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## 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
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