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| author | Kartik Agaram <vc@akkartik.com> | 2018-09-21 22:25:00 -0700 |
|---|---|---|
| committer | Kartik Agaram <vc@akkartik.com> | 2018-09-21 22:25:00 -0700 |
| commit | d47f3a82786c7d3abdb1001c2562780d0e1fab2e (patch) | |
| tree | cd30cf4975ab70ceb427ee0f025f0804885fd8d8 /subx/examples/ex10.subx | |
| parent | ef47911ff39c865aa2e65af442a03ddd8c2b1aa4 (diff) | |
| download | mu-d47f3a82786c7d3abdb1001c2562780d0e1fab2e.tar.gz | |
4584 - discrepancy between SubX and native x86
One of the more painful things I had to debug with machine code. Tricks I used can be seen in ex10.subx: - printing argv[1] in various places - printing a single 'X' in various places to count how many times we get to different instructions - exiting with the current value of EAX in various places I repeatedly went down the wrong trail in several ways: - forgetting that the problem lay in native runs, and accidentally switching to subx runs during debugging. - forgetting to pass commandline args, because ex10 doesn't check its argv - writing the wrong comment for an instruction, and then miscalculating the set of registers that need to be saved. - forgetting that syscalls clobber EAX. Debugging native runs is hard, because you have to write non-trivial code to instrument the binary, and instrumentation can itself be buggy. When we finally tracked it down, I recognized the problem immediately. I'd meant to confirm the behavior of opcode 8a against bare metal, and then forgot. In any case, opcode 8a was inconsistent with 88. Sloppy.
Diffstat (limited to 'subx/examples/ex10.subx')
| -rw-r--r-- | subx/examples/ex10.subx | 77 |
1 files changed, 76 insertions, 1 deletions
diff --git a/subx/examples/ex10.subx b/subx/examples/ex10.subx index 321cee64..1a426e28 100644 --- a/subx/examples/ex10.subx +++ b/subx/examples/ex10.subx @@ -26,9 +26,11 @@ # push args 50/push . . . . . . . . # push EAX 53/push . . . . . . . . # push EBX +#? e8/call write_argv_3/disp32 # call - e8/call argv_equal/disp32 + e8/call argv_equal/disp32 # exit(EAX) +$exit: 89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX b8/copy . . . . . . . 1/imm32 # copy 1 to EAX cd/syscall 0x80/imm8 @@ -39,11 +41,18 @@ argv_equal: # (s1, s2) : null-terminated ascii strings -> EAX : boolean # initialize s1 (ECX) and s2 (EDX) 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 1/r32/ECX 8/disp8 . # copy *(ESP+8) to ECX 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 2/r32/EDX 4/disp8 . # copy *(ESP+4) to EDX +#? # i/ESI = 0 +#? be/copy 0/imm32 # copy 0 to ESI # while (true) $argv_loop: # c1/EAX, c2/EBX = *s1, *s2 + b8/copy 0/imm32 # clear EAX 8a/copy 0/mod/indirect 1/rm32/ECX . . . 0/r32/EAX . . # copy byte at *ECX to lower byte of EAX + bb/copy 0/imm32 # clear EBX 8a/copy 0/mod/indirect 2/rm32/EDX . . . 3/r32/EBX . . # copy byte at *EDX to lower byte of EBX +#? # if (i == _) exit(EAX) +#? 81 7/subop 3/mod/direct 6/rm32/ESI 3/imm32 # compare ESI with _ +#? 74/jump-if-equal $exit/disp8 # if (c1 == 0) break 3d/compare . . . . . . . 0/imm32 # compare EAX with 0 74/jump-if-equal $argv_break/disp8 @@ -53,7 +62,15 @@ $argv_loop: # ++s1, ++s2 41/inc-ECX 42/inc-EDX +#? # ++i +#? 46/inc-ESI # end while +#? 68/push X/imm32 +#? e8/call write_stderr/disp32 +#? 81 0/subop/add 3/mod/direct 4/rm32/ESP 4/imm32 # add 4 to ESP +#? 51/push . . . . . . . . # push ECX +#? e8/call write_argv_3/disp32 +#? 81 0/subop/add 3/mod/direct 4/rm32/ESP 4/imm32 # add 4 to ESP eb/jump $argv_loop/disp8 $argv_break: # if (c2 == 0) return true @@ -65,3 +82,61 @@ $argv_break: $argv_fail: b8/copy . . . . . . . 0/imm32 # copy 0 to EAX c3/return + +write_stderr: # s : (address array byte) -> <void> + # save registers + 50/push . . . . . . . . # push EAX + 51/push . . . . . . . . # push ECX + 52/push . . . . . . . . # push EDX + 53/push . . . . . . . . # push EBX + # write(2/stderr, (data) s+4, (size) *s) + # fd = 2 (stderr) + bb/copy . . . . . . . 2/imm32 # copy 2 to EBX + # x = s+4 + 8b/copy 1/mod/*+disp8 4/rm32/SIB 4/base/ESP 4/index/none . 1/r32/ECX 0x14/disp8 . # copy *(ESP+20) to ECX + 81 0/subop/add 3/mod/direct 1/rm32/ECX . . . . . 4/imm32 # add 4 to ECX + # size = *s + 8b/copy 1/mod/*+disp8 4/rm32/SIB 4/base/ESP 4/index/none . 2/r32/EDX 0x14/disp8 . # copy *(ESP+20) to EDX + 8b/copy 0/mod/indirect 2/rm32/EDX . . . 2/r32/EDX . . # copy *EDX to EDX + # call write() + b8/copy . . . . . . . 4/imm32/write # copy 1 to EAX + cd/syscall 0x80/imm8 + # restore registers + 5b/pop . . . . . . . . # pop EBX + 5a/pop . . . . . . . . # pop EDX + 59/pop . . . . . . . . # pop ECX + 58/pop . . . . . . . . # pop EAX + # end + c3/return + +write_argv_3: # s : null-terminated ascii string of size exactly 3 -> <void> + # save registers + 50/push . . . . . . . . # push EAX + 51/push . . . . . . . . # push ECX + 52/push . . . . . . . . # push EDX + 53/push . . . . . . . . # push EBX + # write(2/stderr, (data) s+4, (size) *s) + # fd = 2 (stderr) + bb/copy . . . . . . . 2/imm32 # copy 2 to EBX + # x = s + 8b/copy 1/mod/*+disp8 4/rm32/SIB 4/base/ESP 4/index/none . 1/r32/ECX 0x14/disp8 . # copy *(ESP+20) to ECX + # size = 3 + ba/copy . . . . . . . 3/imm32 # copy 3 to EDX (hardcoded) + # call write() + b8/copy . . . . . . . 4/imm32/write # copy 1 to EAX + cd/syscall 0x80/imm8 + # restore registers + 5b/pop . . . . . . . . # pop EBX + 5a/pop . . . . . . . . # pop EDX + 59/pop . . . . . . . . # pop ECX + 58/pop . . . . . . . . # pop EAX + # end + c3/return + +== data +X: + 02 00 00 00 + 58/X 0a/newline +Y: + 02 00 00 00 + 59/Y 0a/newline |