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| author | Johannes Stoelp <johannes.stoelp@gmail.com> | 2025-04-10 00:27:54 +0200 |
|---|---|---|
| committer | Johannes Stoelp <johannes.stoelp@gmail.com> | 2025-04-10 00:27:54 +0200 |
| commit | 773d9b46ee3b1b88a94e69f42ea42654c63c48ec (patch) | |
| tree | b46817bdade3fbf0eb78ca74a4b3d580b6f47c63 | |
| parent | 95f599a1cc33e0a73a7595365df84c068820a979 (diff) | |
| download | notes-master.tar.gz notes-master.zip | |
| -rw-r--r-- | src/development/gas.md | 33 |
1 files changed, 33 insertions, 0 deletions
diff --git a/src/development/gas.md b/src/development/gas.md index a3da13f..d0ae284 100644 --- a/src/development/gas.md +++ b/src/development/gas.md @@ -1,6 +1,17 @@ # 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 @@ -31,8 +42,30 @@ 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] |