58 lines
2.6 KiB
Plaintext
58 lines
2.6 KiB
Plaintext
# _write: write to a file descriptor (fd)
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== code
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# instruction effective address register displacement immediate
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# . op subop mod rm32 base index scale r32
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# . 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
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Entry: # just exit; can't test _write just yet
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# . syscall(exit, 0)
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bb/copy-to-ebx 0/imm32
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b8/copy-to-eax 1/imm32/exit
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cd/syscall 0x80/imm8
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_write: # fd : int, s : (addr array byte)
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# . prologue
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55/push-ebp
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89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
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# . save registers
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50/push-eax
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51/push-ecx
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52/push-edx
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53/push-ebx
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# syscall(write, fd, (data) s+4, (size) *s)
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# . fd : ebx
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 3/r32/ebx 8/disp8 . # copy *(ebp+8) to ebx
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# . data : ecx = s+4
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 1/r32/ecx 0xc/disp8 . # copy *(ebp+12) to ecx
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81 0/subop/add 3/mod/direct 1/rm32/ecx . . . . . 4/imm32 # add to ecx
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# . size : edx = *s
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8b/copy 1/mod/*+disp8 5/rm32/ebp . . . 2/r32/edx 0xc/disp8 . # copy *(ebp+12) to edx
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8b/copy 0/mod/indirect 2/rm32/edx . . . 2/r32/edx . . # copy *edx to edx
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# . syscall
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b8/copy-to-eax 4/imm32/write
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cd/syscall 0x80/imm8
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# if (eax < 0) abort
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3d/compare-eax-with 0/imm32
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0f 8c/jump-if-lesser $_write:abort/disp32
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$_write:end:
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# . restore registers
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5b/pop-to-ebx
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5a/pop-to-edx
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59/pop-to-ecx
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58/pop-to-eax
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# . epilogue
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89/copy 3/mod/direct 4/rm32/esp . . . 5/r32/ebp . . # copy ebp to esp
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5d/pop-to-ebp
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c3/return
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$_write:abort:
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# can't write a message here for risk of an infinite loop, so we'll use a special exit code instead
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# . syscall(exit, 255)
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bb/copy-to-ebx 0xff/imm32
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b8/copy-to-eax 1/imm32/exit
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cd/syscall 0x80/imm8
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# never gets here
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# . . vim:nowrap:textwidth=0
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