path: root/117write-int-hex.subx

# Write out the (hex) textual representation of numbers.

== 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

# convert the lowest nibble of eax to ascii and return it in the lowest byte of eax
to-hex-char:  # in/eax: int -> out/eax: int
    # no error checking; accepts argument in eax
    # if (eax <= 9) return eax + '0'
    3d/compare-eax-with  0x9/imm32/9
    7f/jump-if->  $to-hex-char:else/disp8
    05/add-to-eax  0x30/imm32/0
    c3/return
$to-hex-char:else:
    # otherwise return eax + 'a' - 10
    05/add-to-eax  0x57/imm32/a-10
    c3/return

append-byte-hex:  # f: (addr stream byte), n: int
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
    # . save registers
    50/push-eax
    # AL = convert upper nibble to hex
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0xc/disp8       .                 # copy *(ebp+12) to eax
    c1/shift    5/subop/logic-right 3/mod/direct    0/rm32/eax    .           .             .           .           .               4/imm8            # shift eax right by 4 bits, while padding zeroes
    25/and-eax  0xf/imm32
    # . AL = to-hex-char(AL)
    e8/call  to-hex-char/disp32
    # append-byte(f, AL)
    # . . push args
    50/push-eax
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  append-byte/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # AL = convert lower nibble to hex
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0xc/disp8       .                 # copy *(ebp+12) to eax
    25/and-eax  0xf/imm32
    # . AL = to-hex-char(AL)
    e8/call  to-hex-char/disp32
    # append-byte(f, AL)
    # . . push args
    50/push-eax
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  append-byte/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
$append-byte-hex:end:
    # . restore registers
    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-append-byte-hex:
    # - check that append-byte-hex adds the hex textual representation
    # setup
    # . clear-stream(_test-stream)
    # . . push args
    68/push  _test-stream/imm32
    # . . call
    e8/call  clear-stream/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
    # append-byte-hex(_test-stream, 0xa)  # exercises digit, non-digit as well as leading zero
    # . . push args
    68/push  0xa/imm32
    68/push  _test-stream/imm32
    # . . call
    e8/call  append-byte-hex/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-stream-equal(_test-stream, "0a", msg)
    # . . push args
    68/push  "F - test-append-byte-hex"/imm32
    68/push  "0a"/imm32
    68/push  _test-stream/imm32
    # . . call
    e8/call  check-stream-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # . end
    c3/return

write-int32-hex:  # f: (addr stream byte), n: int
    # . prologue
    55/push-ebp
    89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
$write-int32-hex:hex-prefix:
    # write(f, "0x")
    # . . push args
    68/push  "0x"/imm32
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  write/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
$write-int32-hex:rest:
    # write-int32-hex-bits(f, n, 32)
    # . . push args
    68/push  0x20/imm32
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           0xc/disp8       .                 # push *(ebp+12)
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  write-int32-hex-bits/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
$write-int32-hex:end:
    # . epilogue
    89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
    5d/pop-to-ebp
    c3/return

# print rightmost 'bits' of 'n'
# bits must be multiple of 4
write-int32-hex-bits:  # f: (addr stream byte), n: int, bits: int
    # pseudocode:
    #  bits -= 4
    #  while true
    #    if (bits < 0) break
    #    eax = n >> bits
    #    eax = eax & 0xf
    #    append-byte(f, AL)
    #    bits -= 4
    #
    # . 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
    # ecx = bits-4
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           1/r32/ecx   0x10/disp8      .                 # copy *(ebp+16) to ecx
    81          5/subop/subtract    3/mod/direct    1/rm32/ecx    .           .             .           .           .               4/imm32           # subtract from ecx
$write-int32-hex-bits:loop:
    # if (bits < 0) break
    81          7/subop/compare     3/mod/direct    1/rm32/ecx    .           .             .           .           .               0/imm32           # compare ecx
    7c/jump-if-<  $write-int32-hex-bits:end/disp8
    # eax = n >> bits
    8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   0xc/disp8       .                 # copy *(ebp+12) to eax
    d3/>>ecx    5/subop/pad-zeroes  3/mod/direct    0/rm32/eax    .           .             .           .           .               .                 # shift eax right by ecx bits, padding zeroes
    # eax = to-hex-char(AL)
    25/and-eax  0xf/imm32
    e8/call  to-hex-char/disp32
    # append-byte(f, AL)
    # . . push args
    50/push-eax
    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp    .           .             .           .           8/disp8         .                 # push *(ebp+8)
    # . . call
    e8/call  append-byte/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # bits -= 4
    81          5/subop/subtract    3/mod/direct    1/rm32/ecx    .           .             .           .           .               4/imm32           # subtract from ecx
    eb/jump  $write-int32-hex-bits:loop/disp8
$write-int32-hex-bits:end:
    # . restore registers
    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-write-int32-hex:
    # - check that write-int32-hex prints the hex textual representation
    # setup
    # . clear-stream(_test-stream)
    # . . push args
    68/push  _test-stream/imm32
    # . . call
    e8/call  clear-stream/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
    # write-int32-hex(_test-stream, 0x8899aa)
    # . . push args
    68/push  0x8899aa/imm32
    68/push  _test-stream/imm32
    # . . call
    e8/call  write-int32-hex/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
    # check-stream-equal(_test-stream, "0x008899aa", msg)
    # . . push args
    68/push  "F - test-write-int32-hex"/imm32
    68/push  "0x008899aa"/imm32
    68/push  _test-stream/imm32
    # . . call
    e8/call  check-stream-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
    # . end
    c3/return

# . . vim:nowrap:textwidth=0