GNU ARM Assembler Quick Reference
A summary of useful commands and expressions for the ARM architecture using the GNU assembler is
presented briefly in the concluding portion of this Appendix. Each assembly line has the following format:
[<label>:] [<instruction or directive>} @ comment
Unlike the ARM assembler, using the GNU assembler does not require you to indent instructions and
directives. Labels are recognized by the following colon instead of their position at the start of a line. An
example follows showing a simple assembly program defining a function ‘add’ that returns the sum of
two input arguments:
.section .text, “x”
.global add @ give the symbol add external linkage
add:
ADD r0, r0, r1 @ add input arguments
MOV pc, lr @ return from subroutine
@ end of program
GNU Assembler Directives for ARM
The follow is an alphabetical listing of the more command GNU assembler directives.
GNU Assembler Directive Description
.ascii “<string>”
Inserts the string as data into the assembly (like DCB in
armasm).
.asciz “<string>”
Like .ascii, but follows the string with a zero byte.
.balign <power_of_2>
{,<fill_value>
{,<max_padding>} }
Aligns the address to <power_of_2> bytes. The assembler
aligns by adding bytes of value <fill_value> or a suitable default.
The alignment will not occur if more than <max_padding> fill
bytes are required (similar to ALIGN in armasm).
.byte <byte1> {,<byte2>} …
Inserts a list of byte values as data into the assembly (like DCB
in armasm).
.code <number_of_bits>
Sets the instruction width in bits. Use 16 for Thumb and 32 for
ARM assembly (similar to CODE16 and CODE32 in armasm).
.else
Use with .if and .endif (similar to ELSE in armasm).
.end
Marks the end of the assembly file (usually omitted).
.endif
Ends a conditional compilation code block – see .if, .ifdef, .ifndef
(similar to ENDIF in armasm).
.endm
Ends a macro definition – see .macro (similar to MEND in
armasm).
.endr
Ends a repeat loop – see .rept and .irp (similar to WEND in
armasm).
.equ <symbol name>, <value>
This directive sets the value of a symbol (similar to EQU in
armasm)
.err
Causes assembly to halt with an error.
.exitm
Exit a macro partway through – see .macro (similar to MEXIT in
armasm)
.global <symbol>
This directive gives the symbol external linkage (similar to
EXPORT in armasm).
.hword <short1> {,<short2>}
…
Inserts a list of 16-bit values as data into the assembly (similar to
DCW in armasm).
