https://github.com/akkartik/mu/blob/main/linux/ex5.subx

     1 # Read a character from stdin, save it to a local on the stack, write it to stdout.
     2 #
     3 # To run:
     4 #   $ bootstrap/bootstrap translate ex5.subx -o ex5
     5 #   $ bootstrap/bootstrap run ex5
     6
     7 == code
     8 #   instruction                     effective address                                                   register    displacement    immediate
     9 # . op          subop               mod             rm32          base        index         scale       r32
    10 # . 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
    11
    12 Entry:
    13
    14     # allocate x on the stack
    15     81          5/subop/subtract    3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # subtract from esp
    16
    17     # read(stdin, x, 1)
    18     # . fd = 0 (stdin)
    19     bb/copy-to-ebx  0/imm32
    20     # . data = x (location to write result to)
    21     8d/copy-address                 1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none              1/r32/ecx   4/disp8         .                 # copy esp+4 to ecx
    22     # . size = 1 character
    23     ba/copy-to-edx  1/imm32
    24     # . syscall
    25     e8/call  syscall_read/disp32
    26
    27     # syscall(write, stdout, x, 1)
    28     # . fd = 1 (stdout)
    29     bb/copy-to-ebx  1/imm32
    30     # . data = x (location to read from)
    31     8d/copy-address                 1/mod/*+disp8   4/rm32/sib    4/base/esp  4/index/none              1/r32/ecx   4/disp8         .                 # copy esp+4 to ecx
    32     # . size = 1 character
    33     ba/copy-to-edx  1/imm32
    34     # . syscall
    35     e8/call  syscall_write/disp32
    36
    37     # exit(ebx)
    38     e8/call  syscall_exit/disp32
    39
    40 # . . vim:nowrap:textwidth=0