path: root/310copy-bytes.subx

# Fill a region of memory with zeroes.

== code

copy-bytes:  # src: (addr byte), dest: (addr byte), size: int
    # pseudocode:
    #   curr-src/esi = src
    #   curr-dest/edi = dest
    #   i/ecx = 0
    #   while true
    #     if (i >= size) break
    #     *curr-dest = *curr-src
    #     ++curr-src
    #     ++curr-dest
    #     ++i
    #
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # . save registers
    50/push-eax
    51/push-ecx
    52/push-edx
    56/push-esi
    57/push-edi
    # curr-src/esi = src
    8b/-> *(ebp+8) 6/r32/esi
    # curr-dest/edi = dest
    8b/-> *(ebp+0xc) 7/r32/edi
    # var i/ecx: int = 0
    b9/copy-to-ecx 0/imm32
    # edx = size
    8b/-> *(ebp+0x10) 2/r32/edx
    {
      # if (i >= size) break
      39/compare %ecx 2/r32/edx
      7d/jump-if->=  break/disp8
      # *curr-dest = *curr-src
      8a/byte-> *esi 0/r32/AL
      88/byte<- *edi 0/r32/AL
      # update
      46/increment-esi
      47/increment-edi
      41/increment-ecx
      eb/jump loop/disp8
    }
$copy-bytes:end:
    # . restore registers
    5f/pop-to-edi
    5e/pop-to-esi
    5a/pop-to-edx
    59/pop-to-ecx
    58/pop-to-eax
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return