53 lines
2.9 KiB
Plaintext
53 lines
2.9 KiB
Plaintext
# Example showing arg order on the stack.
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#
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# Show difference between ascii codes of first letter of first arg and first
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# letter of second arg.
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#
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# To run:
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# $ bootstrap/bootstrap translate ex9.subx -o ex9
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# $ bootstrap/bootstrap run ex9 z x
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# Expected result:
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# $ echo $?
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# 2
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#
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# At the start of a SubX program:
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# argc: *esp
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# argv[0]: *(esp+4)
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# argv[1]: *(esp+8)
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# ...
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# Locals start from esp-4 downwards.
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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:
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# . prologue
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89/copy 3/mod/direct 5/rm32/ebp . . . 4/r32/esp . . # copy esp to ebp
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# ascii-difference(argv[1], argv[2])
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# . . push argv[2]
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ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 0xc/disp8 . # push *(ebp+12)
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# . . push argv[1]
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ff 6/subop/push 1/mod/*+disp8 5/rm32/ebp . . . . 8/disp8 . # push *(ebp+8)
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# . . call
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e8/call ascii-difference/disp32
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# . . discard args
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81 0/subop/add 3/mod/direct 4/rm32/esp . . . . . 8/imm32 # add to esp
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# exit(eax)
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89/copy 3/mod/direct 3/rm32/ebx . . . 0/r32/eax . . # copy eax to ebx
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e8/call syscall_exit/disp32
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ascii-difference: # (s1, s2): null-terminated ascii strings
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# a = first letter of s1 (ecx)
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8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none . 0/r32/eax 4/disp8 . # copy *(esp+4) to eax
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8b/copy 0/mod/indirect 0/rm32/eax . . . 0/r32/eax . . # copy *eax to eax
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# b = first letter of s2 (edx)
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8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/esp 4/index/none . 1/r32/ecx 8/disp8 # copy *(esp+8) to ecx
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8b/copy 0/mod/indirect 1/rm32/ecx . . . 1/r32/ecx . . # copy *ecx to ecx
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# a-b
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29/subtract 3/mod/direct 0/rm32/eax . . . 1/r32/ecx . . # subtract ecx from eax
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c3/return
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# . . vim:nowrap:textwidth=0
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