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 (from the subx directory):
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# $ subx translate examples/ex9.subx -o examples/ex9
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# $ subx run examples/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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# . prolog
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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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# syscall(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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b8/copy-to-EAX 1/imm32/exit
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cd/syscall 0x80/imm8
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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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