1 # Write a single byte to a stream.
  2 #
  3 # We need to do this in machine code because streams need to be opaque types,
  4 # and we don't yet support opaque types in Mu.
  5 
  6 == code
  7 #   instruction                     effective address                                                   register    displacement    immediate
  8 # . op          subop               mod             rm32          base        index         scale       r32
  9 # . 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
 10 
 11 # Write lower byte of 'n' to 'f'.
 12 append-byte:  # f: (addr stream byte), n: int
 13     # . prologue
 14     55/push-ebp
 15     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
 16     # . save registers
 17     50/push-eax
 18     51/push-ecx
 19     57/push-edi
 20     # edi = f
 21     8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           7/r32/edi   8/disp8         .                 # copy *(ebp+8) to edi
 22     # ecx = f->write
 23     8b/copy                         0/mod/indirect  7/rm32/edi    .           .             .           1/r32/ecx   .               .                 # copy *edi to ecx
 24     # if (f->write >= f->size) abort
 25     3b/compare                      1/mod/*+disp8   7/rm32/edi    .           .             .           1/r32/ecx   8/disp8         .                 # compare ecx with *(edi+8)
 26     7d/jump-if->=  $append-byte:abort/disp8
 27 $append-byte:to-stream:
 28     # write to stream
 29     # f->data[f->write] = LSB(n)
 30     31/xor                          3/mod/direct    0/rm32/eax    .           .             .           0/r32/eax   .               .                 # clear eax
 31     8a/copy-byte                    1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/AL    0xc/disp8       .                 # copy byte at *(ebp+12) to AL
 32     88/copy-byte                    1/mod/*+disp8   4/rm32/sib    7/base/edi  1/index/ecx   .           0/r32/AL    0xc/disp8       .                 # copy AL to *(edi+ecx+12)
 33     # ++f->write
 34     ff          0/subop/increment   0/mod/indirect  7/rm32/edi    .           .             .           .           .               .                 # increment *edi
 35 $append-byte:end:
 36     # . restore registers
 37     5f/pop-to-edi
 38     59/pop-to-ecx
 39     58/pop-to-eax
 40     # . epilogue
 41     89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
 42     5d/pop-to-ebp
 43     c3/return
 44 
 45 $append-byte:abort:
 46     (abort "append-byte: out of space")
 47     # never gets here
 48 
 49 test-append-byte-single:
 50     # - check that append-byte writes to first byte of 'file'
 51     # setup
 52     # . clear-stream(_test-stream)
 53     # . . push args
 54     68/push  _test-stream/imm32
 55     # . . call
 56     e8/call  clear-stream/disp32
 57     # . . discard args
 58     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               4/imm32           # add to esp
 59     # append-byte(_test-stream, 'A')
 60     # . . push args
 61     68/push  0x41/imm32
 62     68/push  _test-stream/imm32
 63     # . . call
 64     e8/call  append-byte/disp32
 65     # . . discard args
 66     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               8/imm32           # add to esp
 67     # check-stream-equal(_test-stream, "A", msg)
 68     # . . push args
 69     68/push  "F - test-append-byte-single"/imm32
 70     68/push  "A"/imm32
 71     68/push  _test-stream/imm32
 72     # . . call
 73     e8/call  check-stream-equal/disp32
 74     # . . discard args
 75     81          0/subop/add         3/mod/direct    4/rm32/esp    .           .             .           .           .               0xc/imm32         # add to esp
 76     # . end
 77     c3/return
 78 
 79 undo-append-byte:  # f: (addr stream byte)
 80     # . prologue
 81     55/push-ebp
 82     89/copy                         3/mod/direct    5/rm32/ebp    .           .             .           4/r32/esp   .               .                 # copy esp to ebp
 83     # . save registers
 84     50/push-eax
 85     # eax = f
 86     8b/copy                         1/mod/*+disp8   5/rm32/ebp    .           .             .           0/r32/eax   8/disp8         .                 # copy *(ebp+8) to eax
 87     # if (f->write <= 0) abort
 88     81          7/subop/compare     0/mod/indirect  0/rm32/eax    .           .             .           .           .               0/imm32           # compare *eax
 89     7e/jump-if-<=  $undo-append-byte:abort/disp8
 90     # --f->write
 91     ff          1/subop/decrement   0/mod/indirect  0/rm32/eax    .           .             .           .           .               .                 # decrement *eax
 92 $undo-append-byte:end:
 93     # . restore registers
 94     58/pop-to-eax
 95     # . epilogue
 96     89/copy                         3/mod/direct    4/rm32/esp    .           .             .           5/r32/ebp   .               .                 # copy ebp to esp
 97     5d/pop-to-ebp
 98     c3/return
 99 
100 $undo-append-byte:abort:
101     (abort "undo-append-byte: empty stream")
102     # never gets here
103 
104 # . . vim:nowrap:textwidth=0