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| author | Kartik Agaram <vc@akkartik.com> | 2018-09-21 16:33:57 -0700 |
|---|---|---|
| committer | Kartik Agaram <vc@akkartik.com> | 2018-09-21 16:38:35 -0700 |
| commit | ddeed58f8524849315286de8ff825c43e5b82f9d (patch) | |
| tree | 682bc948d7fa55d587a06e56b43e941d3fce8524 /subx/examples/ex10.subx | |
| parent | 45967d2106335dbd595cb43ae5732ba88b5d0553 (diff) | |
| download | mu-ddeed58f8524849315286de8ff825c43e5b82f9d.tar.gz | |
4579
New example program: ascii null-terminated string comparison I'd hoped this would be a stepping stone to supporting general ascii comparison, but we're planning to use size-prefixed rather than null-terminated arrays everywhere. The only exception is commandline arguments, which will remain null-terminated to interoperate with Linux. So I'm going to need separate functions for "compare with argv" and for general string comparison.
Diffstat (limited to 'subx/examples/ex10.subx')
| -rw-r--r-- | subx/examples/ex10.subx | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/subx/examples/ex10.subx b/subx/examples/ex10.subx new file mode 100644 index 00000000..1b0b0a29 --- /dev/null +++ b/subx/examples/ex10.subx @@ -0,0 +1,67 @@ +## String comparison: return 1 iff the two args passed in at the commandline are equal. +# +# To run: +# $ subx translate ex10.subx ex10 +# $ subx run ex10 abc abd +# Expected result: +# $ echo $? +# 0 # false + +== code +# instruction effective address operand displacement immediate +# op subop mod rm32 base index scale r32 +# 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes + +# main: return argv_equal(argv[1], argv[2]) +# At the start of a SubX program: +# argc: *ESP +# argv[0]: *(ESP+4) +# argv[1]: *(ESP+8) +# ... + # s1 = argv[1] (EAX) + 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 0/r32/EAX 8/disp8 . # copy *(ESP+8) to EAX + # s2 = argv[2] (EBX) + 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 3/r32/EBX 0xc/disp8 . # copy *(ESP+12) to EBX + # call argv_equal(s1, s2) + # push args + 50/push . . . . . . . . # push EAX + 53/push . . . . . . . . # push EBX + # call + e8/call argv_equal/disp32 + # exit(EAX) + 89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX + b8/copy . . . . . . . 1/imm32 # copy 1 to EAX + cd/syscall . . . . . . . 0x80/imm8 # int 80h + +# compare two null-terminated ascii strings +# reason for the name: the only place we should have null-terminated ascii strings is from commandline args +argv_equal: # (s1, s2) : null-terminated ascii strings -> EAX : boolean + # initialize s1 (ECX) and s2 (EDX) + 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 1/r32/ECX 8/disp8 . # copy *(ESP+8) to ECX + 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 2/r32/EDX 4/disp8 . # copy *(ESP+4) to EDX + # while (true) +$argv_loop: + # c1/EAX, c2/EBX = *s1, *s2 + 8a/copy 0/mod/indirect 1/rm32/ECX . . . 0/r32/EAX . . # copy byte at *ECX to lower byte of EAX + 8a/copy 0/mod/indirect 2/rm32/EDX . . . 3/r32/EBX . . # copy byte at *EDX to lower byte of EBX + # if (c1 == 0) break + 3d/compare . . . . . . . 0/imm32 # compare EAX with 0 + 74/jump-if-equal $argv_break/disp8 + # if (c1 != c2) return false + 39/compare 3/mod/direct 0/rm32/EAX . . . 3/r32/EBX . . # compare EAX with EBX + 75/jump-if-not-equal $argv_fail/disp8 + # ++s1, ++s2 + 41/inc-ECX + 42/inc-EDX + # end while + eb/jump $argv_loop/disp8 +$argv_break: + # if (c2 == 0) return true + 81 7/subop/compare 3/mod/direct 3/rm32/EBX . . . . . 0/imm32 # compare EBX with 0 + 75/jump-if-not-equal $argv_fail/disp8 + b8/copy . . . . . . . 1/imm32 # copy 1 to EAX + c3/return + # return false +$argv_fail: + b8/copy . . . . . . . 0/imm32 # copy 0 to EAX + c3/return |