICERx) on page 107
For more information about hard faults, memory management faults, bus faults, and usage
faults, see Section 2.4: Fault handling on page 39.
2.3.3 Exception handlers
The processor handles exceptions using:
14 -2 PendSV Configurable(3) 0x00000038 Asynchronous
15 -1 SysTick Configurable(3) 0x0000003C Asynchronous
16-83 0-67 Interrupt (IRQ) Configurable(4) 0x00000040 and
above(5) Asynchronous
1. To simplify the software layer, the CMSIS only uses IRQ numbers and therefore uses negative values for exceptions other
than interrupts. The IPSR returns the Exception number, see Interrupt program status register on page 17.
2. See Vector table on page 35 for more information.
3. See System handler priority registers (SHPRx) on page 122.
4. See Interrupt priority registers (NVIC_IPRx) on page 111.
5. Increasing in steps of 4.
Table 16. Properties of the different exception types (continued)
Exception
number(1)
IRQ
number(1)
Exception
type Priority Vector address
or offset(2) Activation
Interrupt Service
Routines (ISRs)
Interrupts IRQ0 to IRQ67 are the exceptions handled by ISRs.
Fault handlers Hard fault, memory management fault, usage fault, bus fault are fault
exceptions handled by the fault handlers.
System handlers NMI, PendSV, SVCall SysTick, and the fault exceptions are all
system exceptions that are handled by system handlers.
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2.3.4 Vector table
The vector table contains the reset value of the stack pointer, and the start addresses, also
called exception vectors, for all exception handlers. Figure 12 on page 35 shows the order of
the exception vectors in the vector table. The least-significant bit of each vector must be 1,
indicating that the exception handler is Thumb code.
Figure 12. Vector table
On system reset, the vector table is fixed at address 0x00000000. Privileged software can
write to the VTOR to relocate the vector table start address to a different memory location, in
the range 0x00000080 to 0x3FFFFF80, see Vector table offset register (SCB_VTOR) on
page 118.
2.3.5 Exception priorities
As Table 16 on page 33 shows, all exceptions have an associated priority, with:
●A lower priority value indicating a higher priority
●Configurable priorities for all exceptions except Reset, Hard fault, and NMI.
Initial SP value
Reset
Hard fault
NMI
Memory management fault
Usage fault
Bus fault
0x0000
0x0004
0x0008
0x000C
0x0010
0x0014
0x0018
Reserved
SVCall
PendSV
Reserved for Debug
Systick
IRQ0
Reserved
0x002C
0x0038
0x003C
0x0040
OffsetException number
2
3
4
5
6
11
12
14
15
16
18
13
7
10
1
Vector
.
.
.
8
9
IRQ1
IRQ2
0x0044
IRQ67
17 0x0048
0x004C
83
.
.
.
.
.
.
0x014C
IRQ number
-14
-13
-12
-11
-10
-5
-2
-1
0
2
1
67
ai15995
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If software does not configure any priorities, then all exceptions with a configurable priority
have a priority of 0. For information about configuring exception priorities see
●System handler priority registers (SHPRx) on page 122
●Interrupt priority registers (NVIC_IPRx) on page 111
Configurable priority values are in the range 0-15. This means that the Reset, Hard fault,
and NMI exceptions, with fixed negative priority values, always have higher priority than any
other exception.
For example, assigning a higher priority value to IRQ[0] and a lower priority value to IRQ[1]
means that IRQ[1] has higher priority than IRQ[0]. If both IRQ[1] and IRQ[0] are asserted,
IRQ[1] is processed before IRQ[0].
If multiple pending exceptions have the same priority, the pending exception with the lowest
exception number takes precedence. For example, if both IRQ[0] and IRQ[1] are pending
and have the same priority, then IRQ[0] is processed before IRQ[1].
When the processor is executing an exception handler, the exception handler is preempted
if a higher priority exception occurs. If an exception occurs with the same priority as the
exception being handled, the handler is not preempted, irrespective of the exception
number. However, the status of the new interrupt changes to pending.
2.3.6 Interrupt priority grouping
To increase priority control in systems with interrupts, the NVIC supports priority grouping.
This divides each interrupt priority register entry into two fields:
●An upper field that defines the group priority
●A lower field that defines a subpriority within the group.
Only the group priority determines preemption of interrupt exceptions. When the processor
is executing an interrupt exception handler, another interrupt with the same group priority as
the interrupt being handled does not preempt the handler,
If multiple pending interrupts have the same group priority, the subpriority field determines
the order in which they are processed. If multiple pending interrupts have the same group
priority and subpriority, the interrupt with the lowest IRQ number is processed first.
For information about splitting the interrupt priority fields into group priority and subpriority,
see Application interrupt and reset control register (SCB_AIRCR) on page 119.
2.3.7 Exception entry and return
Descriptions of exception handling use the following terms:
Preemption When the processor is executing an exception handler, an exception
can preempt the exception handler if its priority is higher than the
priority of the exception being handled. See Section 2.3.6: Interrupt
priority grouping for more information about preemption by an
interrupt.
When one exception preempts another, the exceptions are called
nested exceptions. See Exception entry on page 37 more
information.
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Exception entry
Exception entry occurs when there is a pending exception with sufficient priority and either:
●The processor is in Thread mode
●The new exception is of higher priority than the exception being handled, in which case
the new exception preempts the original exception.
When one exception preempts another, the exceptions are nested.
Sufficient priority means the exception has more priority than any limits set by the mask
registers, see Exception mask registers on page 18. An exception with less priority than this
is pending but is not handled by the processor.
When the processor takes an exception, unless the exception is a tail-chained or a late-
arriving exception, the processor pushes information onto the current stack. This operation
is referred as stacking and the structure of eight data words is referred as stack frame. The
stack frame contains the following information:
●R0-R3, R12
●Return address
●PSR
●LR.
Immediately after stacking, the stack pointer indicates the lowest address in the stack frame.
Unless stack alignment is disabled, the stack frame is aligned to a double-word address. If
the STKALIGN bit of the Configuration Control Register (CCR) is set to 1, stack align
adjustment is performed during stacking.
Return This occurs when the exception handler is completed, and:
●There is no pending exception with sufficient priority to be
serviced
●The completed exception handler was not handling a late-
arriving exception.
The processor pops the stack and restores the processor state to the
state it had before the interrupt occurred. See Exception return on
page 38 for more information.
Tail-chaining This mechanism speeds up exception servicing. On completion of an
exception handler, if there is a pending exception that meets the
requirements for exception entry, the stack pop is skipped and
control transfers to the new exception handler.
Late-arriving This mechanism speeds up preemption. If a higher priority exception
occurs during state saving for a previous exception, the processor
switches to handle the higher priority exception and initiates the
vector fetch for that exception. State saving is not affected by late
arrival because the state saved is the same for both exceptions.
Therefore the state saving continues uninterrupted. The processor
can accept a late arriving exception until the first instruction of the
exception handler of the original exception enters the execute stage
of the processor. On return from the exception handler of the late-
arriving exception, the normal tail-chaining rules apply.
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The stack frame includes the return address. This is the address of the next instruction in
the interrupted program. This value is restored to the PC at exception return so that the
interrupted program resumes.
In parallel to the stacking operation, the processor performs a vector fetch that reads the
exception handler start address from the vector table. When stacking is complete, the
processor starts executing the exception handler. At the same time, the processor writes an
EXC_RETURN value to the LR. This indicates which stack pointer corresponds to the stack
frame and what operation mode the was processor was in before the entry occurred.
If no higher priority exception occurs during exception entry, the processor starts executing
the exception handler and automatically changes the status of the corresponding pending
interrupt to active.
If another higher priority exception occurs during exception entry, the processor starts
executing the exception handler for this exception and does not change the pending status
of the earlier exception. This is the late arrival case.
Exception return
Exception return occurs when the processor is in Handler mode and executes one of the
following instructions to load the EXC_RETURN value into the PC:
●A POP instruction that includes the PC
●A BX instruction with any register.
●An LDR or LDM instruction with the PC as the destination
EXC_RETURN is the value loaded into the LR on exception entry. The exception
mechanism relies on this value to detect when the processor has completed an exception
handler. The lowest four bits of this value provide information on the return stack and
processor mode. Ta bl e 1 7 shows the EXC_RETURN[3:0] values with a description of the
exception return behavior.
The processor sets EXC_RETURN bits[31:4] to 0xFFFFFFF. When this value is loaded into
the PC it indicates to the processor that the exception is complete, and the processor
initiates the exception return sequence.
Table 17. Exception return behavior
EXC_RETURN[3:0] Description
0bxxx0 Reserved.
0b0001
Return to Handler mode.
Exception return gets state from MSP.
Execution uses MSP after return.
0b0011 Reserved.
0b01x1 Reserved.
0b1001
Return to Thread mode.
Exception return gets state from MSP.
Execution uses MSP after return.
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2.4 Fault handling
Faults are a subset of the exceptions, see Exception model on page 32. The following
generate a fault:
●A bus error on:
– An instruction fetch or vector table load
– A data access
●An internally-detected error such as an undefined instruction or an attempt to change
state with a BX instruction
●Attempting to execute an instruction from a memory region marked as Non-Executable
(XN).
2.4.1 Fault types
Ta bl e 1 8 shows the types of fault, the handler used for the fault, the corresponding fault
status register, and the register bit that indicates that the fault has occurred. See
Configurable fault status register (SCB_CFSR) on page 126 for more information about the
fault status registers.
0b1101
Return to Thread mode.
Exception return gets state from PSP.
Execution uses PSP after return.
0b1x11 Reserved.
Table 17. Exception return behavior (continued)
EXC_RETURN[3:0] Description
Table 18. Faults
Fault Handler Bit name Fault status register
Bus error on a vector read Hard fault VECTTBL Hard fault status register
(SCB_HFSR) on page 129
Fault escalated to a hard fault FORCED
Bus error:
Bus fault
--
During exception stacking STKERR
Configurable fault status
register (SCB_CFSR) on
page 126
During exception unstacking UNSTKERR
During instruction prefetch IBUSERR
Precise data bus error PRECISERR
Imprecise data bus error IMPRECISERR
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2.4.2 Fault escalation and hard faults
All faults exceptions except for hard fault have configurable exception priority, see System
handler priority registers (SHPRx) on page 122. Software can disable execution of the
handlers for these faults, see System handler control and state register (SCB_SHCSR) on
page 124.
Usually, the exception priority, together with the values of the exception mask registers,
determines whether the processor enters the fault handler, and whether a fault handler can
preempt another fault handler. as described in Section 2.3: Exception model on page 32.
In some situations, a fault with configurable priority is treated as a hard fault. This is called
priority escalation, and the fault is described as escalated to hard fault. Escalation to hard
fault occurs when:
●A fault handler causes the same kind of fault as the one it is servicing. This escalation
to hard fault occurs because a fault handler cannot preempt itself because it must have
the same priority as the current priority level.
●A fault handler causes a fault with the same or lower priority as the fault it is servicing.
This is because the handler for the new fault cannot preempt the currently executing
fault handler.
●An exception handler causes a fault for which the priority is the same as or lower than
the currently executing exception.
●A fault occurs and the handler for that fault is not enabled.
If a bus fault occurs during a stack push when entering a bus fault handler, the bus fault
does not escalate to a hard fault. This means that if a corrupted stack causes a fault, the
fault handler executes even though the stack push for the handler failed. The fault handler
operates but the stack contents are corrupted.
Only Reset and NMI can preempt the fixed priority hard fault. A hard fault can preempt any
exception other than Reset, NMI, or another hard fault.
Attempt to access a coprocessor
Usage fault
NOCP
Configurable fault status
register (SCB_CFSR) on
page 126
Undefined instruction UNDEFINSTR
Attempt to enter an invalid instruction
set state(1) INVSTATE
Invalid EXC_RETURN value INVPC
Illegal unaligned load or store UNALIGNED
Divide By 0 DIVBYZERO
1. Attempting to use an instruction set other than the Thumb instruction set.
Table 18. Faults (continued)
Fault Handler Bit name Fault status register
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2.4.3 Fault status registers and fault address registers
The fault status registers indicate the cause of a fault. For bus faults and memory
management faults, the fault address register indicates the address accessed by the
operation that caused the fault, as shown in Ta b l e 19 .
2.4.4 Lockup
The processor enters a lockup state if a hard fault occurs when executing the NMI or hard
fault handlers. When the processor is in lockup state it does not execute any instructions.
The processor remains in lockup state until either:
●It is reset
●An NMI occurs
If lockup state occurs from the NMI handler a subsequent NMI does not cause the processor
to leave lockup state.
2.5 Power management
The STM32 and Cortex-M3 processor sleep modes reduce power consumption:
●Sleep mode stops the processor clock. All other system and peripheral clocks may still
be running.
●Deep sleep mode stops most of the STM32 system and peripheral clocks. At product
level, this corresponds to either the Stop or the Standby mode. For more details, please
refer to the “Power modes” Section in the STM32 reference manual.
The SLEEPDEEP bit of the SCR selects which sleep mode is used, see System control
register (SCB_SCR) on page 120. For more information about the behavior of the sleep
modes see the STM32 product reference manual.
This section describes the mechanisms for entering sleep mode, and the conditions for
waking up from sleep mode.
Table 19. Fault status and fault address registers
Handler Status register
name
Address register
name Register description
Hard fault HFSR - Hard fault status register (SCB_HFSR) on
page 129
Memory
management fault MMFSR MMFAR
Configurable fault status register (SCB_CFSR)
on page 126
Memory management fault address register
(SCB_MMFAR) on page 130
Bus fault BFSR BFAR
Configurable fault status register (SCB_CFSR)
on page 126
Bus fault address register (SCB_BFAR) on
page 130
Usage fault UFSR - Configurable fault status register (SCB_CFSR)
on page 126
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2.5.1 Entering sleep mode
This section describes the mechanisms software can use to put the processor into sleep
mode.
The system can generate spurious wakeup events, for example a debug operation wakes up
the processor. Therefore software must be able to put the processor back into sleep mode
after such an event. A program might have an idle loop to put the processor back to sleep
mode.
Wait for interrupt
The wait for interrupt instruction, WFI, causes immediate entry to sleep mode. When the
processor executes a WFI instruction it stops executing instructions and enters sleep mode.
See WFI on page 103for more information.
Wait for event
The wait for event instruction, WFE, causes entry to sleep mode conditional on the value of
an one-bit event register. When the processor executes a WFE instruction, it checks this
register:
●If the register is 0 the processor stops executing instructions and enters sleep mode
●If the register is 1 the processor clears the register to 0 and continues executing
instructions without entering sleep mode.
See WFE on page 102 for more information.
If the event register is 1, this indicate that the processor must not enter sleep mode on
execution of a WFE instruction. Typically, this is because an external event signal is
asserted, or a processor in the system has executed an SEV instruction, see SEV on
page 101. Software cannot access this register directly.
Sleep-on-exit
If the SLEEPONEXIT bit of the SCR is set to 1, when the processor completes the execution
of an exception handler it returns to Thread mode and immediately enters sleep mode. Use
this mechanism in applications that only require the processor to run when an exception
occurs.
2.5.2 Wakeup from sleep mode
The conditions for the processor to wakeup depend on the mechanism that cause it to enter
sleep mode.
Wakeup from WFI or sleep-on-exit
Normally, the processor wakes up only when it detects an exception with sufficient priority to
cause exception entry.
Some embedded systems might have to execute system restore tasks after the processor
wakes up, and before it executes an interrupt handler. To achieve this set the PRIMASK bit
to 1 and the FAULTMASK bit to 0. If an interrupt arrives that is enabled and has a higher
priority than current exception priority, the processor wakes up but does not execute the
interrupt handler until the processor sets PRIMASK to zero. For more information about
PRIMASK and FAULTMASK see Exception mask registers on page 18.
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Wakeup from WFE
The processor wakes up if:
●it detects an exception with sufficient priority to cause exception entry
●it detects an external event signal, see The external event input on page 43
In addition, if the SEVONPEND bit in the SCR is set to 1, any new pending interrupt triggers
an event and wakes up the processor, even if the interrupt is disabled or has insufficient
priority to cause exception entry. For more information about the SCR see System control
register (SCB_SCR) on page 120.
2.5.3 The external event input
The processor provides an external event input signal. This signal can be generated by the
up to 16 external input lines, by the PVD, RTC alarm or by the USB wakeup event,
configured through the external interrupt/event controller (EXTI).
This signal can wakeup the processor from WFE, or set the internal WFE event register to
one to indicate that the processor must not enter sleep mode on a later WFE instruction, see
Wait for event on page 42. Fore more details please refer to the STM32 reference manual,
section 4.3 Low power modes.
2.5.4 Power management programming hints
ANSI C cannot directly generate the WFI and WFE instructions. The CMSIS provides the
following intrinsic functions for these instructions:
void __WFE(void) // Wait for Event
void __WFE(void) // Wait for Interrupt
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3 The Cortex-M3 instruction set
3.1 Instruction set summary
The processor implements a version of the thumb instruction set. Ta b le 2 0 lists the
supported instructions.
In Ta b l e 2 0 :
●Angle brackets, <>, enclose alternative forms of the operand
●Braces, {}, enclose optional operands
●The operands column is not exhaustive
●Op2 is a flexible second operand that can be either a register or a constant
●Most instructions can use an optional condition code suffix
For more information on the instructions and operands, see the instruction descriptions.
Table 20. Cortex-M3 instructions
Mnemonic Operands Brief description Flags Page
ADC, ADCS
{Rd,}
Rn,
Op2
Add with carry N,Z,C,V 3.5.1 on
page 72
ADD, ADDS
{Rd,}
Rn, Op2
Add N,Z,C,V 3.5.1 on
page 72
ADD, ADDW
{Rd,}
Rn, #imm12
Add N,Z,C,V 3.5.1 on
page 72
ADR Rd, label Load PC-relative address — 3.4.1 on
page 59
AND, ANDS {Rd,} Rn, Op2 Logical AND N,Z,C 3.5.2 on
page 74
ASR, ASRS Rd, Rm, <Rs|#n> Arithmetic shift right N,Z,C 3.5.3 on
page 75
B label Branch — 3.8.5 on
page 91
BFC Rd, #lsb, #width Bit field clear — 3.8.1 on
page 88
BFI Rd, Rn, #lsb, #width Bit field insert — 3.8.1 on
page 88
BIC, BICS
{Rd,}
Rn, Op2
Bit clear N,Z,C 3.5.2 on
page 74
BKPT #imm Breakpoint — 3.9.1 on
page 97
BL label Branch with link — 3.8.5 on
page 91
BLX Rm Branch indirect with link — 3.8.5 on
page 91
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BX Rm Branch indirect — 3.8.5 on
page 91
CBNZ Rn, label Compare and branch if non zero — 3.8.6 on
page 92
CBZ Rn, label Compare and branch if zero — 3.8.6 on
page 92
CLREX — Clear exclusive — 3.4.9 on
page 70
CLZ Rd, Rm Count leading zeros — 3.5.4 on
page 76
CMN, CMNS Rn, Op2 Compare negative N,Z,C,V 3.5.5 on
page 77
CMP, CMPS Rn, Op2 Compare N,Z,C,V 3.5.5 on
page 77
CPSID iflags Change processor state, disable
interrupts —3.9.2 on
page 97
CPSIE iflags Change processor state, enable
interrupts —3.9.2 on
page 97
DMB — Data memory barrier — 3.9.4 on
page 98
DSB — Data synchronization barrier — 3.9.4 on
page 98
EOR, EORS {Rd,} Rn, Op2 Exclusive OR N,Z,C 3.5.2 on
page 74
ISB — Instruction synchronization barrier — 3.9.5 on
page 99
IT — If-then condition block — 3.8.7 on
page 93
LDM Rn{!}, reglist Load multiple registers, increment
after —3.4.6 on
page 66
LDMDB,
LDMEA Rn{!}, reglist Load multiple registers, decrement
before —3.4.6 on
page 66
LDMFD,
LDMIA Rn{!}, reglist Load multiple registers, increment
after —3.4.6 on
page 66
LDR Rt, [Rn, #offset] Load register with word — 3.4 on page
59
LDRB,
LDRBT Rt, [Rn, #offset] Load register with byte — 3.4 on page
59
LDRD Rt, Rt2, [Rn, #offset] Load register with two bytes — 3.4.2 on
page 60
LDREX Rt, [Rn, #offset] Load register exclusive — 3.4.8 on
page 69
Table 20. Cortex-M3 instructions (continued)
Mnemonic Operands Brief description Flags Page
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LDREXB Rt, [Rn] Load register exclusive with byte — 3.4.8 on
page 69
LDREXH Rt, [Rn] Load register exclusive with
halfword —3.4.8 on
page 69
LDRH,
LDRHT Rt, [Rn, #offset] Load register with halfword — 3.4 on page
59
LDRSB,
LDRSBT Rt, [Rn, #offset] Load register with signed byte — 3.4 on page
59
LDRSH,
LDRSHT Rt, [Rn, #offset] Load register with signed halfword — 3.4 on page
59
LDRT Rt, [Rn, #offset] Load register with word — 3.4 on page
59
LSL, LSLS Rd, Rm, <Rs|#n> Logical shift left N,Z,C 3.5.3 on
page 75
LSR, LSRS Rd, Rm, <Rs|#n> Logical shift right N,Z,C 3.5.3 on
page 75
MLA Rd, Rn, Rm, Ra Multiply with accumulate, 32-bit
result —3.6.1 on
page 82
MLS Rd, Rn, Rm, Ra Multiply and subtract, 32-bit result — 3.6.1 on
page 82
MOV, MOVS Rd, Op2 Move N,Z,C 3.5.6 on
page 78
MOVT Rd, #imm16 Move top — 3.5.7 on
page 79
MOVW, MOV Rd, #imm16 Move 16-bit constant N,Z,C 3.5.6 on
page 78
MRS Rd, spec_reg Move from special register to
general register —3.9.6 on
page 99
MSR spec_reg, Rm Move from general register to
special register N,Z,C,V 3.9.7 on
page 100
MUL, MULS {Rd,} Rn, Rm Multiply, 32-bit result N,Z 3.6.1 on
page 82
MVN, MVNS Rd, Op2 Move NOT N,Z,C 3.5.6 on
page 78
NOP — No operation — 3.9.8 on
page 101
ORN, ORNS {Rd,} Rn, Op2 Logical OR NOT N,Z,C 3.5.2 on
page 74
ORR, ORRS {Rd,} Rn, Op2 Logical OR N,Z,C 3.5.2 on
page 74
POP reglist Pop registers from stack — 3.4.7 on
page 67
Table 20. Cortex-M3 instructions (continued)
Mnemonic Operands Brief description Flags Page
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PUSH reglist Push registers onto stack — 3.4.7 on
page 67
RBIT Rd, Rn Reverse bits — 3.5.8 on
page 80
REV Rd, Rn Reverse byte order in a word — 3.5.8 on
page 80
REV16 Rd, Rn Reverse byte order in each
halfword —3.5.8 on
page 80
REVSH Rd, Rn Reverse byte order in bottom
halfword and sign extend —3.5.8 on
page 80
ROR, RORS Rd, Rm, <Rs|#n> Rotate right N,Z,C 3.5.3 on
page 75
RRX, RRXS Rd, Rm Rotate right with extend N,Z,C 3.5.3 on
page 75
RSB, RSBS {Rd,} Rn, Op2 Reverse subtract N,Z,C,V 3.5.1 on
page 72
SBC, SBCS {Rd,} Rn, Op2 Subtract with carry N,Z,C,V 3.5.1 on
page 72
SBFX Rd, Rn, #lsb, #width Signed bit field extract — 3.8.2 on
page 88
SDIV {Rd,} Rn, Rm Signed divide — 3.6.3 on
page 85
SEV — Send event — 3.9.9 on
page 101
SMLAL RdLo, RdHi, Rn, Rm Signed multiply with accumulate
(32 x 32 + 64), 64-bit result —3.6.2 on
page 84
SMULL RdLo, RdHi, Rn, Rm Signed multiply (32 x 32), 64-bit
result —3.6.2 on
page 84
SSAT Rd, #n, Rm {,shift #s} Signed saturate Q 3.7.1 on
page 86
STM Rn{!}, reglist Store multiple registers, increment
after —3.4.6 on
page 66
STMDB,
STMEA Rn{!}, reglist Store multiple registers,
decrement before —3.4.6 on
page 66
STMFD,
STMIA Rn{!}, reglist Store multiple registers, increment
after —3.4.6 on
page 66
STR Rt, [Rn, #offset] Store register word — 3.4 on page
59
STRB,
STRBT Rt, [Rn, #offset] Store register byte — 3.4 on page
59
STRD Rt, Rt2, [Rn, #offset] Store register two words — 3.4.2 on
page 60
Table 20. Cortex-M3 instructions (continued)
Mnemonic Operands Brief description Flags Page
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STREX Rd, Rt, [Rn, #offset] Store register exclusive — 3.4.8 on
page 69
STREXB Rd, Rt, [Rn] Store register exclusive byte — 3.4.8 on
page 69
STREXH Rd, Rt, [Rn] Store register exclusive halfword — 3.4.8 on
page 69
STRH,
STRHT Rt, [Rn, #offset] Store register halfword — 3.4 on page
59
STRT Rt, [Rn, #offset] Store register word — 3.4 on page
59
SUB, SUBS {Rd,} Rn, Op2 Subtract N,Z,C,V 3.5.1 on
page 72
SUB, SUBW {Rd,} Rn, #imm12 Subtract N,Z,C,V 3.5.1 on
page 72
SVC #imm Supervisor call — 3.9.10 on
page 102
SXTB {Rd,} Rm {,ROR #n} Sign extend a byte — 3.8.3 on
page 89
SXTH {Rd,} Rm {,ROR #n} Sign extend a halfword — 3.8.3 on
page 89
TBB [Rn, Rm] Table branch byte — 3.8.8 on
page 95
TBH [Rn, Rm, LSL #1] Table branch halfword — 3.8.8 on
page 95
TEQ Rn, Op2 Test equivalence N,Z,C 3.5.9 on
page 81
TST Rn, Op2 Test N,Z,C 3.5.9 on
page 81
UBFX Rd, Rn, #lsb, #width Unsigned bit field extract — 3.8.2 on
page 88
UDIV {Rd,} Rn, Rm Unsigned divide — 3.6.3 on
page 85
UMLAL RdLo, RdHi, Rn, Rm Unsigned multiply with accumulate
(32 x 32 + 64), 64-bit result —3.6.2 on
page 84
UMULL RdLo, RdHi, Rn, Rm Unsigned multiply (32 x 32), 64-bit
result —3.6.2 on
page 84
USAT Rd, #n, Rm {,shift #s} Unsigned saturate Q 3.7.1 on
page 86
UXTB {Rd,} Rm {,ROR #n} Zero extend a byte — 3.8.3 on
page 89
UXTH {Rd,} Rm {,ROR #n} Zero extend a halfword — 3.8.3 on
page 89
Table 20. Cortex-M3 instructions (continued)
Mnemonic Operands Brief description Flags Page
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3.2 Intrinsic functions
ANSI cannot directly access some Cortex-M3 instructions. This section describes intrinsic
functions that can generate these instructions, provided by the CMIS and that might be
provided by a C compiler. If a C compiler does not support an appropriate intrinsic function,
you might have to use an inline assembler to access some instructions.
The CMSIS provides the intrinsic functions listed in Ta b l e 2 1 to generate instructions that
ANSI cannot directly access.
WFE — Wait for event — 3.9.11 on
page 102
WFI — Wait for interrupt — 3.9.12 on
page 103
Table 20. Cortex-M3 instructions (continued)
Mnemonic Operands Brief description Flags Page
Table 21. CMSIS intrinsic functions to generate some Cortex-M3 instructions
Instruction CMSIS intrinsic function
CPSIE I void __enable_irq(void)
CPSID I void __disable_irq(void)
CPSIE F void __enable_fault_irq(void)
CPSID F void __disable_fault_irq(void)
ISB void __ISB(void)
DSB void __DSB(void)
DMB void __DMB(void)
REV uint32_t __REV(uint32_t int value)
REV16 uint32_t __REV16(uint32_t int value)
REVSH uint32_t __REVSH(uint32_t int value)
RBIT uint32_t __RBIT(uint32_t int value)
SEV void __SEV(void)
WFE void __WFE(void)
WFI void __WFI(void)
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The CMSIS also provides a number of functions for accessing the special registers using
MRS
and
MSR
instructions (see Ta bl e 2 2 ).
3.3 About the instruction descriptions
The following sections give more information about using the instructions:
●Operands on page 50
●Restrictions when using PC or SP on page 51
●Flexible second operand on page 51
●Shift operations on page 52
●Address alignment on page 55
●PC-relative expressions on page 55
●Conditional execution on page 56
●Instruction width selection on page 58.
3.3.1 Operands
An instruction operand can be an ARM register, a constant, or another instruction-specific
parameter. Instructions act on the operands and often store the result in a destination
register. When there is a destination register in the instruction, it is usually specified before
the operands.
Operands in some instructions are flexible in that they can either be a register or a constant
(see Flexible second operand).
Table 22. CMSIS intrinsic functions to access the special registers
Special register Access CMSIS function
PRIMASK Read uint32_t __get_PRIMASK (void)
Write void __set_PRIMASK (uint32_t value)
FAULTMASK Read uint32_t __get_FAULTMASK (void)
Write void __set_FAULTMASK (uint32_t value)
BASEPRI Read uint32_t __get_BASEPRI (void)
Write void __set_BASEPRI (uint32_t value)
CONTROL Read uint32_t __get_CONTROL (void)
Write void __set_CONTROL (uint32_t value)
MSP Read uint32_t __get_MSP (void)
Write void __set_MSP (uint32_t TopOfMainStack)
PSP Read uint32_t __get_PSP (void)
Write void __set_PSP (uint32_t TopOfProcStack)
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3.3.2 Restrictions when using PC or SP
Many instructions have restrictions on whether you can use the program counter (PC) or
stack pointer (SP) for the operands or destination register. See instruction descriptions for
more information.
Bit[0] of any address written to the PC with a BX, BLX, LDM, LDR, or POP instruction must
be 1 for correct execution, because this bit indicates the required instruction set, and the
Cortex-M3 processor only supports thumb instructions.
3.3.3 Flexible second operand
Many general data processing instructions have a flexible second operand. This is shown as
operand2 in the descriptions of the syntax of each instruction.
Operand2 can be a:
●Constant
●Register with optional shift
Constant
You specify an operand2 constant in the form #constant, where constant can be:
●Any constant that can be produced by shifting an 8-bit value left by any number of bits
within a 32-bit word.
●Any constant of the form 0x00XY00XY
●Any constant of the form 0xXY00XY00
●Any constant of the form 0xXYXYXYXY
In the constants shown above, X and Y are hexadecimal digits.
In addition, in a small number of instructions, constant can take a wider range of values.
These are described in the individual instruction descriptions.
When an operand2 constant is used with the instructions MOVS, MVNS, ANDS, ORRS,
ORNS, EORS, BICS, TEQ or TST, the carry flag is updated to bit[31] of the constant, if the
constant is greater than 255 and can be produced by shifting an 8-bit value. These
instructions do not affect the carry flag if operand2 is any other constant.
Instruction substitution
Your assembler might be able to produce an equivalent instruction in cases where you
specify a constant that is not permitted. For example, an assembler might assemble the
instruction CMP Rd, #0xFFFFFFFE as the equivalent instruction CMN Rd,
#0x2
.
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Register with optional shift
An operand2 register is specified in the form Rm {, shift}, where:
●Rm is the register holding the data for the second operand
●Shift is an optional shift to be applied to Rm. It can be one of:
ASR #n: Arithmetic shift right n bits, 1 n 32
LSL #n: Logical shift left n bits, 1 n 31
LSR #n: Logical shift right n bits, 1 n 32
ROR #n: Rotate right n bits, 1 n 31
RRX: Rotate right one bit, with extend
—: If omitted, no shift occurs, equivalent to
LSL #0
If you omit the shift, or specify LSL #0, the instruction uses the value in Rm.
If you specify a shift, the shift is applied to the value in Rm, and the resulting 32-bit value is
used by the instruction. However, the contents in the register Rm remains unchanged.
Specifying a register with shift also updates the carry flag when used with certain
instructions. For information on the shift operations and how they affect the carry flag, see
Shift operations.
3.3.4 Shift operations
Register shift operations move the bits in a register left or right by a specified number of bits,
the shift length. Register shift can be performed:
●Directly by the instructions ASR, LSR, LSL, ROR, and RRX. The result is written to a
destination register.
●During the calculation of operand2 by the instructions that specify the second operand
as a register with shift (see Flexible second operand on page 51). The result is used by
the instruction.
The permitted shift lengths depend on the shift type and the instruction (see the individual
instruction description or Flexible second operand). If the shift length is 0, no shift occurs.
Register shift operations update the carry flag except when the specified shift length is 0.
The following sub-sections describe the various shift operations and how they affect the
carry flag. In these descriptions,
Rm
is the register containing the value to be shifted, and
n
is
the shift length.
ASR
Arithmetic shift right by
n
bits moves the left-hand
32
-
n
bits of the register
Rm
, to the right by
n
places, into the right-hand
32
-
n
bits of the result. And it copies the original bit[31] of the
register into the left-hand
n
bits of the result (see Figure 13: ASR#3 on page 53).
You can use the
ASR #n
operation to divide the value in the register
Rm
by 2n, with the result
being rounded towards negative-infinity.
When the instruction is ASRS or when ASR #n is used in operand2 with the instructions
MOVS, MVNS, ANDS, ORRS, ORNS, EORS, BICS, TEQ or TST, the carry flag is updated
to the last bit shifted out, bit[
n
-1], of the register
Rm
.
Note: 1 If
n
is 32 or more, all the bits in the result are set to the value of bit[31] of
Rm
.
2 If
n
is 32 or more and the carry flag is updated, it is updated to the value of bit[31] of
Rm
.
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Figure 13. ASR#3
LSR
Logical shift right by
n
bits moves the left-hand
32
-
n
bits of the register
Rm
, to the right by
n
places, into the right-hand
32
-
n
bits of the result. And it sets the left-hand
n
bits of the result
to 0 (see Figure 14).
You can use the LSR #n operation to divide the value in the register
Rm
by 2n, if the value is
regarded as an unsigned integer.
When the instruction is LSRS or when LSR #n is used in operand2 with the instructions
MOVS, MVNS, ANDS, ORRS, ORNS, EORS, BICS, TEQ or TST, the carry flag is updated
to the last bit shifted out, bit[
n
-1], of the register
Rm
.
Note: 1 If
n
is 32 or more, then all the bits in the result are cleared to 0.
2 If
n
is 33 or more and the carry flag is updated, it is updated to 0.
Figure 14. LSR#3
LSL
Logical shift left by
n
bits moves the right-hand
32
-
n
bits of the register
Rm
, to the left by
n
places, into the left-hand
32
-
n
bits of the result. And it sets the right-hand
n
bits of the result
to 0 (see Figure 15: LSL#3 on page 54).
You can use the LSL #n operation to multiply the value in the register
Rm
by 2n, if the value
is regarded as an unsigned integer or a two’s complement signed integer. Overflow can
occur without warning.
When the instruction is
LSLS
or when
LSL #n
, with non-zero
n
, is used in
operand2
with the
instructions MOVS, MVNS, ANDS, ORRS, ORNS, EORS, BICS, TEQ or TST, the carry flag
is updated to the last bit shifted out, bit[
32
-
n
], of the register
Rm
. These instructions do not
affect the carry flag when used with LSL #0.
Note: 1 If
n
is 32 or more, then all the bits in the result are cleared to 0.
2 If
n
is 33 or more and the carry flag is updated, it is updated to 0.
31 10
Carry
Flag
...
2345
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Figure 15. LSL#3
ROR
Rotate right by
n
bits moves the left-hand
32
-
n
bits of the register
Rm
, to the right by
n
places,
into the right-hand
32
-
n
bits of the result. It also moves the right-hand
n
bits of the register
into the left-hand
n
bits of the result (see Figure 16).
When the instruction is RORS or when ROR
#n
is used in
operand2
with the instructions
MOVS, MVNS, ANDS, ORRS, ORNS, EORS, BICS, TEQ or TST, the carry flag is updated
to the last bit rotation, bit[
n
-1], of the register
Rm
.
Note: 1 If
n
is 32, then the value of the result is same as the value in
Rm
, and if the carry flag is
updated, it is updated to bit[31] of
Rm
.
2
ROR
with shift length,
n
, more than 32 is the same as
ROR
with shift length
n
-32.
Figure 16. ROR #3
RRX
Rotate right with extend moves the bits of the register
Rm
to the right by one bit. And it
copies the carry flag into bit[31] of the result (see Figure 17).
When the instruction is RRXS or when RRX is used in operand2 with the instructions
MOVS, MVNS, ANDS, ORRS, ORNS, EORS, BICS, TEQ or TST, the carry flag is updated
to bit[0] of the register
Rm
.
Figure 17. RRX #3
31 10
Carry
Flag ...
000
2345
31 10
Carry
Flag
...
2345
31 30 10
Carry
Flag
... ...
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3.3.5 Address alignment
An aligned access is an operation where a word-aligned address is used for a word, dual
word, or multiple word access, or where a halfword-aligned address is used for a halfword
access. Byte accesses are always aligned.
The Cortex-M3 processor supports unaligned access only for the following instructions:
●LDR, LDRT
●LDRH, LDRHT
●LDRSH, LDRSHT
●STR, STRT
●STRH, STRHT
All other load and store instructions generate a usage fault exception if they perform an
unaligned access, and therefore their accesses must be address aligned. For more
information about usage faults see Fault handling on page 39.
Unaligned accesses are usually slower than aligned accesses. In addition, some memory
regions might not support unaligned accesses. Therefore, ARM recommends that
programmers ensure that accesses are aligned. To avoid accidental generation of unaligned
accesses, use the UNALIGN_TRP bit in the configuration and control register to trap all
unaligned accesses, see Configuration and control register (SCB_CCR) on page 121.
3.3.6 PC-relative expressions
A PC-relative expression or label is a symbol that represents the address of an instruction or
literal data. It is represented in the instruction as the PC value plus or minus a numeric
offset. The assembler calculates the required offset from the label and the address of the
current instruction. If the offset is too big, the assembler produces an error.
●For the B, BL, CBNZ, and CBZ instructions, the value of the PC is the address of the
current instruction plus four bytes.
●For all other instructions that use labels, the value of the PC is the address of the
current instruction plus four bytes, with bit[1] of the result cleared to 0 to make it word-
aligned.
●Your assembler might permit other syntaxes for PC-relative expressions, such as a
label plus or minus a number, or an expression of the form
[PC, #number]
.
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3.3.7 Conditional execution
Most data processing instructions can optionally update the condition flags in the application
program status register (APSR) according to the result of the operation (see Application
program status register on page 16). Some instructions update all flags, and some only
update a subset. If a flag is not updated, the original value is preserved. See the instruction
descriptions for the flags they affect.
You can execute an instruction conditionally, based on the condition flags set in another
instruction:
●Immediately after the instruction that updated the flags
●After any number of intervening instructions that have not updated the flags
Conditional execution is available by using conditional branches or by adding condition code
suffixes to instructions. See Table 23: Condition code suffixes on page 57 for a list of the
suffixes to add to instructions to make them conditional instructions. The condition code
suffix enables the processor to test a condition based on the flags. If the condition test of a
conditional instruction fails, the instruction:
●Does not execute
●Does not write any value to its destination register
●Does not affect any of the flags
●Does not generate any exception
Conditional instructions, except for conditional branches, must be inside an If-then
instruction block. See IT on page 93 for more information and restrictions when using the
IT
instruction. Depending on the vendor, the assembler might automatically insert an
IT
instruction if you have conditional instructions outside the IT block.
Use the CBZ and CBNZ instructions to compare the value of a register against zero and
branch on the result.
This section describes:
●The condition flags
●Condition code suffixes on page 57
The condition flags
The APSR contains the following condition flags:
●N: Set to 1 when the result of the operation is negative, otherwise cleared to 0
●Z: Set to 1 when the result of the operation is zero, otherwise cleared to 0
●C: Set to 1 when the operation results in a carry, otherwise cleared to 0.
●V: Set to 1 when the operation causes an overflow, otherwise cleared to 0.
For more information about the APSR see Program status register on page 15.
A carry occurs:
●If the result of an addition is greater than or equal to 232
●If the result of a subtraction is positive or zero
●As the result of an inline barrel shifter operation in a move or logical instruction
Overflow occurs if the result of an add, subtract, or compare is greater than or equal to 231,
or less than -231.
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Most instructions update the status flags only if the S suffix is specified. See the instruction
descriptions for more information.
Condition code suffixes
The instructions that can be conditional have an optional condition code, shown in syntax
descriptions as
{cond}
. Conditional execution requires a preceding
IT
instruction. An
instruction with a condition code is only executed if the condition code flags in the APSR
meet the specified condition. Ta b l e 2 3 shows the condition codes to use.
You can use conditional execution with the
IT
instruction to reduce the number of branch
instructions in code.
Ta bl e 2 3 also shows the relationship between condition code suffixes and the N, Z, C, and V
flags.
Specific example 1: Absolute value shows the use of a conditional instruction to find the
absolute value of a number. R0 = ABS(R1).
Specific example 1: Absolute value
MOVSR0, R1; R0 = R1, setting flags
IT MI; IT instruction for the negative condition
RSBMIR0, R1, #0; If negative, R0 = -R1
Table 23. Condition code suffixes
Suffix Flags Meaning
EQ Z = 1 Equal
NE Z = 0 Not equal
CS or HS C = 1 Higher or same, unsigned
CC or LO C = 0 Lower, unsigned <
MI N = 1 Negative
PL N = 0 Positive or zero
VS V = 1 Overflow
VC V = 0 No overflow
HI C = 1 and Z = 0 Higher, unsigned >
LS C = 0 or Z = 1 Lower or same, unsigned
GE N = V Greater than or equal, signed
LT N = V Less than, signed <
GT Z = 0 and N = V Greater than, signed >
LE Z = 1 and N  V Less than or equal, signed
AL Can have any value Always. This is the default when no suffix is specified.
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Specific example 2: Compare and update value shows the use of conditional instructions to
update the value of
R4
if the signed value
R0
and R2 are greater than
R1
and
R3 respectively
.
Specific example 2: Compare and update value
CMP R0, R1 ; compare R0 and R1, setting flags
ITT GT ; IT instruction for the two GT conditions
CMPGT R2, R3; if 'greater than', compare R2 and R3, setting flags
MOVGT R4, R5 ; if still 'greater than', do R4 = R5
3.3.8 Instruction width selection
There are many instructions that can generate either a 16-bit encoding or a 32-bit encoding
depending on the operands and destination register specified. For some of these
instructions, you can force a specific instruction size by using an instruction width suffix.
The
.W
suffix forces a 32-bit instruction encoding. The
.N
suffix forces a 16-bit instruction
encoding.
If you specify an instruction width suffix and the assembler cannot generate an instruction
encoding of the requested width, it generates an error.
In some cases it might be necessary to specify the
.W
suffix, for example if the operand is
the label of an instruction or literal data, as in the case of branch instructions. This is
because the assembler might not automatically generate the right size encoding.
To use an instruction width suffix, place it immediately after the instruction mnemonic and
condition code, if any. Specific example 3: Instruction width selection shows instructions with
the instruction width suffix.
Specific example 3: Instruction width selection
BCS.W label; creates a 32-bit instruction even for a short branch
ADDS.W R0, R0, R1; creates a 32-bit instruction even though the same
; operation can be done by a 16-bit instruction
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3.4 Memory access instructions
Ta bl e 2 4 shows the memory access instructions:
3.4.1 ADR
Load PC-relative address.
Syntax
ADR{cond} Rd, label
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register
●‘label’ is a PC-relative expression (see PC-relative expressions on page 55)
Operation
ADR determines the address by adding an immediate value to the PC. It writes the result to
the destination register.
ADR produces position-independent code, because the address is PC-relative.
If you use ADR to generate a target address for a BX or BLX instruction, you must ensure
that bit[0] of the address you generate is set to1 for correct execution.
Values of
label
must be within the range -4095 to 4095 from the address in the PC.
Table 24. Memory access instructions
Mnemonic Brief description See
ADR Load PC-relative address ADR on page 59
CLREX Clear exclusive CLREX on page 70
LDM{mode} Load multiple registers LDM and STM on page 66
LDR{type} Load register using immediate offset LDR and STR, immediate offset on page 60
LDR{type} Load register using register offset LDR and STR, register offset on page 62
LDR{type}T Load register with unprivileged access LDR and STR, unprivileged on page 63
LDR Load register using PC-relative address LDR, PC-relative on page 64
LDREX{type} Load register exclusive LDREX and STREX on page 69
POP Pop registers from stack PUSH and POP on page 67
PUSH Push registers onto stack PUSH and POP on page 67
STM{mode} Store multiple registers LDM and STM on page 66
STR{type} Store register using immediate offset LDR and STR, immediate offset on page 60
STR{type} Store register using register offset LDR and STR, register offset on page 62
STR{type}T Store register with unprivileged access LDR and STR, unprivileged on page 63
STREX{type} Store register exclusive LDREX and STREX on page 69
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Note: You might have to use the
.W
suffix to get the maximum offset range or to generate
addresses that are not word-aligned (see Instruction width selection on page 58).
Restrictions
Rd
must be neither SP nor PC.
Condition flags
This instruction does not change the flags.
Examples
ADR R1, TextMessage; write address value of a location labelled as
; TextMessage to R1
3.4.2 LDR and STR, immediate offset
Load and store with immediate offset, pre-indexed immediate offset, or post-indexed
immediate offset.
Syntax
op{type}{cond} Rt, [Rn {, #offset}]; immediate offset
op{type}{cond} Rt, [Rn, #offset]!; pre-indexed
op{type}{cond} Rt, [Rn], #offset; post-indexed
opD{cond} Rt, Rt2, [Rn {, #offset}]; immediate offset, two words
opD{cond} Rt, Rt2, [Rn, #offset]!; pre-indexed, two words
opD{cond} Rt, Rt2, [Rn], #offset; post-indexed, two words
where:
●‘op’ is either LDR (load register) or STR (store register)
●‘type’ is one of the following:
B: Unsigned byte, zero extends to 32 bits on loads
SB: Signed byte, sign extends to 32 bits (
LDR
only)
H: Unsigned halfword, zero extends to 32 bits on loads
SH: Signed halfword, sign extends to 32 bits (
LDR
only)
—: Omit, for word
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rt’ is the register to load or store
●‘Rn’ is the register on which the memory address is based
●‘offset’ is an offset from
Rn
. If
offset
is omitted, the address is the contents of
Rn
●‘Rt2’ is the additional register to load or store for two-word operations
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Operation
LDR instructions load one or two registers with a value from memory.
STR
instructions store
one or two register values to memory.
Load and store instructions with immediate offset can use the following addressing modes:
●Offset addressing
The offset value is added to or subtracted from the address obtained from the register
Rn
. The result is used as the address for the memory access. The register
Rn
is
unaltered. The assembly language syntax for this mode is: [Rn, #offset].
●Pre-indexed addressing
The offset value is added to or subtracted from the address obtained from the register
Rn
. The result is used as the address for the memory access and written back into the
register
Rn
. The assembly language syntax for this mode is: [Rn, #offset]!
●Post-indexed addressing
The address obtained from the register
Rn
is used as the address for the memory
access. The offset value is added to or subtracted from the address, and written back
into the register
Rn
. The assembly language syntax for this mode is: [Rn], #offset.
The value to load or store can be a byte, halfword, word, or two words. Bytes and halfwords
can either be signed or unsigned (see Address alignment on page 55).
Ta bl e 2 5 shows the range of offsets for immediate, pre-indexed and post-indexed forms.
Restrictions
●For load instructions
–
Rt
can be SP or PC for word loads only
–
Rt
must be different from
Rt2
for two-word loads
–
Rn
must be different from
Rt
and
Rt2
in the pre-indexed or post-indexed forms
●When
Rt
is PC in a word load instruction
– bit[0] of the loaded value must be 1 for correct execution
– A branch occurs to the address created by changing bit[0] of the loaded value to 0
– If the instruction is conditional, it must be the last instruction in the IT block
●For store instructions
–
Rt
can be SP for word stores only
–
Rt
must not be PC
–
Rn
must not be PC
–
Rn
must be different from
Rt
and
Rt2
in the pre-indexed or post-indexed forms
Table 25. Immediate, pre-indexed and post-indexed offset ranges
Instruction type Immediate offset Pre-indexed Post-indexed
Word, halfword, signed
halfword, byte, or signed byte -255 to 4095 -255 to 255 -255 to 255
Two w o r d s Multiple of 4 in the
range -1020 to 1020
Multiple of 4 in the
range -1020 to 1020
Multiple of 4 in the
range -1020 to 1020
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Condition flags
These instructions do not change the flags.
Examples
LDRR8, [R10]; loads R8 from the address in R10.
LDRNER2, [R5, #960]!; loads (conditionally) R2 from a word
; 960 bytes above the address in R5, and
; increments R5 by 960.
STRR2, [R9,#const-struc]; const-struc is an expression evaluating
; to a constant in the range 0-4095.
STRHR3, [R4], #4; Store R3 as halfword data into address in
; R4, then increment R4 by 4
LDRD R8, R9, [R3, #0x20]; Load R8 from a word 32 bytes above the
; address in R3, and load R9 from a word 36
; bytes above the address in R3
STRDR0, R1, [R8], #-16; Store R0 to address in R8, and store R1 to
; a word 4 bytes above the address in R8,
; and then decrement R8 by 16.
3.4.3 LDR and STR, register offset
Load and store with register offset.
Syntax
op{type}{cond} Rt, [Rn, Rm {, LSL #n}]
where:
●‘op’ is either LDR (load register) or STR (store register)
●‘type’ is one of the following:
B: Unsigned byte, zero extends to 32 bits on loads
SB: Signed byte, sign extends to 32 bits (
LDR
only)
H: Unsigned halfword, zero extends to 32 bits on loads
SH: Signed halfword, sign extends to 32 bits (
LDR
only)
—: Omit, for word
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rt’ is the register to load or store
●‘Rn’ is the register on which the memory address is based
●‘Rm’ is a register containing a value to be used as the offset
●‘LSL #n’ is an optional shift, with n in the range 0 to 3
Operation
LDR instructions load a register with a value from memory. STR instructions store a register
value into memory.
The memory address to load from or store to is at an offset from the register
Rn
. The offset
is specified by the register
Rm
and can be shifted left by up to 3 bits using
LSL
.
The value to load or store can be a byte, halfword, or word. For load instructions, bytes and
halfwords can either be signed or unsigned (see Address alignment on page 55).
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Restrictions
In these instructions:
●
Rn
must not be PC
●
Rm
must be neither SP nor PC
●
Rt
can be SP only for word loads and word stores
●
Rt
can be PC only for word loads
When
Rt
is PC in a word load instruction:
●bit[0] of the loaded value must be 1 for correct execution, and a branch occurs to this
halfword-aligned address
●If the instruction is conditional, it must be the last instruction in the IT block.
Condition flags
These instructions do not change the flags.
Examples
STRR0, [R5, R1]; store value of R0 into an address equal to
; sum of R5 and R1
LDRSBR0, [R5, R1, LSL #1]; read byte value from an address equal to
; sum of R5 and two times R1, sign extended it
; to a word value and put it in R0
STRR0, [R1, R2, LSL #2]; stores R0 to an address equal to sum of R1
; and four times R2
3.4.4 LDR and STR, unprivileged
Load and store with unprivileged access.
Syntax
op{type}T{cond} Rt, [Rn {, #offset}]; immediate offset
where:
●‘op’ is either LDR (load register) or STR (store register)
●‘type’ is one of the following:
B: Unsigned byte, zero extends to 32 bits on loads
SB: Signed byte, sign extends to 32 bits (
LDR
only)
H: Unsigned halfword, zero extends to 32 bits on loads
SH: Signed halfword, sign extends to 32 bits (
LDR
only)
—: Omit, for word
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rt’ is the register to load or store
●‘Rn’ is the register on which the memory address is based
●‘offset’ is an offset from Rn and can be 0 to 255. If offset is omitted, the address is the
value in Rn.
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Operation
These load and store instructions perform the same function as the memory access
instructions with immediate offset (see LDR and STR, immediate offset on page 60). The
difference is that these instructions have only unprivileged access even when used in
privileged software.
When used in unprivileged software, these instructions behave in exactly the same way as
normal memory access instructions with immediate offset.
Restrictions
In these instructions:
●
Rn
must not be PC
●
Rt
must be neither SP nor PC.
Condition flags
These instructions do not change the flags.
Examples
STRBTEQR4, [R7]; conditionally store least significant byte in
; R4 to an address in R7, with unprivileged access
LDRHTR2, [R2, #8]; load halfword value from an address equal to
; sum of R2 and 8 into R2, with unprivileged access
3.4.5 LDR, PC-relative
Load register from memory.
Syntax
LDR{type}{cond} Rt, label
LDRD{cond} Rt, Rt2, label; load two words
where:
●‘type’ is one of the following:
B: Unsigned byte, zero extends to 32 bits
SB: Signed byte, sign extends to 32 bits
H: Unsigned halfword, sign extends to 32 bits
SH: Signed halfword, sign extends to 32 bits
—: Omit, for word
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rt’ is the register to load or store
●‘Rt2’ is the second register to load or store
●‘label’ is a PC-relative expression (see PC-relative expressions on page 55)
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Operation
LDR loads a register with a value from a PC-relative memory address. The memory address
is specified by a label or by an offset from the PC.
The value to load or store can be a byte, halfword, or word. For load instructions, bytes and
halfwords can either be signed or unsigned (see Address alignment on page 55).
‘label
’ must be within a limited range of the current instruction. Ta b l e 2 6 shows the possible
offsets between
label
and the PC.
You might have to use the
.W
suffix to get the maximum offset range (see Instruction width
selection on page 58).
Restrictions
In these instructions:
●Rt can be SP or PC only for word loads
●Rt2 must be neither SP nor PC
●Rt must be different from Rt2
When
Rt
is PC in a word load instruction:
●bit[0] of the loaded value must be 1 for correct execution, and a branch occurs to this
halfword-aligned address
●If the instruction is conditional, it must be the last instruction in the IT block.
Condition flags
These instructions do not change the flags.
Examples
LDRR0, LookUpTable; load R0 with a word of data from an address
; labelled as LookUpTable
LDRSBR7, localdata; load a byte value from an address labelled
; as localdata, sign extend it to a word
; value, and put it in R7
Table 26. label-PC offset ranges
Instruction type Offset range
Word, halfword, signed halfword, byte, signed byte 4095 to 4095
Two w o r d s 1020 to 1020
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3.4.6 LDM and STM
Load and store multiple registers.
Syntax
op{addr_mode}{cond} Rn{!}, reglist
where:
●‘op’ is either LDM (load multiple register) or STM (store multiple register)
●‘addr_mode’ is any of the following:
IA: Increment address after each access (this is the default)
DB: Decrement address before each access
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rn’ is the register on which the memory addresses are based
●‘!’ is an optional writeback suffix. If ! is present, the final address that is loaded from or
stored to is written back into Rn.
●‘reglist’ is a list of one or more registers to be loaded or stored, enclosed in braces. It
can contain register ranges. It must be comma-separated if it contains more than one
register or register range (see Examples on page 67).
LDM and LDMFD are synonyms for LDMIA. LDMFD refers to its use for popping data from
full descending stacks.
LDMEA is a synonym for LDMDB, and refers to its use for popping data from empty
ascending stacks.
STM and STMEA are synonyms for STMIA. STMEA refers to its use for pushing data onto
empty ascending stacks.
STMFD is s synonym for STMDB, and refers to its use for pushing data onto full descending
stacks
Operation
LDM instructions load the registers in reglist with word values from memory addresses
based on Rn.
STM instructions store the word values in the registers in
reglist
to memory addresses based
on Rn.
For LDM, LDMIA, LDMFD, STM, STMIA, and STMEA the memory addresses used for the
accesses are at 4-byte intervals ranging from Rn to Rn + 4 * (
n
-1), where
n
is the number of
registers in reglist. The accesses happen in order of increasing register numbers, with the
lowest numbered register using the lowest memory address and the highest number
register using the highest memory address. If the writeback suffix is specified, the value of
Rn + 4 * (
n
-1) is written back to Rn.
For LDMDB, LDMEA, STMDB, and STMFD the memory addresses used for the accesses
are at 4-byte intervals ranging from Rn to Rn - 4 * (
n
-1), where
n
is the number of registers in
reglist. The accesses happen in order of decreasing register numbers, with the highest
numbered register using the highest memory address and the lowest number register using
the lowest memory address. If the writeback suffix is specified, the value Rn - 4 * (
n
-1) is
written back to Rn.
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The PUSH and POP instructions can be expressed in this form (see PUSH and POP for
details).
Restrictions
In these instructions:
●Rn must not be PC
●reglist must not contain SP
●In any STM instruction, reglist must not contain PC
●In any LDM instruction, reglist must not contain PC if it contains LR
●reglist must not contain
Rn
if you specify the writeback suffix
When PC is in reglist in an LDM instruction:
●bit[0] of the value loaded to the PC must be 1 for correct execution, and a branch
occurs to this halfword-aligned address.
●If the instruction is conditional, it must be the last instruction in the IT block
Condition flags
These instructions do not change the flags.
Examples
LDMR8,{R0,R2,R9}; LDMIA is a synonym for LDM
STMDBR1!,{R3-R6,R11,R12}
Incorrect examples
STMR5!,{R5,R4,R9}; value stored for R5 is unpredictable
LDMR2, {}; there must be at least one register in the list
3.4.7 PUSH and POP
Push registers onto, and pop registers off a full-descending stack.
Syntax
PUSH{cond} reglist
POP{cond} reglist
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘reglist’ is a non-empty list of registers, enclosed in braces. It can contain register
ranges. It must be comma-separated if it contains more than one register or register
range (see Examples on page 67).
PUSH and POP are synonyms for STMDB and LDM (or LDMIA) with the memory
addresses for the access based on SP, and with the final address for the access written
back to the SP. PUSH and POP are the preferred mnemonics in these cases.
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Operation
PUSH stores registers on the stack in order of decreasing register numbers, with the highest
numbered register using the highest memory address and the lowest numbered register
using the lowest memory address.
POP loads registers from the stack in order of increasing register numbers, with the lowest
numbered register using the lowest memory address and the highest numbered register
using the highest memory address.
See LDM and STM on page 66 for more information.
Restrictions
In these instructions:
●
‘reglist’
must not contain SP
●For the PUSH instruction, reglist must not contain PC
●For the POP instruction, reglist must not contain PC if it contains LR
When PC is in reglist in a
POP
instruction:
●bit[0] of the value loaded to the PC must be 1 for correct execution, and a branch
occurs to this halfword-aligned address.
●If the instruction is conditional, it must be the last instruction in the IT block.
Condition flags
These instructions do not change the flags.
Examples
PUSH{R0,R4-R7}
PUSH{R2,LR}
POP{R0,R10,PC}
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3.4.8 LDREX and STREX
Load and store register exclusive.
Syntax
LDREX{cond} Rt, [Rn {, #offset}]
STREX{cond} Rd, Rt, [Rn {, #offset}]
LDREXB{cond} Rt, [Rn]
STREXB{cond} Rd, Rt, [Rn]
LDREXH{cond} Rt, [Rn]
STREXH{cond} Rd, Rt, [Rn]
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register for the returned status
●‘Rt’ is the register to load or store
●‘Rn’ is the register on which the memory address is based
●‘offset’ is an optional offset applied to the value in Rn. If offset is omitted, the address is
the value in Rn.
Operation
LDREX, LDREXB, and LDREXH load a word, byte, and halfword respectively from a
memory address.
STREX, STREXB, and STREXH attempt to store a word, byte, and halfword respectively to
a memory address. The address used in any store-exclusive instruction must be the same
as the address in the most recently executed load-exclusive instruction. The value stored by
the Store-exclusive instruction must also have the same data size as the value loaded by the
preceding load-exclusive instruction. This means software must always use a load-exclusive
instruction and a matching store-exclusive instruction to perform a synchronization
operation, see Synchronization primitives on page 30.
If a store-exclusive instruction performs the store, it writes 0 to its destination register. If it
does not perform the store, it writes 1 to its destination register. If the store-exclusive
instruction writes 0 to the destination register, it is guaranteed that no other process in the
system has accessed the memory location between the load-exclusive and store-exclusive
instructions.
For reasons of performance, keep the number of instructions between corresponding load-
exclusive and store-exclusive instruction to a minimum.
Note: The result of executing a store-exclusive instruction to an address that is different from that
used in the preceding load-exclusive instruction is unpredictable.
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Restrictions
In these instructions:
●Do not use PC
●Do not use SP for Rd and Rt
●For STREX, Rd must be different from both Rt and Rn
●The value of offset must be a multiple of four in the range 0-1020
Condition flags
These instructions do not change the flags.
Examples
MOVR1, #0x1; initialize the ‘lock taken’ value try
LDREXR0, [LockAddr]; load the lock value
CMPR0, #0; is the lock free?
ITTEQ; IT instruction for STREXEQ and CMPEQ
STREXEQR0, R1, [LockAddr]; try and claim the lock
CMPEQR0, #0; did this succeed?
BNEtry; no – try again
; yes – we have the lock
3.4.9 CLREX
Clear exclusive.
Syntax
CLREX{cond}
where:
‘cond’ is an optional condition code (see Conditional execution on page 56)
Operation
Use
CLREX
to make the next
STREX
,
STREXB
, or
STREXH
instruction write 1 to its destination
register and fail to perform the store. It is useful in exception handler code to force the failure
of the store exclusive if the exception occurs between a load exclusive instruction and the
matching store exclusive instruction in a synchronization operation.
See Synchronization primitives on page 30 for more information.
Condition flags
These instructions do not change the flags.
Examples
CLREX
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3.5 General data processing instructions
Ta bl e 2 7 shows the data processing instructions.
Table 27. Data processing instructions
Mnemonic Brief description See
ADC Add with carry ADD, ADC, SUB, SBC, and RSB on page 72
ADD Add ADD, ADC, SUB, SBC, and RSB on page 72
ADDW Add ADD, ADC, SUB, SBC, and RSB on page 72
AND Logical AND AND, ORR, EOR, BIC, and ORN on page 74
ASR Arithmetic shift right ASR, LSL, LSR, ROR, and RRX on page 75
BIC Bit clear AND, ORR, EOR, BIC, and ORN on page 74
CLZ Count leading zeros CLZ on page 76
CMN Compare negative CMP and CMN on page 77
CMP Compare CMP and CMN on page 77
EOR Exclusive OR AND, ORR, EOR, BIC, and ORN on page 74
LSL Logical shift left ASR, LSL, LSR, ROR, and RRX on page 75
LSR Logical shift right ASR, LSL, LSR, ROR, and RRX on page 75
MOV Move MOV and MVN on page 78
MOVT Move top MOVT on page 79
MOVW Move 16-bit constant MOV and MVN on page 78
MVN Move NOT MOV and MVN on page 78
ORN Logical OR NOT AND, ORR, EOR, BIC, and ORN on page 74
ORR Logical OR AND, ORR, EOR, BIC, and ORN on page 74
RBIT Reverse bits REV, REV16, REVSH, and RBIT on page 80
REV Reverse byte order in a word REV, REV16, REVSH, and RBIT on page 80
REV16 Reverse byte order in each halfword REV, REV16, REVSH, and RBIT on page 80
REVSH Reverse byte order in bottom halfword
and sign extend REV, REV16, REVSH, and RBIT on page 80
ROR Rotate right ASR, LSL, LSR, ROR, and RRX on page 75
RRX Rotate right with extend ASR, LSL, LSR, ROR, and RRX on page 75
RSB Reverse subtract ADD, ADC, SUB, SBC, and RSB on page 72
SBC Subtract with carry ADD, ADC, SUB, SBC, and RSB on page 72
SUB Subtract ADD, ADC, SUB, SBC, and RSB on page 72
SUBW Subtract ADD, ADC, SUB, SBC, and RSB on page 72
TEQ Test equivalence TST and TEQ on page 81
TST Test TST and TEQ on page 81
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3.5.1 ADD, ADC, SUB, SBC, and RSB
Add, add with carry, subtract, subtract with carry, and reverse subtract.
Syntax
op{S}{cond} {Rd,} Rn, Operand2
op{cond} {Rd,} Rn, #imm12; ADD and SUB only
where:
●‘op’ is one of:
ADD: Add
ADC: Add with carry
SUB: Subtract
SBC: Subtract with carry
RSB: Reverse subtract
●‘S’ is an optional suffix. If S is specified, the condition code flags are updated on the
result of the operation (see Conditional execution on page 56)
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register. If Rd is omitted, the destination register is Rn
●‘Rn’ is the register holding the first operand
●‘Operand2’ is a flexible second operand (see Flexible second operand on page 51 for
details of the options).
●‘imm12’ is any value in the range 0—4095
Operation
The ADD instruction adds the value of operand2 or imm12 to the value in Rn.
The ADC instruction adds the values in Rn and operand2, together with the carry flag.
The SUB instruction subtracts the value of operand2 or imm12 from the value in Rn.
The SBC instruction subtracts the value of operand2 from the value in Rn. If the carry flag is
clear, the result is reduced by one.
The RSB instruction subtracts the value in Rn from the value of operand2. This is useful
because of the wide range of options for operand2.
Use ADC and SBC to synthesize multiword arithmetic (see Multiword arithmetic examples
on page 73 and ADR on page 59).
ADDW is equivalent to the ADD syntax that uses the imm12 operand. SUBW is equivalent
to the SUB syntax that uses the imm12 operand.
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Restrictions
In these instructions:
●Operand2 must be neither SP nor PC
●Rd can be SP only in ADD and SUB, and only with the following additional restrictions:
–Rn must also be SP
– Any shift in operand2 must be limited to a maximum of three bits using LSL
●Rn can be SP only in ADD and SUB
●Rd can be PC only in the ADD{cond} PC, PC, Rm instruction where:
– You must not specify the S suffix
–Rm must be neither PC nor SP
– If the instruction is conditional, it must be the last instruction in the IT block
●With the exception of the ADD{cond} PC, PC, Rm instruction, Rn can be PC only in
ADD and SUB, and only with the following additional restrictions:
– You must not specify the S suffix
– The second operand must be a constant in the range 0 to 4095
Note: 1 When using the PC for an addition or a subtraction, bits[1:0] of the PC are rounded to b00
before performing the calculation, making the base address for the calculation word-aligned.
2 If you want to generate the address of an instruction, you have to adjust the constant based
on the value of the PC. ARM recommends that you use the
ADR
instruction instead of
ADD
or
SUB
with
Rn
equal to the PC, because your assembler automatically calculates the correct
constant for the
ADR
instruction.
When Rd is PC in the ADD{cond} PC, PC, Rm instruction:
●bit[0] of the value written to the PC is ignored
●A branch occurs to the address created by forcing bit[0] of that value to 0
Condition flags
If
S
is specified, these instructions update the N, Z, C and V flags according to the result.
Examples
ADDR2, R1, R3
SUBSR8, R6, #240; sets the flags on the result
RSBR4, R4, #1280; subtracts contents of R4 from 1280
ADCHIR11, R0, R3; only executed if C flag set and Z
; flag clear
Multiword arithmetic examples
Specific example 4: 64-bit addition shows two instructions that add a 64-bit integer
contained in R2 and R3 to another 64-bit integer contained in R0 and R1, and place the
result in R4 and R5.
Specific example 4: 64-bit addition
ADDSR4, R0, R2; add the least significant words
ADCR5, R1, R3; add the most significant words with carry
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Multiword values do not have to use consecutive registers. Specific example 5: 96-bit
subtraction shows instructions that subtract a 96-bit integer contained in R9, R1, and R11
from another contained in R6, R2, and R8. The example stores the result in R6, R9, and R2.
Specific example 5: 96-bit subtraction
SUBSR6, R6, R9; subtract the least significant words
SBCSR9, R2, R1; subtract the middle words with carry
SBCR2, R8, R11; subtract the most significant words with carry
3.5.2 AND, ORR, EOR, BIC, and ORN
Logical AND, OR, exclusive OR, bit clear, and OR NOT.
Syntax
op{S}{cond} {Rd,} Rn, Operand2
where:
●‘op’ is one of:
AND: Logical AND
ORR: Logical OR or bit set
EOR: Logical exclusive OR
BIC: Logical AND NOT or bit clear
ORN: Logical OR NOT
●‘S’ is an optional suffix. If S is specified, the condition code flags are updated on the
result of the operation (see Conditional execution on page 56).
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register
●‘Rn’ is the register holding the first operand
●‘Operand2’ is a flexible second operand (see Flexible second operand on page 51 for
details of the options).
Operation
The AND, EOR, and ORR instructions perform bitwise AND, exclusive OR, and OR
operations on the values in Rn and operand2.
The BIC instruction performs an AND operation on the bits in Rn with the complements of
the corresponding bits in the value of operand2.
The ORN instruction performs an OR operation on the bits in Rn with the complements of
the corresponding bits in the value of operand2.
Restrictions
Do not use either SP or PC.
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Condition flags
If
S
is specified, these instructions:
●Update the N and Z flags according to the result
●Can update the C flag during the calculation of operand2 (see Flexible second operand
on page 51)
●Do not affect the V flag
Examples
ANDR9, R2,#0xFF00
ORREQR2, R0, R5
ANDSR9, R8, #0x19
EORSR7, R11, #0x18181818
BICR0, R1, #0xab
ORNR7, R11, R14, ROR #4
ORNSR7, R11, R14, ASR #32
3.5.3 ASR, LSL, LSR, ROR, and RRX
Arithmetic shift right, logical shift left, logical shift right, rotate right, and rotate right with
extend.
Syntax
op{S}{cond} Rd, Rm, Rs
op{S}{cond} Rd, Rm, #n
RRX{S}{cond} Rd, Rm
where:
●‘op’ is one of:
ASR: Arithmetic shift right
LSL: Logical shift left
LSR: Logical shift right
ROR: Rotate right
●‘S’ is an optional suffix. If S is specified, the condition code flags are updated on the
result of the operation (see Conditional execution on page 56)
●‘Rd’ is the destination register
●‘Rm’ is the register holding the value to be shifted
●‘Rs’ is the register holding the shift length to apply to the value Rm. Only the least
significant byte is used and can be in the range 0 to 255.
●‘n’ is the shift length. The range of shift lengths depend on the instruction as follows:
ASR: Shift length from 1 to 32
LSL: Shift length from 0 to 31
LSR: Shift length from 1 to 32
ROR: Shift length from 1 to 31
Note: MOV{S}{cond} Rd, Rm is the preferred syntax for LSL{S}{cond} Rd, Rm, #0.
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Operation
ASR, LSL, LSR, and ROR move the bits in the register Rm to the left or right by the number
of places specified by constant n or register Rs.
RRX moves the bits in register Rm to the right by 1.
In all these instructions, the result is written to Rd, but the value in register Rm remains
unchanged. For details on what result is generated by the different instructions (see Shift
operations on page 52).
Restrictions
Do not use either SP or PC.
Condition flags
If
S
is specified:
●These instructions update the N and Z flags according to the result
●The C flag is updated to the last bit shifted out, except when the shift length is 0 (see
Shift operations on page 52).
Examples
ASRR7, R8, #9; arithmetic shift right by 9 bits
LSLSR1, R2, #3; logical shift left by 3 bits with flag update
LSRR4, R5, #6; logical shift right by 6 bits
RORR4, R5, R6; rotate right by the value in the bottom byte of R6
RRXR4, R5; rotate right with extend
3.5.4 CLZ
Count leading zeros.
Syntax
CLZ{cond} Rd, Rm
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register
●‘Rm’ is the operand register
Operation
The CLZ instruction counts the number of leading zeros in the value in Rm and returns the
result in Rd. The result value is 32 if no bits are set in the source register, and zero if bit[31]
is set.
Restrictions
Do not use either SP or PC.
Condition flags
This instruction does not change the flags.
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Examples
CLZR4,R9
CLZNER2,R3
3.5.5 CMP and CMN
Compare and compare negative.
Syntax
CMP{cond} Rn, Operand2
CMN{cond} Rn, Operand2
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rn’ is the register holding the first operand
●‘Operand2’ is a flexible second operand (see Flexible second operand on page 51) for
details of the options.
Operation
These instructions compare the value in a register with operand2. They update the condition
flags on the result, but do not write the result to a register.
The CMP instruction subtracts the value of operand2 from the value in Rn. This is the same
as a SUBS instruction, except that the result is discarded.
The CMN instruction adds the value of operand2 to the value in Rn. This is the same as an
ADDS instruction, except that the result is discarded.
Restrictions
In these instructions:
• Do not use PC
•Operand2 must not be SP
Condition flags
These instructions update the N, Z, C and V flags according to the result.
Examples
CMPR2, R9
CMNR0, #6400
CMPGTSP, R7, LSL #2
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3.5.6 MOV and MVN
Move and move NOT.
Syntax
MOV{S}{cond} Rd, Operand2
MOV{cond} Rd, #imm16
MVN{S}{cond} Rd, Operand2
where:
●‘S’ is an optional suffix. If S is specified, the condition code flags are updated on the
result of the operation (see Conditional execution on page 56).
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register
●‘Operand2’ is a flexible second operand (see Flexible second operand on page 51) for
details of the options.
●‘imm16’ is any value in the range 0—65535
Operation
The MOV instruction copies the value of operand2 into Rd.
When operand2 in a MOV instruction is a register with a shift other than LSL #0, the
preferred syntax is the corresponding shift instruction:
●ASR{S}{cond} Rd, Rm, #n is the preferred syntax for MOV{S}{cond} Rd, Rm, ASR #n
●LSL{S}{cond} Rd, Rm, #n is the preferred syntax for MOV{S}{cond} Rd, Rm, LSL #n if n
!= 0
●LSR{S}{cond} Rd, Rm, #n is the preferred syntax for MOV{S}{cond} Rd, Rm, LSR #n
●ROR{S}{cond} Rd, Rm, #n is the preferred syntax for MOV{S}{cond} Rd, Rm, ROR #n
●RRX{S}{cond} Rd, Rm is the preferred syntax for MOV{S}{cond} Rd, Rm, RRX
Also, the
MOV
instruction permits additional forms of operand2 as synonyms for shift
instructions:
●MOV{S}{cond} Rd, Rm, ASR Rs is a synonym for ASR{S}{cond} Rd, Rm, Rs
●MOV{S}{cond} Rd, Rm, LSL Rs is a synonym for LSL{S}{cond} Rd, Rm, Rs
●MOV{S}{cond} Rd, Rm, LSR Rs is a synonym for LSR{S}{cond} Rd, Rm, Rs
●MOV{S}{cond} Rd, Rm, ROR Rs is a synonym for ROR{S}{cond} Rd, Rm, Rs
See ASR, LSL, LSR, ROR, and RRX on page 75.
The
MVN
instruction takes the value of
operand2
, performs a bitwise logical NOT operation on
the value, and places the result into
Rd
.
Note: The
MOVW
instruction provides the same function as
MOV
, but is restricted to using the
imm16
operand.
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Restrictions
You can use SP and PC only in the MOV instruction, with the following restrictions:
●The second operand must be a register without shift
●You must not specify the S suffix
When Rd is PC in a
MOV
instruction:
●bit[0] of the value written to the PC is ignored
●A branch occurs to the address created by forcing bit[0] of that value to 0.
Note: Though it is possible to use
MOV
as a branch instruction, ARM strongly recommends the use
of a
BX
or
BLX
instruction to branch for software portability to the ARM instruction set.
Condition flags
If
S
is specified, these instructions:
●Update the N and Z flags according to the result
●Can update the C flag during the calculation of
operand2
(see Flexible second operand
on page 51).
●Do not affect the V flag
Example
MOVSR11, #0x000B; write value of 0x000B to R11, flags get updated
MOVR1, #0xFA05; write value of 0xFA05 to R1, flags are not updated
MOVSR10, R12; write value in R12 to R10, flags get updated
MOVR3, #23; write value of 23 to R3
MOVR8, SP; write value of stack pointer to R8
MVNSR2, #0xF; write value of 0xFFFFFFF0 (bitwise inverse of 0xF)
; to the R2 and update flags
3.5.7 MOVT
Move top.
Syntax
MOVT{cond} Rd, #imm16
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register
●‘imm16’ is a 16-bit immediate constant
Operation
MOVT writes a 16-bit immediate value, imm16, to the top halfword, Rd[31:16], of its
destination register. The write does not affect Rd[15:0].
The MOV, MOVT instruction pair enables you to generate any 32-bit constant.
Restrictions
Rd
must be neither SP nor PC.
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Condition flags
This instruction does not change the flags.
Examples
MOVTR3, #0xF123; write 0xF123 to upper halfword of R3, lower halfword
; and APSR are unchanged
3.5.8 REV, REV16, REVSH, and RBIT
Reverse bytes and reverse bits.
Syntax
op{cond} Rd, Rn
where:
●‘op’ is one of:
REV: Reverse byte order in a word
REV16: Reverse byte order in each halfword independently
REVSH: Reverse byte order in the bottom halfword, and sign extends to 32 bits
RBIT: Reverse the bit order in a 32-bit word
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register
●‘Rn’ is the register holding the operand
Operation
Use these instructions to change endianness of data:
●
REV
: Converts 32-bit big-endian data into little-endian data or 32-bit little-endian data
into big-endian data.
●
REV16
: Converts 16-bit big-endian data into little-endian data or 16-bit little-endian data
into big-endian data.
●
REVSH:
Converts either:
– 16-bit signed big-endian data into 32-bit signed little-endian data
– 16-bit signed little-endian data into 32-bit signed big-endian data
Restrictions
Do not use either SP or PC.
Condition flags
These instructions do not change the flags.
Examples
REVR3, R7; reverse byte order of value in R7 and write it to R3
REV16 R0, R0; reverse byte order of each 16-bit halfword in R0
REVSH R0, R5 ; reverse Signed Halfword
REVHS R3, R7 ; reverse with Higher or Same condition
RBIT R7, R8 ; reverse bit order of value in R8 and write the result to R7
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3.5.9 TST and TEQ
Test bits and test equivalence.
Syntax
TST{cond} Rn, Operand2
TEQ{cond} Rn, Operand2
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rn’ is the register holding the first operand
●‘Operand2’ is a flexible second operand (see Flexible second operand on page 51) for
details of the options.
Operation
These instructions test the value in a register against operand2. They update the condition
flags based on the result, but do not write the result to a register.
The
TST
instruction performs a bitwise AND operation on the value in
Rn
and the value of
operand2. This is the same as the
ANDS
instruction, except that it discards the result.
To test whether a bit of Rn is 0 or 1, use the TST instruction with an
operand2
constant that
has that bit set to 1 and all other bits cleared to 0.
The
TEQ
instruction performs a bitwise exclusive OR operation on the value in
Rn
and the
value of operand2. This is the same as the EORS instruction, except that it discards the
result.
Use the TEQ instruction to test if two values are equal without affecting the V or C flags.
TEQ is also useful for testing the sign of a value. After the comparison, the N flag is the
logical exclusive OR of the sign bits of the two operands.
Restrictions
Do not use either SP or PC.
Condition flags
These instructions:
●Update the N and Z flags according to the result
●Can update the C flag during the calculation of operand2 (see Flexible second operand
on page 51).
●Do not affect the V flag
Examples
TSTR0, #0x3F8; perform bitwise AND of R0 value to 0x3F8,
; APSR is updated but result is discarded
TEQEQR10, R9; conditionally test if value in R10 is equal to
; value in R9, APSR is updated but result is discarded
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3.6 Multiply and divide instructions
Ta bl e 2 8 shows the multiply and divide instructions.
3.6.1 MUL, MLA, and MLS
Multiply, multiply with accumulate, and multiply with subtract, using 32-bit operands, and
producing a 32-bit result.
Syntax
MUL{S}{cond} {Rd,} Rn, Rm ; Multiply
MLA{cond} Rd, Rn, Rm, Ra ; Multiply with accumulate
MLS{cond} Rd, Rn, Rm, Ra ; Multiply with subtract
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘S’ is an optional suffix. If S is specified, the condition code flags are updated on the
result of the operation (see Conditional execution on page 56).
●‘Rd’ is the destination register. If Rd is omitted, the destination register is Rn
●‘Rn’, ‘Rm’ are registers holding the values to be multiplied
●‘Ra’ is a register holding the value to be added to or subtracted from
Table 28. Multiply and divide instructions
Mnemonic Brief description See
MLA Multiply with accumulate, 32-bit result MUL, MLA, and MLS on page 82
MLS Multiply and subtract, 32-bit result MUL, MLA, and MLS on page 82
MUL Multiply, 32-bit result MUL, MLA, and MLS on page 82
SDIV Signed divide SDIV and UDIV on page 85
SMLAL Signed multiply with accumulate
(32x32+64), 64-bit result
UMULL, UMLAL, SMULL, and SMLAL on
page 84
SMULL Signed multiply (32x32), 64-bit result UMULL, UMLAL, SMULL, and SMLAL on
page 84
UDIV Unsigned divide SDIV and UDIV on page 85
UMLAL Unsigned multiply with accumulate
(32x32+64), 64-bit result
UMULL, UMLAL, SMULL, and SMLAL on
page 84
UMULL Unsigned multiply (32x32), 64-bit
result
UMULL, UMLAL, SMULL, and SMLAL on
page 84
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Operation
The
MUL
instruction multiplies the values from
Rn
and
Rm
, and places the least significant 32
bits of the result in
Rd
.
The
MLA
instruction multiplies the values from
Rn
and
Rm
, adds the value from
Ra
, and
places the least significant 32 bits of the result in
Rd
.
The
MLS
instruction multiplies the values from
Rn
and
Rm
, subtracts the product from the
value from
Ra
, and places the least significant 32 bits of the result in
Rd
.
The results of these instructions do not depend on whether the operands are signed or
unsigned.
Restrictions
In these instructions, do not use SP and do not use PC.
If you use the S suffix with the
MUL
instruction:
●
Rd
,
Rn
, and
Rm
must all be in the range
R0
to
R7
●
Rd
must be the same as
Rm
●You must not use the
cond
suffix
Condition flags
If
S
is specified, the
MUL
instruction:
●Updates the N and Z flags according to the result
●Does not affect the C and V flags
Examples
MULR10, R2, R5; multiply, R10 = R2 x R5
MLAR10, R2, R1, R5; multiply with accumulate, R10 = (R2 x R1) + R5
MULSR0, R2, R2; multiply with flag update, R0 = R2 x R2
MULLTR2, R3, R2; conditionally multiply, R2 = R3 x R2
MLSR4, R5, R6, R7; multiply with subtract, R4 = R7 - (R5 x R6)
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3.6.2 UMULL, UMLAL, SMULL, and SMLAL
Signed and unsigned long multiply, with optional accumulate, using 32-bit operands and
producing a 64-bit result.
Syntax
op{cond} RdLo, RdHi, Rn, Rm
where:
●‘op’ is one of:
UMULL: Unsigned long multiply
UMLAL: Unsigned long multiply, with accumulate
SMULL: Signed long multiply
SMLAL: Signed long multiply, with accumulate
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘RdHi, RdLo’ are the destination registers. For UMLAL and SMLAL, they also hold the
accumulating value.
●‘Rn, Rm’ are registers holding the operands
Operation
The UMULL instruction interprets the values from
Rn
and
Rm
as unsigned integers. It
multiplies these integers and places the least significant 32 bits of the result in
RdLo
, and the
most significant 32 bits of the result in
RdHi
.
The UMLAL instruction interprets the values from
Rn
and
Rm
as unsigned integers. It
multiplies these integers, adds the 64-bit result to the 64-bit unsigned integer contained in
RdHi
and
RdLo
, and writes the result back to
RdHi
and
RdLo
.
The SMULL instruction interprets the values from
Rn
and
Rm
as two’s complement signed
integers. It multiplies these integers and places the least significant 32 bits of the result in
RdLo
, and the most significant 32 bits of the result in
RdHi
.
The SMLAL instruction interprets the values from
Rn
and
Rm
as two’s complement signed
integers. It multiplies these integers, adds the 64-bit result to the 64-bit signed integer
contained in
RdHi
and
RdLo
, and writes the result back to
RdHi
and
RdLo
.
Restrictions
In these instructions:
●Do not use either SP or PC
●
RdHi
and
RdLo
must be different registers
Condition flags
These instructions do not affect the condition code flags.
Examples
UMULLR0, R4, R5, R6; unsigned (R4,R0) = R5 x R6
SMLALR4, R5, R3, R8; signed (R5,R4) = (R5,R4) + R3 x R8
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3.6.3 SDIV and UDIV
Signed divide and unsigned divide.
Syntax
SDIV{cond} {Rd,} Rn, Rm
UDIV{cond} {Rd,} Rn, Rm
where:
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register. If Rd is omitted, the destination register is Rn
●‘Rn,’ is the register holding the value to be divided
●‘Rm’ is a register holding the divisor
Operation
SDIV performs a signed integer division of the value in
Rn
by the value in
Rm
.
UDIV performs an unsigned integer division of the value in
Rn
by the value in
Rm
.
For both instructions, if the value in
Rn
is not divisible by the value in
Rm
, the result is
rounded towards zero.
Restrictions
Do not use either SP or PC
.
Condition flags
These instructions do not change the flags.
Examples
SDIVR0, R2, R4; signed divide, R0 = R2/R4
UDIVR8, R8, R1; unsigned divide, R8 = R8/R1
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3.7 Saturating instructions
This section describes the saturating instructions,
SSAT
and
USAT
.
3.7.1 SSAT and USAT
Signed saturate and unsigned saturate to any bit position, with optional shift before
saturating.
Syntax
op{cond} Rd, #n, Rm {, shift #s}
where:
●‘op’ is one of the following:
SSAT: Saturates a signed value to a signed range
USAT: Saturates a signed value to an unsigned range
●‘cond’ is an optional condition code (see Conditional execution on page 56)
●‘Rd’ is the destination register.
●‘n’ specifies the bit position to saturate to:
n ranges from 1 to 32 for SSAT
n ranges from 0 to 31 for USAT
●‘Rm’ is the register containing the value to saturate
●‘shift #s’ is an optional shift applied to Rm before saturating. It must be one of the
following:
ASR #s where s is in the range 1 to 31
LSL #s where s is in the range 0 to 31
Operation
These instructions saturate to a signed or unsigned
n
-bit value.
The
SSAT
instruction applies the specified shift, then saturates to the signed range:
-2n–1 x 2n–1-1.
The
USAT
instruction applies the specified shift, then saturates to the unsigned range:
0 x 2n-1.
For signed n-bit saturation using
SSAT
, this means that:
●If the value to be saturated is less than -2n-1, the result returned is -2n-1
●If the value to be saturated is greater than 2n-1-1, the result returned is 2n-1-1
●otherwise, the result returned is the same as the value to be saturated.
For unsigned n-bit saturation using
USAT
, this means that:
●If the value to be saturated is less than 0, the result returned is 0
●If the value to be saturated is greater than 2n-1, the result returned is 2n-1
●Otherwise, the result returned is the same as the value to be saturated.
If the returned result is different from the value to be saturated, it is called saturation. If
saturation occurs, the instruction sets the Q flag to 1 in the APSR. Otherwise, it leaves the Q
flag unchanged. To clear the Q flag to 0, you must use the
MSR
instruction, see MSR on
page 100.
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To read the state of the Q flag, use the
MRS
instruction (see MRS on page 99).
Restrictions
Do not use either SP or PC.
Condition flags
These instructions do not affect the condition code flags.
If saturation occurs, these instructions set the Q flag to 1.
Examples
SSATR7, #16, R7, LSL #4; logical shift left value in R7 by 4, then
; saturate it as a signed 16-bit value and
; write it back to R7
USATNER0, #7, R5; conditionally saturate value in R5 as an
; unsigned 7 bit value and write it to R0
3.8 Bitfield instructions
Ta bl e 2 9 shows the instructions that operate on adjacent sets of bits in registers or bitfields.
Table 29. Packing and unpacking instructions
Mnemonic Brief description See
BFC Bit field clear BFC and BFI on page 88
BFI Bit field insert BFC and BFI on page 88
SBFX Signed bit field extract SBFX and UBFX on page 88
SXTB Sign extend a byte SXT and UXT on page 89
SXTH Sign extend a halfword SXT and UXT on page 89
UBFX Unsigned bit field extract SBFX and UBFX on page 88
UXTB Zero extend a byte SXT and UXT on page 89
UXTH Zero extend a halfword SXT and UXT on page 89
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3.8.1 BFC and BFI
Bit Field Clear and Bit Field Insert.
Syntax
BFC{cond} Rd, #lsb, #width
BFI{cond} Rd, Rn, #lsb, #width
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
●‘Rd’ is the destination register.
●‘Rn’ is the source register.
●‘lsb’ is the position of the least significant bit of the bitfield. lsb must be in the range 0 to
31.
●‘width’ is the width of the bitfield and must be in the range 1 to 32-lsb.
Operation
BFC clears a bitfield in a register. It clears width bits in Rd, starting at the low bit position lsb.
Other bits in Rd are unchanged.
BFI copies a bitfield into one register from another register. It replaces width bits in Rd
starting at the low bit position lsb, with width bits from Rn starting at bit[0]. Other bits in Rd
are unchanged.
Restrictions
Do not use SP and do not use PC.
Condition flags
These instructions do not affect the flags.
Examples
BFC R4, #8, #12 ; Clear bit 8 to bit 19 (12 bits) of R4 to 0
BFI R9, R2, #8, #12 ; Replace bit 8 to bit 19 (12 bits) of R9 with
; bit 0 to bit 11 from R2
3.8.2 SBFX and UBFX
Signed Bit Field Extract and Unsigned Bit Field Extract.
Syntax
SBFX{cond} Rd, Rn, #lsb, #width
UBFX{cond} Rd, Rn, #lsb, #width
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where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
●‘Rd’ is the destination register.
●‘Rn’ is the source register.
●‘lsb’ is the position of the least significant bit of the bitfield. lsb must be in the range 0 to
31.
●‘width’ is the width of the bitfield and must be in the range 1 to 32-lsb.
Operation
SBFX extracts a bitfield from one register, sign extends it to 32 bits, and writes the result to
the destination register.
UBFX extracts a bitfield from one register, zero extends it to 32 bits, and writes the result to
the destination register.
Restrictions
Do not use SP and do not use PC.
Condition flags
These instructions do not affect the flags.
Examples
SBFX R0, R1, #20, #4 ; Extract bit 20 to bit 23 (4 bits) from R1 and sign
; extend to 32 bits and then write the result to R0.
UBFX R8, R11, #9, #10 ; Extract bit 9 to bit 18 (10 bits) from R11 and zero
; extend to 32 bits and then write the result to R8
3.8.3 SXT and UXT
Sign extend and Zero extend.
Syntax
SXTextend{cond} {Rd,} Rm {, ROR #n}
UXTextend{cond} {Rd}, Rm {, ROR #n}
where:
●‘extend’ is one of:
B: Extends an 8-bit value to a 32-bit value.
H: Extends a 16-bit value to a 32-bit value.
●‘cond’ is an optional condition code, see Conditional execution on page 56.
●‘Rd’ is the destination register.
●‘Rm’ is the register holding the value to extend.
●ROR #n is one of:
ROR #8: Value from Rm is rotated right 8 bits.
ROR #16: Value from Rm is rotated right 16 bits.
ROR #24: Value from Rm is rotated right 24 bits.
If ROR #n is omitted, no rotation is performed.
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Operation
These instructions do the following:
1. Rotate the value from Rm right by 0, 8, 16 or 24 bits.
2. Extract bits from the resulting value:
– SXTB extracts bits[7:0] and sign extends to 32 bits.
– UXTB extracts bits[7:0] and zero extends to 32 bits.
– SXTH extracts bits[15:0] and sign extends to 32 bits.
– UXTH extracts bits[15:0] and zero extends to 32 bits.
Restrictions
Do not use SP and do not use PC.
Condition flags
These instructions do not affect the flags.
Examples
SXTH R4, R6, ROR #16 ; Rotate R6 right by 16 bits, then obtain the lower
; halfword of the result and then sign extend to
; 32 bits and write the result to R4.
UXTB R3, R10 ; Extract lowest byte of the value in R10 and zero
; extend it, and write the result to R3
3.8.4 Branch and control instructions
Ta bl e 3 0 shows the branch and control instructions:
Table 30. Branch and control instructions
Mnemonic Brief description See
BBranch B, BL, BX, and BLX on page 91
BL Branch with Link B, BL, BX, and BLX on page 91
BLX Branch indirect with Link B, BL, BX, and BLX on page 91
BX Branch indirect B, BL, BX, and BLX on page 91
CBNZ Compare and Branch if Non
Zero CBZ and CBNZ on page 92
CBZ Compare and Branch if Non
Zero CBZ and CBNZ on page 92
IT If-Then IT on page 93
TBB Table Branch Byte TBB and TBH on page 95
TBH Table Branch Halfword TBB and TBH on page 95
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3.8.5 B, BL, BX, and BLX
Branch instructions.
Syntax
B{cond} label
BL{cond} label
BX{cond} Rm
BLX{cond} Rm
where:
●‘B’ is branch (immediate).
●‘BL’ is branch with link (immediate).
●‘BX’ is branch indirect (register).
●‘BLX’ is branch indirect with link (register).
●‘cond’ is an optional condition code, see Conditional execution on page 56.
●‘label’ is a PC-relative expression. See PC-relative expressions on page 55.
●‘Rm’ is a register that indicates an address to branch to. Bit[0] of the value in Rm must
be 1, but the address to branch to is created by changing bit[0] to 0.
Operation
All these instructions cause a branch to label, or to the address indicated in Rm. In addition:
●The BL and BLX instructions write the address of the next instruction to LR (the link
register, R14).
●The BX and BLX instructions cause a UsageFault exception if bit[0] of Rm is 0.
B cond label is the only conditional instruction that can be either inside or outside an IT
block. All other branch instructions must be conditional inside an IT block, and must be
unconditional outside the IT block, see IT on page 93.
Ta bl e 3 1 shows the ranges for the various branch instructions.
You might have to use the .W suffix to get the maximum branch range. See Instruction width
selection on page 58.
Table 31. Branch ranges
Instruction Branch range
B label 16 MB to +16 MB
Bcond label (outside IT block) 1 MB to +1 MB
Bcond label (inside IT block) 16 MB to +16 MB
BL{cond} label 16 MB to +16 MB
BX{cond} Rm Any value in register
BLX{cond} Rm Any value in register
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Restrictions
The restrictions are:
●Do not use PC in the BLX instruction
●For BX and BLX, bit[0] of Rm must be 1 for correct execution but a branch occurs to the
target address created by changing bit[0] to 0
●When any of these instructions is inside an IT block, it must be the last instruction of the
IT block.
Bcond is the only conditional instruction that is not required to be inside an IT block.
However, it has a longer branch range when it is inside an IT block.
Condition flags
These instructions do not change the flags.
Examples
B loopA ; Branch to loopA
BLE ng ; Conditionally branch to label ng
B.W target ; Branch to target within 16MB range
BEQ target ; Conditionally branch to target
BEQ.W target ; Conditionally branch to target within 1MB
BL funC ; Branch with link (Call) to function funC, return address
; stored in LR
BX LR ; Return from function call
BXNE R0 ; Conditionally branch to address stored in R0
BLX R0 ; Branch with link and exchange (Call) to a address stored
; in R0
3.8.6 CBZ and CBNZ
Compare and branch on zero, compare and branch on non-zero.
Syntax
CBZ Rn, label
CBNZ Rn, label
where:
●‘Rn’ is the register holding the operand.
●‘label’ is the branch destination.
Operation
Use the CBZ or CBNZ instructions to avoid changing the condition code flags and to reduce
the number of instructions.
CBZ Rn, label does not change condition flags but is otherwise equivalent to:
CMP Rn, #0
BEQ label
CBNZ Rn, label does not change condition flags but is otherwise equivalent to:
CMP Rn, #0
BNE label
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Restrictions
The restrictions are:
●Rn must be in the range of R0 to R7
●The branch destination must be within 4 to 130 bytes after the instruction
●These instructions must not be used inside an IT block.
Condition flags
These instructions do not change the flags.
Examples
CBZ R5, target ; Forward branch if R5 is zero
CBNZ R0, target ; Forward branch if R0 is not zero
3.8.7 IT
If-Then condition instruction.
Syntax
IT{x{y{z}}} cond
where:
●‘x’ specifies the condition switch for the second instruction in the IT block.
●‘y’ specifies the condition switch for the third instruction in the IT block.
●‘z’ specifies the condition switch for the fourth instruction in the IT block.
●‘cond’ specifies the condition for the first instruction in the IT block.
The condition switch for the second, third and fourth instruction in the IT block can be either:
T: Then. Applies the condition cond to the instruction.
E: Else. Applies the inverse condition of cond to the instruction.
a) It is possible to use AL (the always condition) for cond in an IT instruction. If this is
done, all of the instructions in the IT block must be unconditional, and each of x, y,
and z must be T or omitted but not E.
Operation
The IT instruction makes up to four following instructions conditional. The conditions can be
all the same, or some of them can be the logical inverse of the others. The conditional
instructions following the IT instruction form the IT block.
The instructions in the IT block, including any branches, must specify the condition in the
{cond} part of their syntax.
Your assembler might be able to generate the required IT instructions for conditional
instructions automatically, so that you do not need to write them yourself. See your
assembler documentation for details.
A BKPT instruction in an IT block is always executed, even if its condition fails.
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Exceptions can be taken between an IT instruction and the corresponding IT block, or within
an IT block. Such an exception results in entry to the appropriate exception handler, with
suitable return information in LR and stacked PSR.
Instructions designed for use for exception returns can be used as normal to return from the
exception, and execution of the IT block resumes correctly. This is the only way that a PC-
modifying instruction is permitted to branch to an instruction in an IT block.
Restrictions
The following instructions are not permitted in an IT block:
●IT
●CBZ and CBNZ
●CPSID and CPSIE.
Other restrictions when using an IT block are:
●a branch or any instruction that modifies the PC must either be outside an IT block or
must be the last instruction inside the IT block. These are:
– ADD PC, PC, Rm
–MOV PC, Rm
– B, BL, BX, BLX
– any LDM, LDR, or POP instruction that writes to the PC
– TBB and TBH
●Do not branch to any instruction inside an IT block, except when returning from an
exception handler
●All conditional instructions except Bcond must be inside an IT block. Bcond can be
either outside or inside an IT block but has a larger branch range if it is inside one
●Each instruction inside the IT block must specify a condition code suffix that is either
the same or logical inverse as for the other instructions in the block.
Your assembler might place extra restrictions on the use of IT blocks, such as prohibiting the
use of assembler directives within them.
Condition flags
This instruction does not change the flags.
Example
ITTE NE ; Next 3 instructions are conditional
ANDNE R0, R0, R1 ; ANDNE does not update condition flags
ADDSNE R2, R2, #1 ; ADDSNE updates condition flags
MOVEQ R2, R3 ; Conditional move
CMP R0, #9 ; Convert R0 hex value (0 to 15) into ASCII
; ('0'-'9', 'A'-'F')
ITE GT ; Next 2 instructions are conditional
ADDGT R1, R0, #55 ; Convert 0xA -> 'A'
ADDLE R1, R0, #48 ; Convert 0x0 -> '0'
IT GT ; IT block with only one conditional instruction
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ADDGT R1, R1, #1 ; Increment R1 conditionally
ITTEE EQ ; Next 4 instructions are conditional
MOVEQ R0, R1 ; Conditional move
ADDEQ R2, R2, #10 ; Conditional add
ANDNE R3, R3, #1 ; Conditional AND
BNE.W dloop ; Branch instruction can only be used in the last
; instruction of an IT block
IT NE ; Next instruction is conditional
ADD R0, R0, R1 ; Syntax error: no condition code used in IT block
3.8.8 TBB and TBH
Table Branch Byte and Table Branch Halfword.
Syntax
TBB [Rn, Rm]
TBH [Rn, Rm, LSL #1]
where:
●‘Rn’ is the register containing the address of the table of branch lengths.
If Rn is PC, then the address of the table is the address of the byte immediately
following the TBB or TBH instruction.
●‘Rm’ is the index register. This contains an index into the table. For halfword tables, LSL
#1 doubles the value in Rm to form the right offset into the table.
Operation
These instructions cause a PC-relative forward branch using a table of single byte offsets for
TBB, or halfword offsets for TBH. Rn provides a pointer to the table, and Rm supplies an
index into the table. For TBB the branch offset is twice the unsigned value of the byte
returned from the table. and for TBH the branch offset is twice the unsigned value of the
halfword returned from the table. The branch occurs to the address at that offset from the
address of the byte immediately after the TBB or TBH instruction.
Restrictions
The restrictions are:
●Rn must not be SP
●Rm must not be SP and must not be PC
●When any of these instructions is used inside an IT block, it must be the last instruction
of the IT block.
Condition flags
These instructions do not change the flags.
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Examples
ADR.W R0, BranchTable_Byte
TBB [R0, R1] ; R1 is the index, R0 is the base address of the
; branch table
Case1
; an instruction sequence follows
Case2
; an instruction sequence follows
Case3
; an instruction sequence follows
BranchTable_Byte
DCB 0 ; Case1 offset calculation
DCB ((Case2-Case1)/2) ; Case2 offset calculation
DCB ((Case3-Case1)/2) ; Case3 offset calculation
TBH [PC, R1, LSL #1] ; R1 is the index, PC is used as base of the
; branch table
BranchTable_H
DCI ((CaseA - BranchTable_H)/2) ; CaseA offset calculation
DCI ((CaseB - BranchTable_H)/2) ; CaseB offset calculation
DCI ((CaseC - BranchTable_H)/2) ; CaseC offset calculation
CaseA
; an instruction sequence follows
CaseB
; an instruction sequence follows
CaseC
; an instruction sequence follows
3.9 Miscellaneous instructions
Ta bl e 3 2 shows the remaining Cortex-M3 instructions:
Table 32. Miscellaneous instructions
Mnemonic Brief description See
BKPT Breakpoint BKPT on page 97
CPSID Change Processor State, Disable Interrupts CPS on page 97
CPSIE Change Processor State, Enable Interrupts CPS on page 97
DMB Data Memory Barrier DMB on page 98
DSB Data Synchronization Barrier DSB on page 98
ISB Instruction Synchronization Barrier ISB on page 99
MRS Move from special register to register MRS on page 99
MSR Move from register to special register MSR on page 100
NOP No Operation NOP on page 101
SEV Send Event SEV on page 101
SVC Supervisor Call SVC on page 102
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3.9.1 BKPT
Breakpoint.
Syntax
BKPT #imm
where:
●‘imm’ is an expression evaluating to an integer in the range 0-255 (8-bit value).
Operation
The BKPT instruction causes the processor to enter Debug state. Debug tools can use this
to investigate system state when the instruction at a particular address is reached.
imm is ignored by the processor. If required, a debugger can use it to store additional
information about the breakpoint.
The BKPT instruction can be placed inside an IT block, but it executes unconditionally,
unaffected by the condition specified by the IT instruction.
Condition flags
This instruction does not change the flags.
Examples
BKPT 0xAB ; Breakpoint with immediate value set to 0xAB (debugger can
; extract the immediate value by locating it using the PC)
3.9.2 CPS
Change Processor State.
Syntax
CPSeffect iflags
where:
●‘effect’ is one of:
IE: Clears the special purpose register.
ID: Sets the special purpose register.
●‘iflags’ is a sequence of one or more flags:
i: Set or clear PRIMASK.
f: Set or clear FAULTMASK.
WFE Wait For Event WFE on page 102
WFI Wait For Interrupt WFI on page 103
Table 32. Miscellaneous instructions (continued)
Mnemonic Brief description See
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Operation
CPS changes the PRIMASK and FAULTMASK special register values. See Exception mask
registers on page 18 for more information about these registers.
Restrictions
The restrictions are:
●Use CPS only from privileged software, it has no effect if used in unprivileged software
●CPS cannot be conditional and so must not be used inside an IT block.
Condition flags
This instruction does not change the condition flags.
Examples
CPSID i ; Disable interrupts and configurable fault handlers (set PRIMASK)
CPSID f ; Disable interrupts and all fault handlers (set FAULTMASK)
CPSIE i ; Enable interrupts and configurable fault handlers (clear PRIMASK)
CPSIE f ; Enable interrupts and fault handlers (clear FAULTMASK)
3.9.3 DMB
Data Memory Barrier.
Syntax
DMB{cond}
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
Operation
DMB acts as a data memory barrier. It ensures that all explicit memory accesses that
appear, in program order, before the DMB instruction are completed before any explicit
memory accesses that appear, in program order, after the DMB instruction. DMB does not
affect the ordering or execution of instructions that do not access memory.
Condition flags
This instruction does not change the flags.
Examples
DMB ; Data Memory Barrier
3.9.4 DSB
Data Synchronization Barrier.
Syntax
DSB{cond}
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where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
Operation
DSB acts as a special data synchronization memory barrier. Instructions that come after the
DSB, in program order, do not execute until the DSB instruction completes. The DSB
instruction completes when all explicit memory accesses before it complete.
Condition flags
This instruction does not change the flags.
Examples
DSB ; Data Synchronisation Barrier
3.9.5 ISB
Instruction Synchronization Barrier.
Syntax
ISB{cond}
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
Operation
ISB acts as an instruction synchronization barrier. It flushes the pipeline of the processor, so
that all instructions following the ISB are fetched from cache or memory again, after the ISB
instruction has been completed.
Condition flags
This instruction does not change the flags.
Examples
ISB ; Instruction Synchronisation Barrier
3.9.6 MRS
Move the contents of a special register to a general-purpose register.
Syntax
MRS{cond} Rd, spec_reg
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where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
●‘Rd’ is the destination register.
●‘spec_reg’ can be any of: APSR, IPSR, EPSR, IEPSR, IAPSR, EAPSR, PSR, MSP,
PSP, PRIMASK, BASEPRI, BASEPRI_MAX, FAULTMASK, or CONTROL.
Operation
Use MRS in combination with MSR as part of a read-modify-write sequence for updating a
PSR, for example to clear the Q flag.
In process swap code, the programmers model state of the process being swapped out
must be saved, including relevant PSR contents. Similarly, the state of the process being
swapped in must also be restored. These operations use MRS in the state-saving
instruction sequence and MSR in the state-restoring instruction sequence.
BASEPRI_MAX is an alias of BASEPRI when used with the MRS instruction.
See MSR on page 100.
Restrictions
Rd must not be SP and must not be PC.
Condition flags
This instruction does not change the flags.
Examples
MRS R0, PRIMASK ; Read PRIMASK value and write it to R0
3.9.7 MSR
Move the contents of a general-purpose register into the specified special register.
Syntax
MSR{cond} spec_reg, Rn
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
●‘Rn’ is the source register.
●‘spec_reg’ can be any of: APSR, IPSR, EPSR, IEPSR, IAPSR, EAPSR, PSR, MSP,
PSP, PRIMASK, BASEPRI, BASEPRI_MAX, FAULTMASK, or CONTROL.
Operation
The register access operation in MSR depends on the privilege level. Unprivileged software
can only access the APSR, see Table 4: APSR bit definitions on page 16. Privileged
software can access all special registers.
In unprivileged software writes to unallocated or execution state bits in the PSR are ignored.
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When you write to BASEPRI_MAX, the instruction writes to BASEPRI only if either:
●Rn is non-zero and the current BASEPRI value is 0
●Rn is non-zero and less than the current BASEPRI value.
See MRS on page 99.
Restrictions
Rn must not be SP and must not be PC.
Condition flags
This instruction updates the flags explicitly based on the value in Rn.
Examples
MSR CONTROL, R1 ; Read R1 value and write it to the CONTROL register
3.9.8 NOP
No Operation.
Syntax
NOP{cond}
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
Operation
NOP does nothing. NOP is not necessarily a time-consuming NOP. The processor might
remove it from the pipeline before it reaches the execution stage.
Use NOP for padding, for example to place the following instruction on a 64-bit boundary.
Condition flags
This instruction does not change the flags.
Examples
NOP ; No operation
3.9.9 SEV
Send Event.
Syntax
SEV{cond}
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
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Operation
SEV is a hint instruction that causes an event to be signaled to all processors within a
multiprocessor system. It also sets the local event register to 1, see Power management on
page 41.
Condition flags
This instruction does not change the flags.
Examples
SEV ; Send Event
3.9.10 SVC
Supervisor Call.
Syntax
SVC{cond} #imm
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
●‘imm’ is an expression evaluating to an integer in the range 0-255 (8-bit value).
Operation
The SVC instruction causes the SVC exception.
imm is ignored by the processor. If required, it can be retrieved by the exception handler to
determine what service is being requested.
Condition flags
This instruction does not change the flags.
Examples
SVC 0x32 ; Supervisor Call (SVC handler can extract the immediate value
; by locating it via the stacked PC)
3.9.11 WFE
Wait For Event.
Syntax
WFE{cond}
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
Operation
WFE is a hint instruction.
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If the event register is 0, WFE suspends execution until one of the following events occurs:
●An exception, unless masked by the exception mask registers or the current priority
level
●An exception enters the Pending state, if SEVONPEND in the System Control Register
is set
●A Debug Entry request, if Debug is enabled
●An event signaled by a peripheral or another processor in a multiprocessor system
using the SEV instruction.
If the event register is 1, WFE clears it to 0 and returns immediately.
For more information see Power management on page 41.
Condition flags
This instruction does not change the flags.
Examples
WFE ; Wait for event
3.9.12 WFI
Wait for Interrupt.
Syntax
WFI{cond}
where:
●‘cond’ is an optional condition code, see Conditional execution on page 56.
Operation
WFI is a hint instruction that suspends execution until one of the following events occurs:
●An exception
●A Debug Entry request, regardless of whether Debug is enabled.
Condition flags
This instruction does not change the flags.
Examples
WFI ; Wait for interrupt
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4 Core peripherals
4.1 About the STM32 core peripherals
The address map of the Private peripheral bus (PPB) is:
In register descriptions:
●The required privilege gives the privilege level required to access the register, as
follows:
4.2 Nested vectored interrupt controller (NVIC)
This section describes the Nested Vectored Interrupt Controller (NVIC) and the registers it
uses. The NVIC supports:
●up to 68 interrupts
●A programmable priority level of 0-15 for each interrupt. A higher level corresponds to a
lower priority, so level 0 is the highest interrupt priority
●Level and pulse detection of interrupt signals
●Dynamic reprioritization of interrupts
●Grouping of priority values into group priority and subpriority fields
●Interrupt tail-chaining
●An external Non-maskable interrupt (NMI)
The processor automatically stacks its state on exception entry and unstacks this state on
exception exit, with no instruction overhead. This provides low latency exception handling.
The hardware implementation of the NVIC registers is:
Table 33. STM32 core peripheral register regions
Address Core peripheral Description
0xE000E010-0xE000E01F System timer Table 41 on page 135
0xE000E100-0xE000E4EF Nested vectored interrupt
controller Table 37 on page 114
0xE000ED00-0xE000ED3F System control block Table 40 on page 131
0xE000ED90-0xE000ED93 MPU type register Reads as zero, indicating no MPU is
implemented(1)
1. Software can read the MPU Type Register at 0xE000ED90 to test for the presence of a memory protection
unit (MPU).
0xE000EF00-0xE000EF03 Nested vectored interrupt
controller Table 37 on page 114
Privileged Only privileged software can access the register.
Unprivileged Both unprivileged and privileged software can access the register.
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4.2.1 The CMSIS mapping of the Cortex-M3 NVIC registers
To improve software efficiency, the CMSIS simplifies the NVIC register presentation. In the
CMSIS:
●The Set-enable, Clear-enable, Set-pending, Clear-pending and Active Bit registers map
to arrays of 32-bit integers, so that:
– The array ISER[0] to ISER[2] corresponds to the registers ISER0-ISER2
– The array ICER[0] to ICER[2] corresponds to the registers ICER0-ICER2
– The array ISPR[0] to ISPR[2] corresponds to the registers ISPR0-ISPR2
– The array ICPR[0] to ICPR[2] corresponds to the registers ICPR0-ICPR2
– The array IABR[0] to IABR[2] corresponds to the registers IABR0-IABR2.
●The 8-bit fields of the Interrupt Priority Registers map to an array of 8-bit integers, so
that the array IP[0] to IP[67] corresponds to the registers IPR0-IPR67, and the array
entry IP[n] holds the interrupt priority for interrupt n.
The CMSIS provides thread-safe code that gives atomic access to the Interrupt Priority
Registers. For more information see the description of the NVIC_SetPriority function in
NVIC programming hints on page 113. Ta bl e 3 4 shows how the interrupts, or IRQ numbers,
map onto the interrupt registers and corresponding CMSIS variables that have one bit per
interrupt.
Table 34. Mapping of interrupts to the interrupt variables
Interrupts
CMSIS array elements(1)
1. Each array element corresponds to a single NVIC register, for example the element ICER[1] corresponds
to the ICER1 register.
Set-enable Clear-enable Set-pending Clear-pending Active Bit
0-31 ISER[0] ICER[0] ISPR[0] ICPR[0] IABR[0]
32-63 ISER[1] ICER[1] ISPR[1] ICPR[1] IABR[1]
64-67 ISER[2] ICER[2] ISPR[2] ICPR[2] IABR[2]
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4.2.2 Interrupt set-enable registers (NVIC_ISERx)
Address offset: 0x00 - 0x0B
Reset value: 0x0000 0000
Required privilege: Privileged
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
SETENA[31:16]
rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs
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SETENA[15:0]
rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs
Bits 31:0 SETENA[31:0]: Interrupt set-enable bits.
Write:
0: No effect
1: Enable interrupt
Read:
0: Interrupt disabled
1: Interrupt enabled.
See Table 34: Mapping of interrupts to the interrupt variables on page 105 for the
correspondence of interrupts to each register bit.
If a pending interrupt is enabled, the NVIC activates the interrupt based on its priority. If an
interrupt is not enabled, asserting its interrupt signal changes the interrupt state to pending,
but the NVIC never activates the interrupt, regardless of its priority.
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4.2.3 Interrupt clear-enable registers (NVIC_ICERx)
Address offset: 0x00 - 0x0B
Reset value: 0x0000 0000
Required privilege: Privileged
The ICER0-ICER2 registers disable interrupts, and show which interrupts are enabled.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
CLRENA[31:16]
rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1
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CLRENA[15:0]
rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1
Bits 31:0 CLRENA[31:0]: Interrupt clear-enable bits.
Write:
0: No effect
1: Disable interrupt
Read:
0: Interrupt disabled
1: Interrupt enabled.
See Table 34: Mapping of interrupts to the interrupt variables on page 105 for the
correspondence of interrupts to each register bit.
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4.2.4 Interrupt set-pending registers (NVIC_ISPRx)
Address offset: 0x00 - 0x0B
Reset value: 0x0000 0000
Required privilege: Privileged
The ISPR0-ISPR2 registers force interrupts into the pending state, and show which
interrupts are pending.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
SETPEND[31:16]
rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs
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SETPEND[15:0]
rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs
Bits 31:0 SETPEND[31:0]: Interrupt set-pending bits
Write:
0: No effect
1: Changes interrupt state to pending
Read:
0: Interrupt is not pending
1: Interrupt is pending
See Table 34: Mapping of interrupts to the interrupt variables on page 105 for the
correspondence of interrupts to each register bit.
Writing 1 to the ISPR bit corresponding to an interrupt that is pending:
– has no effect.
Writing 1 to the ISPR bit corresponding to a disabled interrupt:
– sets the state of that interrupt to pending.
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4.2.5 Interrupt clear-pending registers (NVIC_ICPRx)
Address offset: 0x00 - 0x0B
Reset value: 0x0000 0000
Required privilege: Privileged
The ICPR0-ICPR2 registers remove the pending state from interrupts, and show which
interrupts are pending.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
CLRPEND[31:16]
rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1
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CLRPEND[15:0]
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Bits 31:0 CLRPEND[31:0]: Interrupt clear-pending bits
Write:
0: No effect
1: Removes the pending state of an interrupt
Read:
0: Interrupt is not pending
1: Interrupt is pending
See Table 34: Mapping of interrupts to the interrupt variables on page 105 for the
correspondence of interrupts to each register bit.
Writing 1 to an ICPR bit does not affect the active state of the corresponding interrupt.
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4.2.6 Interrupt active bit registers (NVIC_IABRx)
Address offset: 0x00- 0x0B
Reset value: 0x0000 0000
Required privilege: Privileged
The IABR0-IABR2 registers indicate which interrupts are active.
The bit assignments are:
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
ACTIVE[31:16]
rrrrrr r r r r rrrrrr
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ACTIVE[15:0]
rrrrrr r r r r rrrrrr
Bits 31:0 ACTIVE[31:0]: Interrupt active flags
0: Interrupt not active
1: Interrupt active
See Table 34: Mapping of interrupts to the interrupt variables on page 105 for the
correspondence of interrupts to each register bit.
A bit reads as 1 if the status of the corresponding interrupt is active or active and pending.
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4.2.7 Interrupt priority registers (NVIC_IPRx)
Address offset: 0x00- 0x0B
Reset value: 0x0000 0000
Required privilege: Privileged
The IPR0-IPR16 registers provide a 4-bit priority field for each interrupt. These registers are
byte-accessible. Each register holds four priority fields, that map to four elements in the
CMSIS interrupt priority array IP[0] to IP[67], as shown in Figure 18.
Figure 18. NVIC__IPRx register mapping
See The CMSIS mapping of the Cortex-M3 NVIC registers on page 105 for more
information about the IP[0] to IP[67] interrupt priority array, that provides the software view of
the interrupt priorities.
Find the IPR number and byte offset for interrupt N as follows:
●The corresponding IPR number, M, is given by M = N DIV 4
●The byte offset of the required Priority field in this register is N MOD 4, where:
– byte offset 0 refers to register bits[7:0]
– byte offset 1 refers to register bits[15:8]
– byte offset 2 refers to register bits[23:16]
– byte offset 3 refers to register bits[31:24].
Table 35. IPR bit assignments
Bits Name Function
[31:24] Priority, byte offset 3
Each priority field holds a priority value, 0-255. The lower the value,
the greater the priority of the corresponding interrupt. The processor
implements only bits[7:4] of each field, bits[3:0] read as zero and
ignore writes.
[23:16] Priority, byte offset 2
[15:8] Priority, byte offset 1
[7:0] Priority, byte offset 0
IP[67]
31 24 23 16 15 8 7 0
IP[66] IP[65] IP[64]
IPR16
IP[4m+3] IP[4m+2] IP[4m+1] IP[4m]
IPRm
IP[3] IP[2] IP[1] IP[0]
IPR0
. . . . . .
. . . . . .
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4.2.8 Software trigger interrupt register (NVIC_STIR)
Address offset: 0xE00
Reset value: 0x0000 0000
Required privilege: When the USERSETMPEND bit in the SCR is set to 1, unprivileged
software can access the STIR, see Section 4.3.5: System control register (SCB_SCR). Only
privileged software can enable unprivileged access to the STIR.
4.2.9 Level-sensitive and pulse interrupts
STM32 interrupts are both level-sensitive and pulse-sensitive. Pulse interrupts are also
described as edge-triggered interrupts.
A level-sensitive interrupt is held asserted until the peripheral deasserts the interrupt signal.
Typically this happens because the ISR accesses the peripheral, causing it to clear the
interrupt request. A pulse interrupt is an interrupt signal sampled synchronously on the
rising edge of the processor clock. To ensure the NVIC detects the interrupt, the peripheral
must assert the interrupt signal for at least one clock cycle, during which the NVIC detects
the pulse and latches the interrupt.
When the processor enters the ISR, it automatically removes the pending state from the
interrupt, see Hardware and software control of interrupts. For a level-sensitive interrupt, if
the signal is not deasserted before the processor returns from the ISR, the interrupt
becomes pending again, and the processor must execute its ISR again. This means that the
peripheral can hold the interrupt signal asserted until it no longer needs servicing.
Hardware and software control of interrupts
The Cortex-M3 latches all interrupts. A peripheral interrupt becomes pending for one of the
following reasons:
●The NVIC detects that the interrupt signal is HIGH and the interrupt is not active
●The NVIC detects a rising edge on the interrupt signal
●Software writes to the corresponding interrupt set-pending register bit, see
Section 4.2.4: Interrupt set-pending registers (NVIC_ISPRx), or to the STIR to make an
SGI pending, see Section 4.2.8: Software trigger interrupt register (NVIC_STIR).
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved
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Reserved INTID[8:0]
wwwwwwwww
Bits 31:9 Reserved, must be kept cleared.
Bits 8:0 NTID[8:0] Software generated interrupt ID
Write to the STIR to generate a Software Generated Interrupt (SGI). The value to be written is
the Interrupt ID of the required SGI, in the range 0-239. For example, a value of 0b000000011
specifies interrupt IRQ3.
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A pending interrupt remains pending until one of the following:
●The processor enters the ISR for the interrupt. This changes the state of the interrupt
from pending to active. Then:
– For a level-sensitive interrupt, when the processor returns from the ISR, the NVIC
samples the interrupt signal. If the signal is asserted, the state of the interrupt
changes to pending, which might cause the processor to immediately re-enter the
ISR. Otherwise, the state of the interrupt changes to inactive.
– For a pulse interrupt, the NVIC continues to monitor the interrupt signal, and if this
is pulsed the state of the interrupt changes to pending and active. In this case,
when the processor returns from the ISR the state of the interrupt changes to
pending, which might cause the processor to immediately re-enter the ISR. If the
interrupt signal is not pulsed while the processor is in the ISR, when the processor
returns from the ISR the state of the interrupt changes to inactive.
●Software writes to the corresponding interrupt clear-pending register bit.
For a level-sensitive interrupt, if the interrupt signal is still asserted, the state of the
interrupt does not change. Otherwise, the state of the interrupt changes to inactive.
For a pulse interrupt, state of the interrupt changes to:
– Inactive, if the state was pending
– Active, if the state was active and pending.
4.2.10 NVIC design hints and tips
Ensure software uses correctly aligned register accesses. The processor does not support
unaligned accesses to NVIC registers. See the individual register descriptions for the
supported access sizes.
A interrupt can enter pending state even it is disabled.
Before programming VTOR to relocate the vector table, ensure the vector table entries of
the new vector table are setup for fault handlers, NMI and all enabled exception like
interrupts. For more information see Section 4.3.3: Vector table offset register (SCB_VTOR)
on page 118.
NVIC programming hints
Software uses the CPSIE I and CPSID I instructions to enable and disable interrupts. The
CMSIS provides the following intrinsic functions for these instructions:
void __disable_irq(void) // Disable Interrupts
void __enable_irq(void) // Enable Interrupts
In addition, the CMSIS provides a number of functions for NVIC control, including:
Table 36. CMSIS functions for NVIC control
CMSIS interrupt control function Description
void NVIC_SetPriorityGrouping(uint32_t priority_grouping) Set the priority grouping
void NVIC_EnableIRQ(IRQn_t IRQn) Enable IRQn
void NVIC_DisableIRQ(IRQn_t IRQn) Disable IRQn
uint32_t NVIC_GetPendingIRQ (IRQn_t IRQn) Return true (IRQ-Number) if IRQn is
pending
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For more information about these functions see the CMSIS documentation.
4.2.11 NVIC register map
The table provides shows the NVIC register map and reset values. The base address of the
main NVIC register block is 0xE000E100. The NVIC_STIR register is located in a separate
block at 0xE000EF00.
void NVIC_SetPendingIRQ (IRQn_t IRQn) Set IRQn pending
void NVIC_ClearPendingIRQ (IRQn_t IRQn) Clear IRQn pending status
uint32_t NVIC_GetActive (IRQn_t IRQn) Return the IRQ number of the active
interrupt
void NVIC_SetPriority (IRQn_t IRQn, uint32_t priority) Set priority for IRQn
uint32_t NVIC_GetPriority (IRQn_t IRQn) Read priority of IRQn
void NVIC_SystemReset (void) Reset the system
Table 36. CMSIS functions for NVIC control (continued)
CMSIS interrupt control function Description
Table 37. NVIC register map and reset values
Offset Register
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0x000 NVIC_ISER0 SETENA[31:0]
Reset Value 00000000000000000000000000000000
0x004 NVIC_ISER1 SETENA[63:32]
Reset Value 00000000000000000000000000000000
0x008 NVIC_ISER2 Reserved
SETENA
[67:64]
Reset Value 0000
0x080 NVIC_ICER0 CLRENA[31:0]
Reset Value 00000000000000000000000000000000
0x084 NVIC_ICER1 CLRENA[63:32]
Reset Value 00000000000000000000000000000000
0x088 NVIC_ICER2 Reserved
CLRENA
[67:64]
Reset Value 0000
0x200 NVIC_ISPR0 SETPEND[31:0]
Reset Value 00000000000000000000000000000000
0x204 NVIC_ISPR1 SETPEND[63:32]
Reset Value 00000000000000000000000000000000
0x208 NVIC_ISPR2 Reserved
SETPEND
[67:64]
Reset Value 0000
0x280 NVIC_ICPR0 CLRPEND[31:0]
Reset Value 00000000000000000000000000000000
0x284 NVIC_ICPR1 CLRPEND[63:32]
Reset Value 00000000000000000000000000000000
0x288 NVIC_ICPR2 Reserved
CLRPEND
[67:64]
Reset Value 0000
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4.3 System control block (SCB)
The System control block (SCB) provides system implementation information, and system
control. This includes configuration, control, and reporting of the system exceptions. The
CMSIS mapping of the Cortex-M3 SCB registers
To improve software efficiency, the CMSIS simplifies the SCB register presentation. In the
CMSIS, the byte array SHP[0] to SHP[12] corresponds to the registers SHPR1-SHPR3.
4.3.1 CPUID base register (SCB_CPUID)
Address offset: 0x00
Reset value: 0x411F C231
Required privilege: Privileged
The CPUID register contains the processor part number, version, and implementation
information.
0x300 NVIC_IABR0 ACTIVE[31:0]
Reset Value 00000000000000000000000000000000
0x304 NVIC_IABR1 ACTIVE[63:32]
Reset Value 00000000000000000000000000000000
0x308 NVIC_IABR2 Reserved
ACTIVE
[67:64]
Reset Value 0000
0x400 NVIC_IPR0 IP[3] IP[2] IP[1] IP[0]
Reset Value 00000000000000000000000000000000
:::
0x410 NVIC_IPR16 IP[67] IP[66] IP[65] IP[64]
Reset Value 00000000000000000000000000000000
SCB registers
Reserved
0xE00 NVIC_STIR Reserved INTID[8:0]
Reset Value 000000000
Table 37. NVIC register map and reset values (continued)
Offset Register
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Implementer Variant Constant
rrrrrr r r r r rrrrrr
1514131211109876543210
PartNo Revision
rrrrrr r r r r rrrrrr
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4.3.2 Interrupt control and state register (SCB_ICSR)
Address offset: 0x04
Reset value: 0x0000 0000
Required privilege: Privileged
The ICSR:
●Provides:
– A set-pending bit for the Non-Maskable Interrupt (NMI) exception
– Set-pending and clear-pending bits for the PendSV and SysTick exceptions
●Indicates:
– The exception number of the exception being processed
– Whether there are preempted active exceptions
– The exception number of the highest priority pending exception
– Whether any interrupts are pending.
Caution: When you write to the ICSR, the effect is unpredictable if you:
●Write 1 to the PENDSVSET bit and write 1 to the PENDSVCLR bit
●Write 1 to the PENDSTSET bit and write 1 to the PENDSTCLR bit.
Bits 31:24 Implementer: Implementer code
0x41: ARM
Bits 23:20 Variant: Variant number
The r value in the rnpn product revision identifier
0x1: r1p1
Bits 19:16 Constant: Reads as 0xF
Bits 15:4 PartNo: Part number of the processor
0xC23: = Cortex-M3
Bits 3:0 Revision: Revision number
The p value in the rnpn product revision identifier
0x1: = r1p1
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
NMIPE
NDSET Reserved
PEND
SVSET
PEND
SVCLR
PEND
STSET
PENDS
TCLR Reserved
ISRPE
NDING VECTPENDING[9:4]
rw rwwrww rrrrrrr
1514131211109876543210
VECTPENDING[3:0] RETOB
ASE Reserved VECTACTIVE[8:0]
rrrrr rwrwrwrwrwrwrwrwrw
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Bit 31 NMIPENDSET: NMI set-pending bit.
Write:
0: No effect
1: Change NMI exception state to pending.
Read:
0: NMI exception is not pending
1: NMI exception is pending
Because NMI is the highest-priority exception, normally the processor enter the NMI
exception handler as soon as it registers a write of 1 to this bit, and entering the handler clears
this bit to 0. A read of this bit by the NMI exception handler returns 1 only if the NMI signal is
reasserted while the processor is executing that handler.
Bits 30:29 Reserved, must be kept cleared
Bit 28 PENDSVSET: PendSV set-pending bit.
Write:
0: No effect
1: Change PendSV exception state to pending.
Read:
0: PendSV exception is not pending
1: PendSV exception is pending
Writing 1 to this bit is the only way to set the PendSV exception state to pending.
Bit 27 PENDSVCLR: PendSV clear-pending bit.
Write:
0: No effect
1: Removes the pending state from the PendSV exception.
Bit 26 PENDSTSET: SysTick exception set-pending bit.
Write:
0: No effect
1: Change SysTick exception state to pending
Read:
0: SysTick exception is not pending
1: SysTick exception is pending
Bit 25 PENDSTCLR: SysTick exception clear-pending bit.
Write:
0: No effect
1: Removes the pending state from the SysTick exception.
This bit is write-only. On a register read its value is unknown.
Bit 24 Reserved, must be kept cleared.
Bit 23 This bit is reserved for Debug use and reads-as-zero when the processor is not in Debug.
Bit 22 ISRPENDING: Interrupt pending flag, excluding NMI and Faults
0: Interrupt not pending
1: Interrupt pending
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4.3.3 Vector table offset register (SCB_VTOR)
Address offset: 0x08
Reset value: 0x0000 0000
Required privilege: Privileged
Bits 21:12 VECTPENDING[9:0] Pending vector
Indicates the exception number of the highest priority pending enabled exception.
0: No pending exceptions
Other values: The exception number of the highest priority pending enabled exception.
The value indicated by this field includes the effect of the BASEPRI and FAULTMASK
registers, but not any effect of the PRIMASK register.
Bit 11 RETOBASE: Return to base level
Indicates whether there are preempted active exceptions:
0: There are preempted active exceptions to execute
1: There are no active exceptions, or the currently-executing exception is the only active
exception.
Bits 10:9 Reserved, must be kept cleared
Bits 8:0 VECTACTIVE[8:0] Active vector
Contains the active exception number:
0: Thread mode
Other values: The exception number(1) of the currently active exception.
Note: Subtract 16 from this value to obtain the IRQ number required to index into the
Interrupt Clear-Enable, Set-Enable, Clear-Pending, Set-Pending, or Priority Registers,
see Table 5 on page 17.
1. This is the same value as IPSR bits[8:0], see Interrupt program status register on page 17.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved TBLOFF[29:16]
rw rw rw rw rw rw rw rw rw rw rw rw rw rw
1514131211109876543210
TBLOFF[15:9] Reserved
rw rw rw rw rw rw rw
Bits 31:30 Reserved, must be kept cleared
Bits 29:11 TBLOFF[29:9]: Vector table base offset field.
It contains bits [29:9] of the offset of the table base from memory address 0x00000000. When
setting TBLOFF, you must align the offset to the number of exception entries in the vector table.
The minimum alignment is 128 words. Table alignment requirements mean that bits[8:0] of the
table offset are always zero.
Bit 29 determines whether the vector table is in the code or SRAM memory region.
0: Code
1: SRAM
Note: Bit 29 is sometimes called the TBLBASE bit.
Bits 10:0 Reserved, must be kept cleared
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4.3.4 Application interrupt and reset control register (SCB_AIRCR)
Address offset: 0x0C
Reset value: 0xFA05 0000
Required privilege: Privileged
The AIRCR provides priority grouping control for the exception model, endian status for data
accesses, and reset control of the system.
To write to this register, you must write 0x5VA to the VECTKEY field, otherwise the
processor ignores the write.
Binary point
The PRIGROUP field indicates the position of the binary point that splits the PRI_n fields in
the Interrupt Priority Registers into separate group priority and subpriority fields. Tabl e 3 8
shows how the PRIGROUP value controls this split.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
VECTKEYSTAT[15:0](read)/ VECTKEY[15:0](write)
rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
1514131211109876543210
ENDIA
NESS Reserved PRIGROUP Reserved
SYS
RESET
REQ
VECT
CLR
ACTIVE
VECT
RESET
rrwrwrw www
Bits 31:16 VECTKEYSTAT[15:0]/ VECTKEY[15:0] Register key
Reads as 0x05FA
On writes, write 0x5FA to VECTKEY, otherwise the write is ignored.
Bit 15 ENDIANESS Data endianness bit
Reads as 0.
0: Little-endian
Bits 14:11 Reserved, must be kept cleared
Bits 10:8 PRIGROUP[2:0]: Interrupt priority grouping field
This field determines the split of group priority from subpriority, see Binary point on page 119.
Bits 7:3 Reserved, must be kept cleared
Bit 2 SYSRESETREQ System reset request
This is intended to force a large system reset of all major components except for debug.
This bit reads as 0.
0: No system reset request
1: Asserts a signal to the outer system that requests a reset.
Bit 1 VECTCLRACTIVE
Reserved for Debug use. This bit reads as 0. When writing to the register you must write 0 to
this bit, otherwise behavior is unpredictable.
Bit 0 VECTRESET
Reserved for Debug use. This bit reads as 0. When writing to the register you must write 0 to
this bit, otherwise behavior is unpredictable.
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Determining preemption of an exception uses only the group priority field, see Section 2.3.6:
Interrupt priority grouping on page 36.
4.3.5 System control register (SCB_SCR)
Address offset: 0x10
Reset value: 0x0000 0000
Required privilege: Privileged
The SCR controls features of entry to and exit from low power state.
Table 38. Priority grouping
PRIGROUP
[2:0]
Interrupt priority level value, PRI_N[7:4] Number of
Binary
point(1)
1. PRI_n[7:4] field showing the binary point. x denotes a group priority field bit, and y denotes a subpriority
field bit.
Group priority
bits
Subpriority
bits
Group
priorities
Sub
priorities
0b011 0bxxxx [7:4] None 16 None
0b100 0bxxx.y [7:5] [4] 8 2
0b101 0bxx.yy [7:6] [5:4] 4 4
0b110 0bx.yyy [7] [6:4] 2 8
0b111 0b.yyyy None [7:4] None 16
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved
1514131211109876543210
Reserved
SEVON
PEND Res.
SLEEP
DEEP
SLEEP
ON
EXIT Res.
rw rw rw
Bits 31:5 Reserved, must be kept cleared
Bit 4 SEVEONPEND Send Event on Pending bit
When an event or interrupt enters pending state, the event signal wakes up the processor from
WFE. If the processor is not waiting for an event, the event is registered and affects the next
WFE.
The processor also wakes up on execution of an SEV instruction or an external event
0: Only enabled interrupts or events can wakeup the processor, disabled interrupts are
excluded
1: Enabled events and all interrupts, including disabled interrupts, can wakeup the
processor.
Bit 3 Reserved, must be kept cleared
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4.3.6 Configuration and control register (SCB_CCR)
Address offset: 0x14
Reset value: 0x0000 0200
Required privilege: Privileged
The CCR controls entry to Thread mode and enables:
●The handlers for NMI, hard fault and faults escalated by FAULTMASK to ignore bus
faults
●Trapping of divide by zero and unaligned accesses
●Access to the STIR by unprivileged software, see Software trigger interrupt register
(NVIC_STIR) on page 112.
Bit 2 SLEEPDEEP
Controls whether the processor uses sleep or deep sleep as its low power mode:
0: Sleep
1: Deep sleep.
Bit 2 SLEEPONEXIT
Configures sleep-on-exit when returning from Handler mode to Thread mode. Setting this bit to
1 enables an interrupt-driven application to avoid returning to an empty main application.
0: Do not sleep when returning to Thread mode.
1: Enter sleep, or deep sleep, on return from an interrupt service routine.
Bit 0 Reserved, must be kept cleared
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved
1514131211109876543210
Reserved
STK
ALIGN
BFHF
NMIGN Reserved
DIV_0_
TRP
UN
ALIGN_
TRP Res.
USER
SET
MPEND
NON
BASE
THRD
ENA
rw rw rw rw rw rw
Bits 31:10 Reserved, must be kept cleared
Bit 9 STKALIGN
Configures stack alignment on exception entry. On exception entry, the processor uses bit 9 of
the stacked PSR to indicate the stack alignment. On return from the exception it uses this
stacked bit to restore the correct stack alignment.
0: 4-byte aligned
1: 8-byte aligned
Bit 8 BFHFNMIGN
Enables handlers with priority -1 or -2 to ignore data bus faults caused by load and store
instructions. This applies to the hard fault, NMI, and FAULTMASK escalated handlers. Set this
bit to 1 only when the handler and its data are in absolutely safe memory. The normal use of
this bit is to probe system devices and bridges to detect control path problems and fix them.
0: Data bus faults caused by load and store instructions cause a lock-up
1: Handlers running at priority -1 and -2 ignore data bus faults caused by load and store
instructions.
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4.3.7 System handler priority registers (SHPRx)
The SHPR1-SHPR3 registers set the priority level, 0 to 15 of the exception handlers that
have configurable priority.
SHPR1-SHPR3 are byte accessible.
The system fault handlers and the priority field and register for each handler are:
Each PRI_N field is 8 bits wide, but the processor implements only bits[7:4] of each field,
and bits[3:0] read as zero and ignore writes.
Bits 7:5 Reserved, must be kept cleared
Bit 4 DIV_0_TRP
Enables faulting or halting when the processor executes an SDIV or UDIV instruction with a
divisor of 0:
0: Do not trap divide by 0
1: Trap divide by 0.
When this bit is set to 0, a divide by zero returns a quotient of 0.
Bit 3 UNALIGN_ TRP
Enables unaligned access traps:
0: Do not trap unaligned halfword and word accesses
1: Trap unaligned halfword and word accesses.
If this bit is set to 1, an unaligned access generates a usage fault.
Unaligned LDM, STM, LDRD, and STRD instructions always fault irrespective of whether
UNALIGN_TRP is set to 1.
Bit 2 Reserved, must be kept cleared
Bit 1 USERSETMPEND
Enables unprivileged software access to the STIR, see Software trigger interrupt register
(NVIC_STIR) on page 112:
0: Disable
1: Enable.
Bit 0 NONBASETHRDENA
Configures how the processor enters Thread mode.
0: Processor can enter Thread mode only when no exception is active.
1: Processor can enter Thread mode from any level under the control of an EXC_RETURN
value, see Exception return on page 38.
Table 39. System fault handler priority fields
Handler Field Register description
Memory management fault PRI_4
System handler priority register 1 (SCB_SHPR1)Bus fault PRI_5
Usage fault PRI_6
SVCall PRI_11 System handler priority register 2 (SCB_SHPR2) on
page 123
PendSV PRI_14 System handler priority register 3 (SCB_SHPR3) on
page 124
SysTick PRI_15
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System handler priority register 1 (SCB_SHPR1)
Address offset: 0x18
Reset value: 0x0000 0000
Required privilege: Privileged
System handler priority register 2 (SCB_SHPR2)
Address offset: 0x1C
Reset value: 0x0000 0000
Required privilege: Privileged
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved PRI_6[7:4] PRI_6[3:0]
rw rw rw rw r r r r
1514131211109876543210
PRI_5[7:4] PRI_5[3:0] PRI_4[7:4] PRI_4[7:4]
rw rw rw rw r r r r rw rw rw rw r r r r
Bits 31:24 Reserved, must be kept cleared
Bits 23:16 PRI_6[7:0]: Priority of system handler 6, usage fault
Bits 15:8 PRI_5[7:0]: Priority of system handler 5, bus fault
Bits 7:0 PRI_4[7:0]: Priority of system handler 4, memory management fault
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
PRI_11[7:4] PRI_11[3:0] Reserved
rw rw rw rw r r r r
1514131211109876543210
Reserved
Bits 31:24 PRI_11[7:0]: Priority of system handler 11, SVCall
Bits 23:0 Reserved, must be kept cleared
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System handler priority register 3 (SCB_SHPR3)
Address: 0xE000 ED20
Reset value: 0x0000 0000
Required privilege: Privileged
4.3.8 System handler control and state register (SCB_SHCSR)
Address offset: 0x24
Reset value: 0x0000 0000
Required privilege: Privileged
The SHCSR enables the system handlers, and indicates:
●The pending status of the bus fault, memory management fault, and SVC exceptions
●The active status of the system handlers.
If you disable a system handler and the corresponding fault occurs, the processor treats the
fault as a hard fault.
You can write to this register to change the pending or active status of system exceptions.
An OS kernel can write to the active bits to perform a context switch that changes the
current exception type.
●Software that changes the value of an active bit in this register without correct
adjustment to the stacked content can cause the processor to generate a fault
exception. Ensure software that writes to this register retains and subsequently
restores the current active status.
●After you have enabled the system handlers, if you have to change the value of a bit in
this register you must use a read-modify-write procedure to ensure that you change
only the required bit.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
PRI_15[7:4] PRI_15[3:0] PRI_14[7:4] PRI_14[3:0]
rw rw rw rw r r r r rw rw rw rw r r r r
1514131211109876543210
Reserved
Bits 31:24 PRI_15[7:0]: Priority of system handler 15, SysTick exception
Bits 23:16 PRI_14[7:0]: Priority of system handler 14, PendSV
Bits 15:0 Reserved, must be kept cleared
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31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved
USG
FAULT
ENA
BUS
FAULT
ENA
MEM
FAULT
ENA
rw rw rw
1514131211109876543210
SV
CALL
PEND
ED
BUS
FAULT
PEND
ED
MEM
FAULT
PEND
ED
USG
FAULT
PEND
ED
SYS
TICK
ACT
PEND
SV
ACT Res.
MONIT
OR
ACT
SV
CALL
ACT Reserved
USG
FAULT
ACT Res.
BUS
FAULT
ACT
MEM
FAULT
ACT
rw rw rw rw rw rw rw rw rw rw rw
Bits 31:19 Reserved, must be kept cleared
Bit 18 USGFAULTENA: Usage fault enable bit, set to 1 to enable(1)
Bit 17 BUSFAULTENA: Bus fault enable bit, set to 1 to enable(1)
Bit 16 MEMFAULTENA: Memory management fault enable bit, set to 1 to enable(1)
Bit 15 SVCALLPENDED: SVC call pending bit, reads as 1 if exception is pending(2)
Bit 14 BUSFAULTPENDED: Bus fault exception pending bit, reads as 1 if exception is pending(2)
Bit 13 MEMFAULTPENDED: Memory management fault exception pending bit, reads as 1 if
exception is pending(2)
Bit 12 USGFAULTPENDED: Usage fault exception pending bit, reads as 1 if exception is pending(2)
Bit 11 SYSTICKACT: SysTick exception active bit, reads as 1 if exception is active(3)
Bit 10 PENDSVACT: PendSV exception active bit, reads as 1 if exception is active
Bit 9 Reserved, must be kept cleared
Bit 8 MONITORACT: Debug monitor active bit, reads as 1 if Debug monitor is active
Bit 7 SVCALLACT: SVC call active bit, reads as 1 if SVC call is active
Bits 6:4 Reserved, must be kept cleared
Bit 3 USGFAULTACT: Usage fault exception active bit, reads as 1 if exception is active
Bit 2 Reserved, must be kept cleared
Bit 1 BUSFAULTACT: Bus fault exception active bit, reads as 1 if exception is active
Bit 0 MEMFAULTACT: Memory management fault exception active bit, reads as 1 if exception is
active
1. Enable bits, set to 1 to enable the exception, or set to 0 to disable the exception.
2. Pending bits, read as 1 if the exception is pending, or as 0 if it is not pending. You can write to these bits to change the
pending status of the exceptions.
3. Active bits, read as 1 if the exception is active, or as 0 if it is not active. You can write to these bits to change the active
status of the exceptions, but see the Caution in this section.
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4.3.9 Configurable fault status register (SCB_CFSR)
Address offset: 0x28
Reset value: 0x0000 0000
Required privilege: Privileged
The CFSR is byte accessible. You can access the CFSR or its subregisters as follows:
●Access the complete CFSR with a word access to 0xE000ED28
●Access the MMFSR with a byte access to 0xE000ED28
●Access the MMFSR and BFSR with a halfword access to 0xE000ED28
●Access the BFSR with a byte access to 0xE000ED29
●Access the UFSR with a halfword access to 0xE000ED2A.
The CFSR indicates the cause of a memory management fault, bus fault, or usage fault.
Figure 19. CFSR subregisters
Memory Management
Fault Status Register
31 16 15 8 7 0
Usage Fault Status Register Bus Fault Status
Register
UFSR BFSR MMFSR
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved
DIVBY
ZERO
UNALI
GNED Reserved NOCP INVPC INV
STATE
UNDEF
INSTR
rc_w1 rc_w1 rc_w1 rc_w1 rc_w1 rc_w1
1514131211109876543210
BFARV
ALID Reserved
STK
ERR
UNSTK
ERR
IMPRE
CIS
ERR
PRECI
S ERR
IBUS
ERR
MMAR
VALID Reserved
MSTK
ERR
M
UNSTK
ERR Res.
DACC
VIOL
IACC
VIOL
rw rw rw rw rw rw rw rw rw rw rw
Bits 31:26 Reserved, must be kept cleared
Bit 25 DIVBYZERO: Divide by zero usage fault
When the processor sets this bit to 1, the PC value stacked for the exception return points to
the instruction that performed the divide by zero.
Enable trapping of divide by zero by setting the DIV_0_TRP bit in the CCR to 1, see
Configuration and control register (SCB_CCR) on page 121.
0: No divide by zero fault, or divide by zero trapping not enabled
1: The processor has executed an SDIV or UDIV instruction with a divisor of 0.
Bit 24 UNALIGNED: Unaligned access usage fault:
Enable trapping of unaligned accesses by setting the UNALIGN_TRP bit in the CCR to 1, see
Configuration and control register (SCB_CCR) on page 121.
Unaligned LDM, STM, LDRD, and STRD instructions always fault irrespective of the setting of
UNALIGN_TRP.
0: No unaligned access fault, or unaligned access trapping not enabled
1: the processor has made an unaligned memory access.
Bits 23:20 Reserved, must be kept cleared
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Bit 19 NOCP: No coprocessor usage fault. The processor does not support coprocessor instructions:
0: No usage fault caused by attempting to access a coprocessor
1: the processor has attempted to access a coprocessor.
Bit 18 INVPC: Invalid PC load usage fault, caused by an invalid PC load by EXC_RETURN:
When this bit is set to 1, the PC value stacked for the exception return points to the instruction
that tried to perform the illegal load of the PC.
0: No invalid PC load usage fault
1: The processor has attempted an illegal load of EXC_RETURN to the PC, as a result of an
invalid context, or an invalid EXC_RETURN value.
Bit 17 INVSTATE: Invalid state usage fault:
When this bit is set to 1, the PC value stacked for the exception return points to the instruction
that attempted the illegal use of the EPSR.
This bit is not set to 1 if an undefined instruction uses the EPSR.
0: No invalid state usage fault
1: The processor has attempted to execute an instruction that makes illegal use of the
EPSR.
Bit 16 UNDEFINSTR: Undefined instruction usage fault:
When this bit is set to 1, the PC value stacked for the exception return points to the undefined
instruction.
An undefined instruction is an instruction that the processor cannot decode.
0: No undefined instruction usage fault
1: The processor has attempted to execute an undefined instruction.
Bit 15 BFARVALID: Bus Fault Address Register (BFAR) valid flag:
The processor sets this bit to 1 after a bus fault where the address is known. Other faults can
set this bit to 0, such as a memory management fault occurring later.
If a bus fault occurs and is escalated to a hard fault because of priority, the hard fault handler
must set this bit to 0. This prevents problems if returning to a stacked active bus fault handler
whose BFAR value has been overwritten.
0: Value in BFAR is not a valid fault address
1: BFAR holds a valid fault address.
Bits 14:13 Reserved, must be kept cleared
Bit 12 STKERR: Bus fault on stacking for exception entry
When the processor sets this bit to 1, the SP is still adjusted but the values in the context area
on the stack might be incorrect. The processor does not write a fault address to the BFAR.
0: No stacking fault
1: Stacking for an exception entry has caused one or more bus faults.
Bit 11 UNSTKERR: Bus fault on unstacking for a return from exception
This fault is chained to the handler. This means that when the processor sets this bit to 1, the
original return stack is still present. The processor does not adjust the SP from the failing
return, does not performed a new save, and does not write a fault address to the BFAR.
0: No unstacking fault
1: Unstack for an exception return has caused one or more bus faults.
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Bit 10 IMPRECISERR: Imprecise data bus error
When the processor sets this bit to 1, it does not write a fault address to the BFAR.
This is an asynchronous fault. Therefore, if it is detected when the priority of the current
process is higher than the bus fault priority, the bus fault becomes pending and becomes
active only when the processor returns from all higher priority processes. If a precise fault
occurs before the processor enters the handler for the imprecise bus fault, the handler detects
both IMPRECISERR set to 1 and one of the precise fault status bits set to 1.
0: No imprecise data bus error
1: A data bus error has occurred, but the return address in the stack frame is not related to
the instruction that caused the error.
Bit 9 PRECISERR: Precise data bus error
When the processor sets this bit is 1, it writes the faulting address to the BFAR.
0: No precise data bus error
1: A data bus error has occurred, and the PC value stacked for the exception return points to
the instruction that caused the fault.
Bit 8 IBUSERR: Instruction bus error
The processor detects the instruction bus error on prefetching an instruction, but it sets the
IBUSERR flag to 1 only if it attempts to issue the faulting instruction.
When the processor sets this bit is 1, it does not write a fault address to the BFAR.
0: No instruction bus error
1: Instruction bus error.
Bit 7 MMARVALID: Memory Management Fault Address Register (MMAR) valid flag
If a memory management fault occurs and is escalated to a hard fault because of priority, the
hard fault handler must set this bit to 0. This prevents problems on return to a stacked active
memory management fault handler whose MMAR value has been overwritten.
0: Value in MMAR is not a valid fault address
1: MMAR holds a valid fault address.
Bits 6:5 Reserved, must be kept cleared
Bit 4 MSTKERR: Memory manager fault on stacking for exception entry
When this bit is 1, the SP is still adjusted but the values in the context area on the stack might
be incorrect. The processor has not written a fault address to the MMAR.
0: No stacking fault
1: Stacking for an exception entry has caused one or more access violations.
Bit 3 MUNSTKERR: Memory manager fault on unstacking for a return from exception
This fault is chained to the handler. This means that when this bit is 1, the original return stack
is still present. The processor has not adjusted the SP from the failing return, and has not
performed a new save. The processor has not written a fault address to the MMAR.
0: No unstacking fault
1: Unstack for an exception return has caused one or more access violations.
Bit 2 Reserved, must be kept cleared
Bit 1 DACCVIOL: Data access violation flag
When this bit is 1, the PC value stacked for the exception return points to the faulting
instruction. The processor has loaded the MMAR with the address of the attempted access.
0: No data access violation fault
1: The processor attempted a load or store at a location that does not permit the operation.
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4.3.10 Hard fault status register (SCB_HFSR)
Address offset: 0x2C
Reset value: 0x0000 0000
Required privilege: Privileged
The HFSR gives information about events that activate the hard fault handler.
This register is read, write to clear. This means that bits in the register read normally, but
writing 1 to any bit clears that bit to 0.
Bit 1 IACCVIOL: Instruction access violation flag
This fault occurs on any access to an XN region.
When this bit is 1, the PC value stacked for the exception return points to the faulting
instruction. The processor has not written a fault address to the MMAR.
0: No instruction access violation fault
1: The processor attempted an instruction fetch from a location that does not permit
execution.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
DEBU
G_VT
FORC
ED Reserved
rc_w1 rc_w1
1514131211109876543210
Reserved
VECT
TBL Res.
rc_w1
Bit 31 DEBUG_VT:
Reserved for Debug use. When writing to the register you must write 0 to this bit, otherwise
behavior is unpredictable.
Bit 30 FORCED: Forced hard fault
Indicates a forced hard fault, generated by escalation of a fault with configurable priority that
cannot be handles, either because of priority or because it is disabled:
When this bit is set to 1, the hard fault handler must read the other fault status registers to find
the cause of the fault.
0: No forced hard fault
1: Forced hard fault.
Bits 29:2 Reserved, must be kept cleared
Bit 1 VECTTBL: Vector table hard fault
Indicates a bus fault on a vector table read during exception processing:
This error is always handled by the hard fault handler.
When this bit is set to 1, the PC value stacked for the exception return points to the instruction
that was preempted by the exception.
0: No bus fault on vector table read
1: Bus fault on vector table read.
Bit 0 Reserved, must be kept cleared
Core peripherals PM0056
130/137 Doc ID 15491 Rev 2
4.3.11 Memory management fault address register (SCB_MMFAR)
Address offset: 0x34
Reset value: undefined
Required privilege: Privileged
4.3.12 Bus fault address register (SCB_BFAR)
Address offset: 0x38
Reset value: undefined
Required privilege: Privileged
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
MMFAR[31:16]
rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
1514131211109876543210
MMFAR[15:0]
rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
Bits 31:0 MMFAR[31:0]: Memory management fault address
When the MMARVALID bit of the MMFSR is set to 1, this field holds the address of the
location that generated the memory management fault.
When an unaligned access faults, the address is the actual address that faulted. Because a
single read or write instruction can be split into multiple aligned accesses, the fault address
can be any address in the range of the requested access size.
Flags in the MMFSR register indicate the cause of the fault, and whether the value in the
MMFAR is valid. See Configurable fault status register (SCB_CFSR) on page 126.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
BFAR[31:16]
rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
1514131211109876543210
BFAR[15:0]
rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
Bits 31:0 BFAR[31:0]: Bus fault address
When the BFARVALID bit of the BFSR is set to 1, this field holds the address of the location
that generated the bus fault.
When an unaligned access faults the address in the BFAR is the one requested by the
instruction, even if it is not the address of the fault.
Flags in the BFSR register indicate the cause of the fault, and whether the value in the BFAR
is valid. See Configurable fault status register (SCB_CFSR) on page 126.
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4.3.13 System control block design hints and tips
Ensure software uses aligned accesses of the correct size to access the system control
block registers:
●except for the CFSR and SHPR1-SHPR3, it must use aligned word accesses
●for the CFSR and SHPR1-SHPR3 it can use byte or aligned halfword or word
accesses.
The processor does not support unaligned accesses to system control block registers.
In a fault handler. to determine the true faulting address:
1. Read and save the MMFAR or BFAR value.
2. Read the MMARVALID bit in the MMFSR, or the BFARVALID bit in the BFSR. The
MMFAR or BFAR address is valid only if this bit is 1.
Software must follow this sequence because another higher priority exception might change
the MMFAR or BFAR value. For example, if a higher priority handler preempts the current
fault handler, the other fault might change the MMFAR or BFAR value.
4.3.14 SCB register map
The table provides shows the System control block register map and reset values. The base
address of the SCB register block is 0xE000 ED00.
Table 40. SCB register map and reset values
Offset Register
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0x00 SCB_CPUID Implementer Variant Constant PartNo Revision
Reset Value 01000001000111111100001000110001
0x04 SCB_ICSR
NMIPENDSET
Reser
ved
PENDSVSET
PENDSVCLR
PENDSTSET
PENDSTCLR
Reser
ved
ISRPENDING
VECTPENDING[9:0]
RETOBASE
Reser
ved
VECTACTIVE[8:0]
Reset Value 0 0000 000000000000 000000000
0x08 SCB_VTOR Reser
ved
TABLEOFF[29:9] Reserved
Reset Value 000000000000000000000
0x0C SCB_AIRCR VECTKEY[15:0]
ENDIANESS
Reserved
PRIGRO
UP[2:0] Reserved
SYSRESETREQ
VECTCLRACTIVE
VECTRESET
Reset Value 11111010000001010 000 000
0x10 SCB_SCR Reserved
SEVONPEND
Reserved
SLEEPDEEP
SLEEPONEXIT
Reserved
Reset Value 000
0x14 SCB_CCR Reserved
STKALIGN
BFHFNIGN
Reserved
DIV_0_TRP
UNALIGN_TRP
Reserved
USERSETMPEND
NONBASETHRDENA
Reset Value 10 00 00
0x18 SCB_SHPR1 Reserved PRI6 PRI5 PRI4
Reset Value 000000000000000000000000
Core peripherals PM0056
132/137 Doc ID 15491 Rev 2
4.4 SysTick timer (STK)
The processor has a 24-bit system timer, SysTick, that counts down from the reload value to
zero, reloads (wraps to) the value in the LOAD register on the next clock edge, then counts
down on subsequent clocks.
When the processor is halted for debugging the counter does not decrement.
4.4.1 SysTick control and status register (STK_CTRL)
Address offset: 0x00
Reset value: 0x0000 0004
Required privilege: Privileged
The SysTick CTRL register enables the SysTick features.
0x1C SCB_SHPR2 PRI11 Reserved
Reset Value 00000000
0x20 SCB_SHPR3 PRI15 PRI14 Reserved
Reset Value 0000000000000000
0x24 SCB_SHCRS Reserved
USG FAULT ENA
BUS FAULT ENA
MEM FAULT ENA
SV CALL PENDED
BUS FAULT PENDED
MEM FAULT PENDED
USG FAULT PENDED
SYS TICK ACT
PENDSV ACT
Reserved
MONITOR ACT
SV CALL ACT
Reserved
USG FAULT ACT
Reserved
BUS FAULT ACT
MEM FAULT ACT
Reset Value 000000000 00 0 00
0x28 SCB_CFSR UFSR BFSR MMFSR
Reset Value 00000000000000000000000000000000
0x2C SCB_HFSR
DEBUG_VT
FORCED
Reserved
VECTTBL
Reserved
Reset Value 0 0 0
0x34 SCB_MMAR MMAR[31:0]
Reset Value xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
0x38 SCB_BFAR BFAR[31:0]
Reset Value xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Table 40. SCB register map and reset values (continued)
Offset Register
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved
COUNT
FLAG
rw
1514131211109876543210
Reserved
CLKSO
URCE
TICK
INT
EN
ABLE
rw rw rw
Bits 31:17 Reserved, must be kept cleared.
Bit 16 COUNTFLAG:
Returns 1 if timer counted to 0 since last time this was read.
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4.4.2 SysTick reload value register (STK_LOAD)
Address offset: 0x04
Reset value: 0x0000 0000
Required privilege: Privileged
Bits 15:3 Reserved, must be kept cleared.
Bit 2 CLKSOURCE: Clock source selection
Selects the clock source.
0: AHB/8
1: Processor clock (AHB)
Bit 1 TICKINT: SysTick exception request enable
0: Counting down to zero does not assert the SysTick exception request
1: Counting down to zero to asserts the SysTick exception request.
Note: Software can use COUNTFLAG to determine if SysTick has ever counted to zero.
Bit 0 ENABLE: Counter enable
Enables the counter. When ENABLE is set to 1, the counter loads the RELOAD value from the
LOAD register and then counts down. On reaching 0, it sets the COUNTFLAG to 1 and
optionally asserts the SysTick depending on the value of TICKINT. It then loads the RELOAD
value again, and begins counting.
0: Counter disabled
1: Counter enabled
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved RELOAD[23:16]
rw rw rw rw rw rw rw rw
1514131211109876543210
RELOAD[15:0]
rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
Bits 31:24 Reserved, must be kept cleared.
Bits 23:0 RELOAD[23:0]: RELOAD value
The LOAD register specifies the start value to load into the VAL register when the counter is
enabled and when it reaches 0.
Calculating the RELOAD value
The RELOAD value can be any value in the range 0x00000001-0x00FFFFFF. A start value of
0 is possible, but has no effect because the SysTick exception request and COUNTFLAG are
activated when counting from 1 to 0.
The RELOAD value is calculated according to its use:
●To generate a multi-shot timer with a period of N processor clock cycles, use a RELOAD
value of N-1. For example, if the SysTick interrupt is required every 100 clock pulses, set
RELOAD to 99.
●To deliver a single SysTick interrupt after a delay of N processor clock cycles, use a
RELOAD of value N. For example, if a SysTick interrupt is required after 400 clock
pulses, set RELOAD to 400.
Core peripherals PM0056
134/137 Doc ID 15491 Rev 2
4.4.3 SysTick current value register (STK_VAL)
Address offset: 0x08
Reset value: 0x0000 0000
Required privilege: Privileged
4.4.4 SysTick calibration value register (STK_CALIB)
Address offset: 0x0C
Reset value: 0x0002328
Required privilege: Privileged
The CALIB register indicates the SysTick calibration properties.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
Reserved CURRENT[23:16]
rw rw rw rw rw rw rw rw
1514131211109876543210
CURRENT[15:0]
rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw rw
Bits 31:24 Reserved, must be kept cleared.
Bits 23:0 CURRENT[23:0]: Current counter value
The VAL register contains the current value of the SysTick counter.
Reads return the current value of the SysTick counter.
A write of any value clears the field to 0, and also clears the COUNTFLAG bit in the
STK_CTRL register to 0.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
NO
REF SKEW Reserved TENMS[23:16]
rr rrrrrrrr
1514131211109876543210
TENMS[15:0]
rrrrrr r r r r rrrrrr
Bit 31 NOREF: NOREF flag
Reads as zero. Indicates that a separate reference clock is provided. The frequency of this
clock is HCLK/8.
Bit 30 SKEW: SKEW flag
Reads as one. Calibration value for the 1 ms inexact timing is not known because TENMS is
not known. This can affect the suitability of SysTick as a software real time clock.
Bits 29:24 Reserved, must be kept cleared.
Bits 23:0 TENMS[23:0]: Calibration value
Reads as 9000. Indicates the calibration value when the SysTick counter runs on HCLK max/8
as external clock. For 72 MHz HCLK clock, the SysTick period is 1ms.
If calibration information is not known, calculate the calibration value required from the
frequency of the processor clock or external clock.
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Doc ID 15491 Rev 2 135/137
4.4.5 SysTick design hints and tips
The SysTick counter runs on the processor clock. If this clock signal is stopped for low
power mode, the SysTick counter stops.
Ensure software uses aligned word accesses to access the SysTick registers.
4.4.6 SysTick register map
The table provides shows the SysTick register map and reset values. The base address of
the SysTick register block is 0xE000 E010.
Table 41. SysTick register map and reset values
Offset Register
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0x00 STK_CTRL Reserved
COUNTFLAG
Reserved
CLKSOURCE
TICK INT
EN ABLE
Reset Value 0 1 0 0
0x04 STK_LOAD Reserved RELOAD[23:0]
Reset Value 000000000000000000000000
0x08 STK_VAL Reserved CURRENT[23:0]
Reset Value 000000000000000000000000
0x0C STK_CALIB Reserved TENMS[23:0]
Reset Value 000000000000000000000000
Revision history PM0056
136/137 Doc ID 15491 Rev 2
5 Revision history
Table 42. Document revision history
Date Revision Changes
03-Apr-2009 1 Initial release.
26-Oct-2009 2 Table 38: Priority grouping modified.
PM0056
Doc ID 15491 Rev 2 137/137
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