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