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:
[
:] [} @ 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 “” Inserts the string as data into the assembly (like DCB in
armasm).
.asciz “” Like .ascii, but follows the string with a zero byte.
.balign
{,
{,} }
Aligns the address to bytes. The assembler
aligns by adding bytes of value or a suitable default.
The alignment will not occur if more than fill
bytes are required (similar to ALIGN in armasm).
.byte {,} … Inserts a list of byte values as data into the assembly (like DCB
in armasm).
.code 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 , 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 This directive gives the symbol external linkage (similar to
EXPORT in armasm).
.hword {,}
…
Inserts a list of 16-bit values as data into the assembly (similar to
DCW in armasm).
GNU Assembler Directive Description
.if Makes a block of code conditional. End the block using .endif
(similar to IF in armasm). See also .else.
.ifdef Include a block of code if is defined. End the block
with .endif.
.ifndef Include a block of code if is not defined. End the block
with .endif.
.include “” Includes the indicated source file (similar to INCLUDE in
armasm or #include in C).
.irp }
{,} …
Repeats a block of code, once for each value in the value list.
Mark the end of the block using a .endr directive. In the
repeated code block, use \ {} … {,}
Defines an assembler macro called with N parameters.
The macro definition must end with .endm. To escape from the
macro at an earlier point, use .exitm. These directives are
similar to MACRO, MEND, and MEXIT in armasm. You must
precede the dummy macro parameters by \. For example:
.macro SHIFTLEFT a, b
.if \b < 0
MOV \a, \a, ASR #-\b
.exitm
.endif
MOV \a, \a, LSL #\b
.endm
.rept Repeats a block of code the given number of times. End with
.endr.
.req
This directive names a register. It is similar to the RN directive
in armasm except that you must supply a name rather than a
number on the right (e.g., acc .req r0).
.section
{,””}
Starts a new code or data section. Sections in GNU are called
.text, a code section, .data, an initialized data section, and
.bss, an uninitialized data section. These sections have default
flags, and the linker understands the default names (similar
directive to the armasm directive AREA). The following are
allowable .section flags for ELF format files:
Meaning
a allowable section
w writable section
x executable section
.set ,
This directive sets the value of a variable. It is similar to SETA
in armasm.
.space
{,}
Reserves the given number of bytes. The bytes are filled with
zero or if specified (similar to SPACE in armasm).
.word {,} … Inserts a list of 32-bit word values as data into the assembly
(similar to DCD in armasm).
Assembler Special Characters / Syntax
Inline comment char: ‘@’
Line comment char: ‘#’
Statement separator: ‘;’
Immediate operand prefix: ‘#’ or ‘$’
Register Names
General registers: %r0 - %r15 ($0 = const 0)
FP registers: %f0 - %f7
Non-saved (temp) regs: %r0 - %r3, %r12
Saved registers: %r4 - %r10
Stack ptr register: %sp
Frame ptr register: %fp
Link (retn) register: %lr
Program counter: %ip
Status register: $psw
Status register flags: xPSR
(x = C current) xPSR_all
(x = S saved ) xPSR_f
xPSR_x
xPSR_ctl
xPSR_fs
xPSR_fx
xPSR_fc
xPSR_cs
xPSR_cf
xPSR_cx
.. and so on
Arm Procedure Call Standard (APCS) Conventions
Argument registers: %a0 - %a4 (aliased to %r0 - %r4)
Returned value regs: %v1 - %v6 (aliased to %r4 - %r9)
Addressing Modes
‘rn’ in the following refers to any of the numbered registers, but not the control registers.
addr Absolute addressing mode
%rn Register direct
[%rn] Register indirect or indexed
[%rn,#n] Register based with offset
#imm Immediate data
Machine Dependent Directives
.arm Assemble using arm mode
.thumb Assemble using thumb mode
.code16 Assemble using thumb mode
.code32 Assemble using arm mode
.force_thumb Force thumb mode (even if not supported)
.thumb_func Mark entry point as thumb coded (force bx entry)
.ltorg Start a new literal pool
Opcodes
For detailed information on the machine instruction set, see this manual:
ARM Architecture Reference Manual, Addison-Wesley ISBN 0-201-73719-1
Here is a recommended book to get a lot of system developer information on the ARM architecture.
ARM System Developer’s Guide, Morgan Kaufmann Publishers ISBN 1-55860-874-5 (alk.paper), authors:
Andrew N. Sloss, Dominic Symes, Chris Wright, 2004
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