gas

Frequently used directives

• .section to define a section (elf files)

  .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

  .byte 0xaa
  .2byte 0xaabb                 # .word
  .2byte 0xaa, 0xbb
  .4byte 0xaabbccdd             # .long
  .8byte 0xaabbccdd11223344     # .quad
  

• .ascii to define an ascii string

  .ascii "foo"   ; allocates 3 bytes
  

• .asciz to define an ascii string with '\0' terminator

  .asciz "foo"   ; allocates 4 bytes (str + \0)
  

• .macro to define assembler macros. Arguments are accessed with the \arg syntax.

  .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.

  .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).

  .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
• GNU Assembler Directives
• GNU Assembler x86_64 dependent features