# _write: write to a file descriptor (fd)

== 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:  # just exit; can't test _write just yet
    # . syscall(exit, 0)
    bb/copy-to-EBX  0/imm32
    b8/copy-to-EAX  1/imm32/exit
    cd/syscall  0x80/imm8

_write:  # fd : int, s : (address array byte) -> <void>
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # . save registers
    50/push-EAX
    51/push-ECX
    52/push-EDX
    53/push-EBX
    # syscall(write, fd, (data) s+4, (size) *s)
    # . fd : EBX
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           3/r32/EBX   8/disp8         .                 # copy *(EBP+8) to EBX
    # . data : ECX = s+4
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           1/r32/ECX   0xc/disp8       .                 # copy *(EBP+12) to ECX
    81          0/subop/add         3/mod/direct    1/rm32/ECX    .           .             .           .           .               4/imm32           # add to ECX
    # . size : EDX = *s
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           2/r32/EDX   0xc/disp8       .                 # copy *(EBP+12) to EDX
    8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
    # . syscall
    b8/copy-to-EAX  4/imm32/write
    cd/syscall  0x80/imm8
    # if (EAX < 0) abort
    3d/compare-EAX-with  0/imm32
    0f 8c/jump-if-lesser  $_write:abort/disp32
$_write:end:
    # . restore registers
    5b/pop-to-EBX
    5a/pop-to-EDX
    59/pop-to-ECX
    58/pop-to-EAX
    # . epilog
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

$_write:abort:
    # can't write a message here for risk of an infinite loop, so we'll use a special exit code instead
    # . syscall(exit, 255)
    bb/copy-to-EBX  0xff/imm32
    b8/copy-to-EAX  1/imm32/exit
    cd/syscall  0x80/imm8
    # never gets here

# . . vim:nowrap:textwidth=0