https://github.com/akkartik/mu/blob/main/linux/apps/ex7.subx

     1 # Example showing file syscalls.
     2 #
     3 # Create a file, open it for writing, write a character to it, close it, open
     4 # it for reading, read a character from it, close it, delete it, and return
     5 # the character read.
     6 #
     7 # To run:
     8 #   $ bootstrap/bootstrap translate apps/ex7.subx -o ex7
     9 #   $ bootstrap/bootstrap run ex7
    10 # Expected result:
    11 #   $ echo $?
    12 #   97
    13
    14 == code
    15 #   instruction                     effective address                                                   register    displacement    immediate
    16 # . op          subop               mod             rm32          base        index         scale       r32
    17 # . 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
    18
    19 Entry:
    20     # creat(Filename)
    21     bb/copy-to-ebx  Filename/imm32
    22     b9/copy-to-ecx  0x180/imm32/fixed-perms
    23     e8/call  syscall_creat/disp32
    24
    25     # stream = open(Filename, O_WRONLY, 0)  # we can't use 'fd' because it looks like a hex byte
    26     bb/copy-to-ebx  Filename/imm32
    27     b9/copy-to-ecx  1/imm32/wronly
    28     ba/copy-to-edx  0x180/imm32/fixed-perms
    29     e8/call  syscall_open/disp32
    30     # save stream
    31     bb/copy-to-ebx  Stream/imm32
    32     89/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to *ebx
    33
    34     # write(Stream, "a", 1)
    35     # . load stream
    36     bb/copy-to-ebx  Stream/imm32
    37     8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    38     # .
    39     b9/copy-to-ecx  A/imm32
    40     ba/copy-to-edx  1/imm32/size
    41     e8/call  syscall_write/disp32
    42
    43     # close(Stream)
    44     # . load stream
    45     bb/copy-to-ebx  Stream/imm32
    46     8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    47     # .
    48     e8/call  syscall_close/disp32
    49
    50     # stream = open(Filename, O_RDONLY, 0)
    51     bb/copy-to-ebx  Filename/imm32
    52     b9/copy-to-ecx  0/imm32/rdonly
    53     ba/copy-to-edx  0x180/imm32/fixed-perms
    54     e8/call  syscall_open/disp32
    55     # . save Stream
    56     bb/copy-to-ebx  Stream/imm32
    57     89/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           0/r32/eax   .               .                 # copy eax to *ebx
    58
    59     # read(Stream, B, 1)
    60     # . load stream
    61     bb/copy-to-ebx  Stream/imm32
    62     8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    63     # .
    64     b9/copy-to-ecx  B/imm32
    65     ba/copy-to-edx  1/imm32/size
    66     e8/call  syscall_read/disp32
    67
    68     # close(Stream)
    69     # . load stream
    70     bb/copy-to-ebx  Stream/imm32
    71     8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    72     #
    73     e8/call  syscall_close/disp32
    74
    75     # unlink(filename)
    76     bb/copy-to-ebx  Filename/imm32
    77     e8/call  syscall_unlink/disp32
    78
    79     # exit(b)
    80     # . load b
    81     bb/copy-to-ebx  B/imm32
    82     8b/copy                         0/mod/indirect  3/rm32/ebx    .           .             .           3/r32/ebx   .               .                 # copy *ebx to ebx
    83     #
    84     e8/call  syscall_exit/disp32
    85
    86 == data
    87
    88 Stream:
    89     0/imm32
    90 A:
    91     61/imm32/A
    92 B:
    93     0/imm32
    94 Filename:
    95     2e 66 6f 6f 00 00 00 00
    96 #   .  f  o  o  null
    97
    98 # . . vim:nowrap:textwidth=0