LC 3 Instructions
User Manual: Pdf
Open the PDF directly: View PDF 
.
Page Count: 16
| Download | |
| Open PDF In Browser | View PDF |
LC-3 Instruction Summary ADD* 15 14 13 12 11 10 9 0001 DR 8 7 6 SR1 5 0 4 0 3 0 ADD* 0001 DR SR1 1 AND* 0101 DR SR1 0 AND* 0101 DR SR1 1 BR 0000 n z p JMP 1100 0 0 0 JSR 0100 1 JSRR 0100 0 LD* 0010 DR PCoffset9 LDI* 1010 DR PCoffset9 LDR* 0110 DR LEA* 1110 DR NOT* 1001 DR RET 1100 0 0 0 1 1 RTI 1000 0 0 0 0 0 ST 0011 SR PCoffset9 STI 1011 SR PCoffset9 STR 0111 SR TRAP 1111 0 2 1 0 SR2 imm5 0 0 SR2 imm5 PCoffset9 BaseR 0 0 0 0 0 0 0 0 0 0 PCoffset11 0 0 0 BaseR 0 0 BaseR offset6 PCoffset9 SR 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 BaseR 0 0 offset6 trapvect8 x20 = GetC x21 = Out x22 = PutS x23 = In x25 = Halt Note: * indicates instructions that modify the condition codes (CC). LC-3 Instructions Page 1 of 16 Instruction Assembler Format Example Operation Addition ADD ADD dr, sr1, sr2 ADD dr, sr1, imm5 ADD R2, R3, R4 ADD R2, R3, #7 R2 ← R3 + R4 R2 ← R3 + 7 Logical AND AND AND dr, sr1, sr2 AND dr, sr1, imm5 AND R2, R3, R4 AND R2, R3, #7 R2 ← R3 AND R4 R2 ← R3 AND 7 Conditional Branch BR label BRn label BRz label BRp label BRnz label BRnp label BRzp label BRnzp label Unconditional branch BRnz loop Branch if the CC is negative or zero. BRnzp loop Unconditional branch Jump JMP JMP baseR JMP foo Jump to foo. PC ← baseR Jump to Subroutine JSR / JSRR JSR PCoffset11 JSR Sort R7 ← PC+1 Jump to Sort JSRR baseR JSRR R2 R7 ← PC+1 Jump to address in R2 Load Direct LD LD dr, label LD R4, count R4 ← mem[count] Load Indirect LDI LDI dr, label LDI R4, pointer R4 ← mem[mem[pointer]] Load Base + Offset LDR LDR dr, baseR, offset6 LDR R4, R2, #10 R4 ← contents of mem[R2+#10] Load Effective Address LEA LEA dr, label LEA R4, foo R4 ← address of foo Complement NOT NOT dr, sr NOT R4, R2 R4 ← NOT(R2) Return from Subroutine RET RET RET PC ← R7 Return from Interrupt RTI RTI RTI NZP, PC ← top two values popped off stack Store Direct ST ST sr, label ST R4, count mem[count] ← R4 Store Indirect STI STI SR, label STI R4, pointer mem[mem[pointer]] ← R4 LC-3 Instructions Page 2 of 16 Store Base + Offset STR STR sr, baseR, offset6 STR R4, R2, #10 mem[R2+#10] ← R4 Operating System Call TRAP TRAP x20 GETC TRAP x21 OUT TRAP x22 PUTS TRAP x23 IN TRAP x25 HALT Get a character from keyboard. The character is not echoed onto the screen. Its ASCII code is copied into R0. The high eight bits of R0 are cleared. Write a character in R0[7:0] to the screen Write a string pointed to by R0 to the screen. Print a prompt on the screen and read a single character from the keyboard. The character is echoed onto the screen, and its ASCII code is copied into R0. The high eight bits of R0 are cleared. Halt execution General purpose registers: The LC-3 has eight 16-bit general purpose registers R0 to R7. Special memory locations: xF3FC CRT status register (CRTSR). The ready bit (bit 15) indicates if the video device is ready to receive another character to print on the screen. xF3FF CRT data register (CRTDR). A character written in the low byte of this register will be displayed on the screen. xF400 Keyboard status register (KBSR). The ready bit (bit 15) indicates if the keyboard has received a new character. xF401 Keyboard data register (KBDR). Bits [7:0] contain the last character typed on the keyboard. xF402 Machine control register (MCR). Bit [15] is the clock enable bit. When cleared, instruction processing stops. Notations: baseR dr imm5 mem[address] PCoffset9 offset6 sr CC – base register – destination register – a five-bit immediate value – denotes the contents of memory at the given address – a 9-bit immediate value used in an offset relative to the incremented PC – a 6-bit immediate value used in a Base+Offset instruction – source register – condition code register (N,Z,P) LC-3 Instructions Page 3 of 16 1. Operate instructions ADD 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0 0 1 DR SR1 0 0 0 SR2 0 0 0 1 DR SR1 1 imm5 if (bit[5] == 0) DR = SR1 + SR2 else DR = SR1 + sign-extend(imm5) set cc(DR) Example: ADD R2, R3, R4 ADD R2, R3, #7 ; R2 ← R3 + R4 ; R2 ← R3 + 7 AND 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 1 0 1 DR SR1 0 0 0 SR2 0 1 0 1 DR SR1 1 imm5 if (bit[5] == 0) DR = SR1 AND SR2 else DR = SR1 AND sign-extend(imm5) set cc(DR) NOT 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 0 0 1 DR SR 1 1 1 1 1 1 There is no OR instruction. However, using DeMorgan’s law A OR B is: A OR B = (A' AND B' )' LC-3 Instructions Page 4 of 16 2. Data movement instructions Load and Store Format 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 opcode DR or SR operand specifier The load and store instructions are for copying data between a register and a memory location. The load instruction copies data from a memory location to a register, whereas, the store instruction copies data from a register to a memory location. There are four different versions of the load and store instructions. They differ in how the address of the memory location to be accessed is calculated. This is referred to as the different addressing modes of the instruction. Addressing Modes Address modes specify how the memory address is calculated. LC-3 Instructions Page 5 of 16 PC-Relative Addressing Mode LD (0010) and ST (0011) specify the PC-relative addressing mode. It loads (LD) or stores (ST) the value that is found in the memory address that is formed by sign-extending bits [8:0] to 16 bits and adding this value to the incremented PC. The content in memory at this address is loaded into DR for the LD instruction. For the ST instruction, the content of the SR is stored into the memory at this computed address. memory address x4018 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0 1 0 1 0 1 1 1 0 1 0 1 1 1 1 LD DR or SR PCoffset9 The PC is x4018. The incremented PC, i.e. PC+1, is x4019. The 9-bit offset in the instruction x1AF is sign-extended to 16 bits giving FFAF. The incremented PC (x4019) is added to the sign-extended offset (xFFAF) giving x3FC8. For the LD instruction, the value in memory location x3FC8 is loaded into register R5. For the ST instruction, the value in R5 is stored into memory location x3FC8. PC PC+1 Memory Address Memory Content 0100 0000 0001 1000 0010 101 110101111 0100 0000 0001 1001 sign extend to 16 bits 1111 1111 1010 1111 + 0011 1111 1100 1000 Memory Address Memory Content 0011 1111 1100 1000 1010 1010 1010 1010 LD R5 1010 1010 1010 1010 ST Example LD R5, offset ST R5, offset LC-3 Instructions ; R5 ← mem[PC+1+SEXT(offset)] ; mem[PC+1+SEXT(offset)] ← R5 Page 6 of 16 Base+Offset Addressing Mode LDR (0110) and STR (0111) specify the base+offset addressing mode. The address of the operand is obtained by adding the sign-extended 6-bit offset to the content of the specified base register. The result is the effective address of the memory location to be accessed. memory address x4018 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 1 1 0 1 0 1 1 1 1 0 1 1 1 0 1 LDR DR or SR BaseR Offset6 Memory Address Memory Content 0100 0000 0001 1000 0110 101 111 011101 R7 0010 0011 0100 0101 LDR 0010 0011 0110 0010 1111 0000 1111 0000 0010 0011 0100 0101 + 0000 0000 0001 1101 0010 0011 0110 0010 R5 1111 0000 1111 0000 STR Example LDR R5, R7, offset STR R5, R7, offset LC-3 Instructions ;R5 ← mem[R7 + SEXT(offset)] ;mem[R7 + SEXT(offset)] ← R5 Page 7 of 16 Indirect Addressing Mode LDI (1010) and ST I(1011) specify the indirect addressing mode. An address is first formed like the LD and ST instructions. However, the contents from this memory location form the address of the operand to be loaded or stored. memory address x4018 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 0 1 0 1 0 1 1 1 0 1 0 1 1 1 1 LDI DR or SR PCoffset9 PC PC+1 Memory Address Memory Content 0100 0000 0001 1000 1010 101 110101111 0100 0000 0001 1001 sign extend to 16 bits 1111 1111 1010 1111 + 0011 1111 1100 1000 Memory Address Memory Content 0011 1111 1100 1000 0011 1101 1001 1001 Memory Address Memory Content 0011 1101 1001 1001 1010 1010 1010 1010 LDI R5 1010 1010 1010 1010 STI Example LDI R5, offset STI R5, offset LC-3 Instructions ; R5 ← mem[mem[PC+1+SEXT(offset)]] ; mem[mem[PC+1+SEXT(offset)]] ← R5 Page 8 of 16 Immediate Addressing Mode The LEA (1110) instruction loads the immediate value formed by adding the incremented PC to the signextended 9-bit offset. memory address x4018 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 1 0 1 0 1 1 1 0 1 0 1 1 1 1 LEA DR PCoffset9 PC PC+1 Memory Address Memory Content 0100 0000 0001 1000 0010 101 110101111 0100 0000 0001 1001 sign extend to 16 bits 1111 1111 1010 1111 + 0011 1111 1100 1000 R5 0011 1111 1100 1000 Example LEA R5, offset LC-3 Instructions ; R5 ← PC+1+SEXT(offset) Page 9 of 16 3. Control Instructions Branch The conditional branch BR (0000) instruction format is memory address x4C18 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 1 BR N Z P PCoffset9 When a condition bit [11:9] (N, Z, P) is set, that corresponding condition code is checked. If that corresponding condition is set (i.e. true), then the PC is loaded with the value formed by adding the incremented PC to the sign-extended 9-bit offset [8:0]. All instructions that write values into registers set the three condition code registers (i.e., the single-bit registers N, Z, P) depending on whether the value written is negative, zero, or positive. These instructions are ADD, AND, NOT, LD, LDI, LDR, and LEA. BR BRn BRz BRp Label Label Label Label BRnz Label BRnp Label BRzp Label BRnzp Label (unconditional jump) ; loop 10 times 3000 AND ADD R0,R0,#0 R0,R0,#10 ; beginning of loop NOP ADD R0,R0,#-1 ; decrement by 1 BRp #-3 ; loop back if positive LC-3 Instructions Page 10 of 16 JMP The unconditional jump JMP (1100) instruction format is memory address x4018 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 0 0 0 0 0 1 1 1 0 0 0 0 0 0 JMP BaseR Unconditionally jumps to the location specified by the contents of the base register. Loads the PC with the value in the BaseR. LC-3 Instructions Page 11 of 16 JSR (Jump Subroutine) The Jump Subroutine instruction is used to implement function calls. The JSR (0100) instruction format is memory address x4018 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 1 0 0 1 0 0 1 1 1 0 1 1 1 0 1 JSR PCoffset11 Bit 11 for the JSR instruction is a 1. Save the incremented PC in R7; This is used to return from subroutine Loads the PC with the value formed by adding the incremented PC to the sign-extended 11-bit offset [10:0]. JSRR (Jump Subroutine Register) The JSRR (0100) instruction format is memory address x4018 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 1 0 0 0 0 0 1 1 1 0 0 0 0 0 0 JSRR BaseR Bit 11 for the JSRR instruction is a 0. Save the incremented PC in R7; This is used to return from subroutine Loads the PC with the contents of the base register. RET (Return) The Return instruction is used to return from a function to the caller. It simply copies R7 to the PC. It is the same as JMP R7 memory address x4018 LC-3 Instructions 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 0 0 0 0 0 1 1 1 0 0 0 0 0 0 JMP BaseR Page 12 of 16 Trap The Trap (1111) instruction invokes a system routine. When the OS is finished performing the service call, the program counter is set to the address of the instruction following the TRAP instruction and the program continues. 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 1 1 0 0 0 0 0 0 1 0 0 0 1 1 TRAP trap vector R7 ← PC; PC ← mem[ZEXT(trapvector8) Trap Number x20 Assembler Name GETC x21 x22 x23 OUT PUTS IN x25 HALT LC-3 Instructions Description Read a single character from the keyboard. The character is not echoed onto the console. Its ASCII code is copied into R0. The high eight bits of R0 are cleared. Write a character in R0[7:0] to the console. Write a string pointed to by R0 to the console. Print a prompt on the screen and read a single character from the keyboard. The character is echoed onto the console, and its ASCII code is copied into R0. The high eight bits of R0 are cleared. Halt execution and print a message on the console. Page 13 of 16 Compiler Directives • • Commands to tell the assembler what to do. These are not LC3 commands. All directives start with a period (.). Directive .ORIG .FILL Description Where to start placing code in memory Allocate one memory location and initialize it with a value. .BLKW Allocate a block of memory (array). .STRINGZ Allocate and initialize memory with a null terminated string. .END Tells assembler the end of your source listing Example .ORIG .FILL .BLKW .STRINGZ $3000 x30 #5 “Hello World” .END Examples .ORIG $3000 Thirty .FILL x30 Array .BLKW 20 Hi .STRINGZ “Hello World” ; allocate and initialize memory with the string “Hello World” ; allocate a memory location, initialize it to x30, and label it #0 ; allocate 20 locations and initialize them all to zero. ; the starting location is labeled “Array” .END LC-3 Instructions Page 14 of 16 Examples ; Print Hello World on the console .ORIG x3000 LEA R0, Hi PUTS HALT Hi .STRINGZ "Hello World" .END ; Output the numbers from 0 to 9 to the console .ORIG x3000 LD R3,Thirty ; R3 = x30 AND R1,R1,#0 ; R1 = 0 Loop ADD R0,R1,#-10 ; subtract 10 to test for the ending condition BRz Stop ADD R0,R1,R3 ; convert R1 to ASCII OUT ADD R1,R1,#1 BR Loop Stop HALT Thirty .FILL x30 .END Exercises Write LC-3 assembly programs for the following: 1) Output the numbers from 0 to 9 with one number per line. Hint: Use carriage return (CR). 2) Use .FILL to put two numbers in the range 0 to 4 in memory. Write a program to calculate and output the sum of these two numbers on the console. 3) Use .FILL to put a two-digit decimal number in memory. Print out this number on the console. 4) Output the numbers from 0 to 19. 5) Same as 2) but the two numbers are in the range 0 to 9. 6) Use .FILL to put ten numbers in memory. Write a program to print out the largest of these ten numbers. LC-3 Instructions Page 15 of 16 ; input two one-digit numbers and print out the sum. ; Correct only if the sum is less than 10 .ORIG x3000 LD R3,nThirty ; load constant x-30 LEA R0,Prompt ; print prompt to enter number PUTS GETC ; get first number OUT ; echo it ADD R0,R0,R3 ; convert ASCII to value ADD R1,R0,#0 ; save first number in R1 LD OUT R0,CR ; print Return LEA PUTS GETC OUT ADD ADD ADD R0,Prompt ; print prompt to enter number R0,R0,R3 R2,R0,#0 R2,R1,R2 ; get second number ; echo it ; convert ASCII to value ; save second number in R2 ; add R2 <- R1+R2 LD OUT R0,CR ; print Return LEA PUTS JSR OUT HALT R0,Sum Convert ; call function to convert number to ASCII ; end of main program ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; subroutine to convert number in R2 to ASCII ; need to add x30 Convert LD R0,Thirty ADD R0,R2,R0 ; RET ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; start of constants nThirty.FILL Thirty .FILL CR .FILL Prompt .STRINGZ Sum .STRINGZ x-30 x30 x0D "Enter a number? " "The sum is " .END LC-3 Instructions Page 16 of 16
Source Exif Data:
File Type : PDF File Type Extension : pdf MIME Type : application/pdf PDF Version : 1.3 Linearized : No XMP Toolkit : XMP Core 5.4.0 Modify Date : 2014:11:19 14:38:52-08:00 Creator Tool : PScript5.dll Version 5.2.2 Create Date : 2014:11:19 14:38:52-08:00 Producer : Acrobat Distiller 7.0.5 (Windows) Creator : admin Format : application/pdf Title : Microsoft Word - LC-3 Instructions Document ID : uuid:d328a387-ad2e-49cb-b5e4-959b92cb9f56 Instance ID : uuid:a29e473d-e41e-437b-8d5f-6ad4d92050ba Page Count : 16 Author : adminEXIF Metadata provided by EXIF.tools