GNU AS ARM Reference Gas ARMRef

User Manual: GasARMRef

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Macro Directives

Directive Description Syntax Example

.macro Define a macro. .macro name {args, …}

A macro can be defined without arguments, .macro NoArgsMacro

and can be called simply by specifying its name. name {args, …} NoArgsMacro

A macro can also be defined with arguments, .macro ArgMacro arg, arg2

and can be called the same way with commas seperating its

arguments. ArgMacro 10, 11

Arguments can be accessed by their name prefixed with a \. \arg mov r0, \arg

You can define default macro arguments. .macro ArgMacro arg=1, arg2

Arguments are omited by simply placing a comma and no

value, or ignoring them all together (trailing only). ArgMacro , 11

Arguments can be set in a modified order by referencing

them by name. ArgMacro arg2=11, arg=10

Macros can be recursive.

.endm Mark the end of a macro. .endm .endm

.exitm Exit a macro early. .exitm .exitm

\@ Psudo variable that contains the macro number executed.

Can be used for a unique number on every macro definition.

\@ MyLabel\@:

.purgem Undefine a macro, so that further uses do not evaluate. .purgem name .purgem NoArgsMacro

Expression Operators

Precedence Symbol Name Operation

prefix -Negate Negate argument.

prefix ~Compliment Compliment argument.

Highest *Multiplication Multiply arg1 by arg2.

Highest /Division Divide arg1 by arg2.

Highest %Remainder Divide arg1 by arg2, and return the remainder.

Highest << or < Left Shift Shift arg1 left by arg2.

Highest >> or > Right Shift Shift arg1 right by arg2.

Intermediate |Bitwise OR OR arg1 with arg2.

Intermediate &Bitwise AND AND arg1 with arg2.

Intermediate ^Bitwise XOR XOR arg1 with arg2.

Intermediate !Bitwise OR NOT OR arg1 with arg2, and NOT result.

Lowest +Addition Add arg1 to arg2.

Lowest -Subtraction Subtract arg2 from arg1.

Precedence Symbol Name Operation

Digit Encoding Formats

Number Type Base Prefix Digits Example

Decimal Intege r 10 0 - 925

Hexadecimal Integer 16 0x or 0X 0 - 9, A - F (10 - 15) 0xD7

Octa l Integer 800 - 7027

Binary Integer 10b or 0B 0 - 10b11010

Floating Point Number 10 0f or 0F 0 - 90f+24.112E-25

Cha ra cter n/a 'Ascii Symbol 'c

String n/a " and "Ascii Symbol(s) "MyString\n"

Number Type Base Prefix Digits Example

Escape Codes

\ Description Ascii \ Description

\b Backspace 8 \### Octal Character Code

\f Form Feed 12 \x## Hex Character Code

\n New Line 10 \\ \ character

\r Carrige Return 13 \" " character

\t Horizontol Tab 9

Constant Definition Directives

Directive Description Syntax Example

.byte Define byte expr (8bit numbers) .byte expr {, …} .byte 25, 0x11, 031, 'A

.hword Define half-word expr (16bit numbers) .hword expr {, …} .hword 2, 0xFFE0

.short Same as .hword .short expr {, …} .short 257

.word Define word expr (32bit numbers) .word expr {, …} .word 144511, 0x 11223

.int Same as .word .int expr {, …} .int 21

.long Same as .word .long expr {, …} .long 1923, 0b10010101

.ascii Define string expr (non zero terminated array of bytes) .asc ii ex pr {, …} .ascii "Ascii text is here"

.asciz Define string expr (zero terminated array of bytes) .asciz expr {, …} .asciz "Zero Terminated Text"

.string Same as .asciz .string expr {, …} .string "My Cool String\n"

.quad Define bignum expr (break at 8bit increments) .quad expr {, …} .qua d 0xDAFADAFA911

.octa Define bignum expr (break at 16bit increments) .octa expr {, …} .octa 0xFEDCBA987654321

.float Define 32bit IEEE flonum expr (floating point numbers) .float expr {, …} .float 0f3.14, 0F359.2e11

.single Same as .float .single expr {, …} .single 0f12341243.14E2

.double Define 64bit IEEE flonum expr (floating point numbers) .double ex pr {, …} .double 0f2E1

.fill Generate repeat copies of value that is of size size . size

defaults to 1, and value defaults to 0. .fill repeat {, size} {, value} .fill 32, 4, 0xFFFFFFFF

.zero Fills in size bytes with 0. .zero size .zero 400

.space Fills in size bytes with value . value defaults to 0. .space size {, value} .space 25, 0b11001100

.skip Same as .space .skip size {, value} .skip 22

Assembly Listing Directives

Directive Description Syntax Example

.eject Force a page break when generating assembly listings. .eject .eject

.psize

Set the number of lines to generate for each page of the

assembly listing and the number of columns . Lines

defaults to 60, Columns defaults to 200. A page break is

generated when the number of lines hits lines . If lines is

0, then no page breaks are generated (excluding ones by

.eject).

.psize lines {, columns} .psize 40, 80

.list Start generation of an assembly listings from .list to

.nolist..list .list

.nolist End generation of an assembly listing. Listings can be re-

started with .list again. .nolist .nolist

.title Uses heading as the title (2nd line, under filename and

page number) .title "heading" .title "My Asm Output"

.sbttl Uses heading as the title (3rd line, under .title) .sbttl "heading .sbttl "Part 1: Cool stuff"

Conditional Directives

Directive Description Syntax Example

.if Assembles if absolute_expression does not equal zero.

For all If's, if absolute_expres s ion is omited, it equals 0. .if {absolute_expression} .if (2+2)

.elseif

Assembles if abs olute_expression does not equal zero.

Used in .if blocks to provide alternates when previous .if's

or .elseif's fail.

.elseif {absolute_expression} .elseif (2+3) - 5

.else Assembles if all previous .if and .elseif blocks failed. .else .else

.endif Ends an .if block. .endif .endif

.ifdef Assembles if symb ol exists. .ifdef symbol .ifdef _test_i_

.ifndef Assembles if symb ol does not exist. .ifndef symbol .ifndef _test_i_

.ifnotdef Same as .ifndef .ifnotdef s y mbol .ifnotdef _test_i_

.ifc Assembles if the strings are the same. .ifc string1, string2 .ifc "this", "that"

.ifnc Assembles if the strings are not the same. .ifnc string1, string2 .ifnc "this", "that"

.ifeqs Same as .ifc .ifeqs string1, string2 .ifeqs "those", "this"

.ifnes Same as .ifeqs .ifnes string1, string2 .ifnes "those", "this"

.ifeq Assembles if absolute_expression equals zero. .ifeq {absolute_expression} .ifeq (2+2) - 4

.ifne Assembles if absolute_expression does not equal zero. .ifne {absolute_expression} .ifne (2+2) - 5

.ifge Assembles if absolute_expression is greater than or equal

to zero. .ifge {absolute_expression} .ifge 10

.ifgt Assembles if absolute_expression is greater than zero. .ifgt {absolute_expression} .ifgt

.ifle Assembles if absolute_expression is less than or equal to

zero. .ifle {abs olute_express ion} .ifle

.iflt Assembles if absolute_expression is less than z ero. .iflt {absolute_express ion} .iflt -10

Debug Directives

Directive Description Syntax Example

.func Generate debug information for code as a function. If label

is omited, label is assumed to be name ..func name {, label} .func CoolFunc

.endfunc Mark the end of a function. .endfunc .endfunc

.stabs See GAS documentation for info. Not very useful unless

your generate assembly listings from another source. .stabs string, type, other, desc, value

Looping Directives

Directive Description Syntax Example

.rept Repeat the sequence of lines between .rept and .endr

count number of times . .rept count .rept 10

.irp Evaluate a comma delimited sequence of statements to

assign to the value of symbol..irp symbol, values… .irp newval, 1, 2, 3

.irpc For each character in values , assign its value. .irp symbol, value .irp newval, 123

Symbol can be referenced with \symbol. \symbol .byte 0xC\newval

.endr End .re pt, .irp, and .irpc sequences. .endr .endr

Assembler Control Directives

Directive Description Syntax Example

.arm Following opcodes use the ARM instruction set .arm .arm

.thumb Following opc odes us e t he THUMB instruc tion subs et .thumb .thumb

.code 16 Same as .thumb .code 16 .code 16

.code 32 Same as .arm .code 32 .code 32

.include Include a file. .include "file" .include "hardware.i"

.align

Byte align the following code to alignment byte boundary

(default=4). Fill skipped bytes with fill (de fa u l t=0 or

NOP). If the number of bytes skipped is greater than max,

then don't align (default=ali gnment ).

.align {alignment} {, fill} {, max} .align

.balign Same as .align .balign {alignment} {, fill} {, max} .balign 8, 0

.balignw

Half-word align the following code to alignment byte

boundary (default=4). Fill skipped half-words with fill

(default=0 or NOP). If the number of bytes skipped is

greater than max, then don't align (default=alignm ent ).

.balignw {alignment} {, fill} {, max} .balignw 2

.balignl

Word align the following code to alignment byte boundary

(default=4). Fill skipped words with fill (default=0 or

NOP). If the number of bytes skipped is greater than max,

then don't align (default=ali gnment ).

.balignl {alignment} {, fill} {, max} .balignl

.end Marks the end of the assembly file. Data following this

directive is not processed. .end .end

.fail

Generates errors or warnings durring assembly . If expr is

greater than or equal to 500, it prints a warning message.

If less than it prints an error message.

.fail expr .fail 1

.err Generate an error durring assembly. .err .err

.print Print a string to standard output durring assembly. .print string .print "Som et hing is broken"

.section Tell the ass embler to assemble the following in section

expr . expr can be either .text, .data, or .bss

..s ec tion ex pr .section .bss

.text

Tell assembler to assemble the following in the "text"

(code) section. You can also specify a subsection of "text"

with sub section .

.text {s ubsection} .text

.data

Tell assembler to ass emble the following in the "data"

section. You can also specify a subsection of "data" with

subs ection .

.data {subsection} .data 0

.bss

Tell assembler to ass emble the following in the "bss"

(variables) section. You can also specify a subsection of

"bss" with subsection .

.bss {subsection} .bss

.struct

Tell assembler to assemble the following in an absolute

section. Be sure to switch sections before you get back to

code or data.

.struct expr

.struct 0

feild1 is 0. field1:

.struct field1 + 4

field2 is 4. field2:

.org Following code is inserted at the start of the section plus

new-lc ..org new-lc {, fill} .org 0x 20000

.pool

Tell the assembler where it can safely place data for

immediate 32bit loads (ideally after your return). Use the =

prefix operator to pool the value. ldr r0, =0x4000002

.pool .pool

Symbol Directives

Directive Description Syntax Example

.equ Set the value of symbol equal to expr . .equ symbol, expr .equ Version, "0.1"

.set Same as .equ .s et sy m bol, ex pr .set Flavor, "CHERRY"

.equiv Set the value of symbol equal to expr . Generates an error

if the symbol has been previously defined. .equiv symbol, ex pr .equiv Version, "0.2"

.global Makes symbol visible to the linker. .global symbol .globa l MyAsmFunc

.globl Same as .global .globl symbol .globl MyOtherAsmFunc