diff options
Diffstat (limited to 'linux/101_write.subx')
| -rw-r--r-- | linux/101_write.subx | 53 |
1 files changed, 53 insertions, 0 deletions
diff --git a/linux/101_write.subx b/linux/101_write.subx new file mode 100644 index 00000000..2567f444 --- /dev/null +++ b/linux/101_write.subx @@ -0,0 +1,53 @@ +# _write: write to a file descriptor (fd) + +== code +# instruction effective address register displacement immediate +# . op subop mod rm32 base index scale r32 +# . 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes + +Entry: # just exit; can't test _write just yet + bb/copy-to-ebx 0/imm32 + e8/call syscall_exit/disp32 + +_write: # fd: int, s: (addr array byte) + # . prologue + 55/push-ebp + 89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp + # . save registers + 50/push-eax + 51/push-ecx + 52/push-edx + 53/push-ebx + # syscall(write, fd, (data) s+4, (size) *s) + # . ebx = fd + 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 3/r32/ebx 8/disp8 . # copy *(ebp+8) to ebx + # . var data/ecx: (addr byte) = s+4 + 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 1/r32/ecx 0xc/disp8 . # copy *(ebp+12) to ecx + 81 0/subop/add 3/mod/direct 1/rm32/ecx . . . . . 4/imm32 # add to ecx + # . var size/edx: int = *s + 8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx + 8b/copy 0/mod/indirect 2/rm32/edx . . . 2/r32/edx . . # copy *edx to edx + # . syscall + e8/call syscall_write/disp32 + # if (eax < 0) abort + 3d/compare-eax-with 0/imm32 + 0f 8c/jump-if-< $_write:abort/disp32 +$_write:end: + # . restore registers + 5b/pop-to-ebx + 5a/pop-to-edx + 59/pop-to-ecx + 58/pop-to-eax + # . epilogue + 89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp + 5d/pop-to-ebp + c3/return + +$_write:abort: + # can't write a message here for risk of an infinite loop, so we'll use a special exit code instead + # . syscall(exit, 255) + bb/copy-to-ebx 0xff/imm32 + e8/call syscall_exit/disp32 + # never gets here + +# . . vim:nowrap:textwidth=0 |