path: root/080zero-out.subx

# 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: (addr byte), len: int
    # pseudocode:
    #   curr/esi = start
    #   i/ecx = 0
    #   while true
    #     if (i >= len) break
    #     *curr = 0
    #     ++curr
    #     ++i
    #
    # . prologue
    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
    # var i/ecx: int = 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->=  $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
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

test-zero-out:
    # . prologue
    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
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# . . vim:nowrap:textwidth=0