https://github.com/akkartik/mu/blob/master/subx/053new-segment.subx

     1 # Create a new segment (for data) using mmap().
     2
     3 == code
     4 #   instruction                     effective address                                                   register    displacement    immediate
     5 # . op          subop               mod             rm32          base        index         scale       r32
     6 # . 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
     7
     8 Entry:   # manual test
     9     # EAX = new-segment(0x1000)
    10     # . . push args
    11     68/push  0x1000/imm32
    12     # . . call
    13     e8/call  new-segment/disp32
    14     # . . discard args
    15     81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
    16
    17     # write to *EAX to check that we have access to the newly-allocated segment
    18     c7          0/subop/copy        0/mod/direct    0/rm32/EAX    .           .             .           .           .               0x34/imm32        # copy to *EAX
    19
    20     # syscall(exit, EAX)
    21     89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
    22     b8/copy-to-EAX  1/imm32/exit
    23     cd/syscall  0x80/imm8
    24
    25 new-segment:  # len : int -> address
    26     # . prolog
    27     55/push-EBP
    28     89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    29     53/push-EBX
    30     # copy len to _mmap-new-segment->len
    31     # TODO: compute _mmap-new-segment+4 before runtime
    32     8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           0/r32/EAX   8/disp8         .                 # copy *(EBP+8) to EAX
    33     bb/copy-to-EBX  _mmap-new-segment/imm32
    34     89/copy                         1/mod/*+disp8   3/rm32/EBX    .           .             .           0/r32/EAX   4/disp8         .                 # copy EAX to *(EBX+4)
    35     # mmap(_mmap-new-segment)
    36     bb/copy-to-EBX  _mmap-new-segment/imm32
    37     b8/copy-to-EAX  0x5a/imm32/mmap
    38     cd/syscall  0x80/imm8
    39 $new-segment:end:
    40     # . epilog
    41     5b/pop-to-EBX
    42     89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    43     5d/pop-to-EBP
    44     c3/return
    45
    46 == data
    47
    48 # various constants used here were found in the Linux sources (search for file mman-common.h)
    49 _mmap-new-segment:  # type mmap_arg_struct
    50     # addr
    51     0/imm32
    52     # len
    53     0/imm32
    54     # protection flags
    55     3/imm32  # PROT_READ | PROT_WRITE
    56     # sharing flags
    57     0x22/imm32  # MAP_PRIVATE | MAP_ANONYMOUS
    58     # fd
    59     -1/imm32  # since MAP_ANONYMOUS is specified
    60     # offset
    61     0/imm32  # since MAP_ANONYMOUS is specified
    62
    63 # . . vim:nowrap:textwidth=0