1 ## Example reading commandline arguments: compute length of first arg. 2 # 3 # To run (from the subx directory): 4 # $ subx translate examples/ex8.subx -o examples/ex8 5 # $ subx run examples/ex8 abc de fghi 6 # Expected result: 7 # $ echo $? 8 # 3 # length of 'abc' 9 # 10 # At the start of a SubX program: 11 # argc: *ESP 12 # argv[0]: *(ESP+4) 13 # argv[1]: *(ESP+8) 14 # ... 15 # Locals start from ESP-4 downwards. 16 17 == code 18 # instruction effective address operand displacement immediate 19 # op subop mod rm32 base index scale r32 20 # 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 21 # prolog 22 89/copy 3/mod/direct 5/rm32/EBP . . . 4/r32/ESP . . # copy ESP to EBP 23 # call ascii_length(argv[1]) 24 # push args 25 ff 6/subop/push 1/mod/*+disp8 4/rm32/sib 5/base/EBP 4/index/none . . 0x8/disp8 . # push *(EBP+8) 26 # call 27 e8/call ascii_length/disp32 28 # discard args 29 81 0/subop/add 3/mod/direct 4/rm32/ESP . . . . . 4/imm32 # add to ESP 30 31 # exit(EAX) 32 89/copy 3/mod/direct 3/rm32/EBX . . . 0/r32/EAX . . # copy EAX to EBX 33 b8/copy . . . . . . . 1/imm32/exit # copy to EAX 34 cd/syscall 0x80/imm8 35 36 ascii_length: # (s) 37 # initialize s (EDX) 38 8b/copy 1/mod/*+disp8 4/rm32/sib 4/base/ESP 4/index/none 2/r32/EDX 4/disp8 # copy *(ESP+4) to EDX 39 # var result = 0 (EAX) 40 b8/copy . . . . . . . 0/imm32 # copy to EAX 41 $ascii_length_loop: 42 # var c = *s (ECX) 43 8a/copy 0/mod/* 2/rm32/EDX . . . 1/r32/ECX . . # copy byte at *EDX to lower byte of ECX 44 # if c == '\0' break 45 81 7/subop/compare 3/mod/direct 1/rm32/ECX . . . . . 0/imm32 # compare ECX 46 74/jump-if-equal $ascii_length_ret/disp8 47 # ++s 48 81 0/subop/add 3/mod/direct 2/rm32/EDX . . . . . 1/imm32 # add to EDX 49 # ++result 50 40/inc-EAX 51 # loop 52 eb/jump $ascii_length_loop/disp8 53 $ascii_length_ret: 54 # return (result in EAX) 55 c3/return