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# gas
## Frequently used directives
- `.section` to define a section (elf files)
```x86asm
.section .text.foo, "ax", @progbits
; defines section named .text.foo with alloc+exec perms
.section .data.foo, "aw", @progbits
; defines section named .data.foo with alloc+write perms
.section .rodata.foo, "a", @progbits
; defines section named .rodata.foo with alloc perms
```
- `.byte`, `.2byte`, `.4byte`, `.8byte` to define a N byte value
```x86asm
.byte 0xaa
.2byte 0xaabb
.2byte 0xaa, 0xbb
.4byte 0xaabbccdd
.8byte 0xaabbccdd11223344
```
- `.ascii` to define an ascii string
```x86asm
.ascii "foo" ; allocates 3 bytes
```
- `.asciz` to define an ascii string with `'\0'` terminator
```x86asm
.asciz "foo" ; allocates 4 bytes (str + \0)
```
- `.macro` to define assembler macros. Arguments are accessed with the
`\arg` syntax.
```x86asm
.macro defstr name str
\name:
.ascii "\str"
\name\()_len:
.8byte . - \name
.endm
; use as
defstr foo, "foobar"
```
> Use `\()` to concatenate macro argument and literal.
- `.rept` to repeat a sequence of lines between `.rept` and `.endr`.
```x86asm
.rept 4
.4byte 123
.endr
```
- `.fill cnt, elem_size, val` write `cnt` times `val` with element size `elem_size`. For example one can use it to create a mbr boot record (magic number 0xaa55 at byte 511, 512).
```x86asm
.section .boot, "ax", @progbits
; some code ..
.4byte 0xff
.fill 510 - (. - .boot), 1, 0x00
.2byte 0xaa55
; as foo.s && objdump -j .boot -s
; Contents of section .boot:
; 0000 ff000000 00000000 00000000 00000000
; ..
; 01f0 00000000 00000000 00000000 000055aa
```
> Here `.` stands for the current location counter.
## References
- [GNU Assembler][gas_doc]
- [GNU Assembler Directives][gas_directives]
- [GNU Assembler `x86_64` dependent features][gas_x86_64]
[gas_doc]: https://sourceware.org/binutils/docs/as
[gas_directives]: https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
[gas_x86_64]: https://sourceware.org/binutils/docs/as/i386_002dDependent.html
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