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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