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# Example reading commandline arguments: compute length of first arg.
#
# To run (from the subx directory):
# $ subx translate examples/ex8.subx -o examples/ex8
# $ subx run examples/ex8 abc de fghi
# Expected result:
# $ echo $?
# 3 # length of 'abc'
#
# 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
# . prolog
89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP
# EAX = ascii-length(argv[1])
# . . push args
ff 6/subop/push 1/mod/*+disp8 5/rm32/EBP . . . . 8/disp8 . # push *(EBP+8)
# . . call
e8/call ascii-length/disp32
# . . discard args
81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP
# exit(EAX)
89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX
b8/copy-to-EAX 1/imm32/exit
cd/syscall 0x80/imm8
ascii-length: # s : (address array byte) -> n/EAX
# EDX = s
8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none . 2/r32/EDX 4/disp8 . # copy *(ESP+4) to EDX
# var result/EAX = 0
b8/copy-to-EAX 0/imm32
$ascii-length-loop:
# var c/ECX = *s
8a/copy 0/mod/* 2/rm32/EDX . . . 1/r32/ECX . . # copy byte at *EDX to lower byte of ECX
# if c == '\0' break
81 7/subop/compare 3/mod/direct 1/rm32/ECX . . . . . 0/imm32 # compare ECX
74/jump-if-equal $ascii-length-ret/disp8
# ++s
81 0/subop/add 3/mod/direct 2/rm32/EDX . . . . . 1/imm32 # add to EDX
# ++result
40/inc-EAX
# loop
eb/jump $ascii-length-loop/disp8
$ascii-length-ret:
# return EAX
c3/return
# . . vim:nowrap:textwidth=0
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