diff options
Diffstat (limited to 'subx/examples/ex9.subx')
| -rw-r--r-- | subx/examples/ex9.subx | 61 |
1 files changed, 61 insertions, 0 deletions
diff --git a/subx/examples/ex9.subx b/subx/examples/ex9.subx new file mode 100644 index 00000000..9ea547f1 --- /dev/null +++ b/subx/examples/ex9.subx @@ -0,0 +1,61 @@ +# Example reading commandline arguments: compute length of first arg. +# +# To run: +# $ subx translate ex9.subx ex9 +# $ subx run ex9 abc de fghi +# Expected result: +# $ echo $? +# 3 # length of 'abc' +# +# At the start of a SubX program: +# argc: *ESP +# argv[0]: *(ESP+4) +# argv[1]: *(ESP+8) +# ... +# Locals start from ESP-4 downwards. + +== 0x08048054 # code segment, after leaving room for ELF header and segment headers +# instruction effective address operand displacement immediate +# op subop mod rm32 base index scale r32 +# 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes + # var s = argv[1] (EBX) + 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 3/r32/EBX 8/disp8 . # copy *(ESP+8) to EBX + # call ascii_length(EBX) + # prepare call + 55/push . . . . . . . . # push EBP + 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP + # push args + 53/push . . . . . . . . # push EBX + # call + e8/call . . . . . . ascii_length/disp32 + # discard args + 5a/pop . . . . . . . . # pop into EDX + # clean up after call + 89/copy 3/mod/direct 4/rm32/ESP . . . 5/r32/EBP . . # copy EBP to ESP + 5d/pop . . . . . . . . # pop to EBP + + # exit(EAX) + 89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX + b8/copy . . . . . . . 1/imm32/exit # copy 1 to EAX + cd/syscall . . . . . . . 0x80/imm8 # int 80h + +ascii_length: # (s) + # initialize s (EDX) + 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 2/r32/EDX 4/disp8 # copy *(ESP+4) to EDX + # var result = 0 (EAX) + b8/copy . . . . . . . 0/imm32 # copy 1 to EAX +$al_loop: + # var c = *s (ECX) + 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 with 0 + 74/jump-if-zero . . . . . . . $al_ret/disp8 # jump if equal + # ++s + 81 0/subop/add 3/mod/direct 2/rm32/EDX . . . . . 1/imm32 # add 1 to EDX + # ++result + 81 0/subop/add 3/mod/direct 0/rm32/EAX . . . . . 1/imm32 # add 1 to EAX + # loop + eb/jump . . . . . . . $al_loop/disp8 # jump $al_loop +$al_ret: + # return (result in EAX) + c3/return |