# Fill a region of memory with zeroes.

== 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

zero-out:  # start : address, len : int
    # pseudocode:
    #   curr/ESI = start
    #   i/ECX = 0
    #   while true
    #     if (i >= len) break
    #     *curr = 0
    #     ++curr
    #     ++i
    #
    # . 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
    56/push-ESI
    # curr/ESI = start
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           6/r32/ESI   8/disp8         .                 # copy *(EBP+8) to ESI
    # i/ECX = 0
    31/xor                          3/mod/direct    1/rm32/ECX    .           .             .           1/r32/ECX   .               .                 # clear ECX
    # EDX = len
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           2/r32/EDX   0xc/disp8       .                 # copy *(EBP+12) to EDX
$zero-out:loop:
    # if (i >= len) break
    39/compare                      3/mod/direct    1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # compare ECX with EDX
    7d/jump-if-greater-or-equal  $zero-out:end/disp8
    # *curr = 0
    c6          0/subop/copy        0/mod/direct    6/rm32/ESI    .           .             .           .           .               0/imm8            # copy byte to *ESI
    # ++curr
    46/increment-ESI
    # ++i
    41/increment-ECX
    eb/jump  $zero-out:loop/disp8
$zero-out:end:
    # . restore registers
    5e/pop-to-ESI
    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

test-zero-out:
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # region/ECX = 34, 35, 36, 37
    68/push  0x37363534/imm32
    89/copy                         3/mod/direct    1/rm32/ECX    .           .             .           4/r32/ESP   .               .                 # copy ESP to ECX
    # zero-out(ECX, 3)
    # . . push args
    68/push  3/imm32/len
    51/push-ECX
    # . . call
    e8/call  zero-out/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # first 3 bytes cleared, fourth left alone
    # . check-ints-equal(*ECX, 0x37000000, msg)
    # . . push args
    68/push  "F - test-zero-out"/imm32
    68/push  0x37000000/imm32
    ff          6/subop/push        0/mod/indirect  1/rm32/ECX    .           .             .           .           .               .                 # push *ECX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    # . epilog
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

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