path: root/subx/053new-segment.subx

# Create a new segment (for data) using mmap().

== 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

Entry:   # manual test
    # EAX = new-segment(0x1000)
    # . . push args
    68/push  0x1000/imm32
    # . . call
    e8/call  new-segment/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP

    # write to *EAX to check that we have access to the newly-allocated segment
    c7          0/subop/copy        0/mod/direct    0/rm32/EAX    .           .             .           .           .               0x34/imm32        # copy to *EAX

    # syscall(exit, EAX)
    89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
    b8/copy-to-EAX  1/imm32/exit
    cd/syscall  0x80/imm8

new-segment:  # len : int -> address
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    53/push-EBX
    # copy len to _mmap-new-segment->len
    # TODO: compute _mmap-new-segment+4 before runtime
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           0/r32/EAX   8/disp8         .                 # copy *(EBP+8) to EAX
    bb/copy-to-EBX  _mmap-new-segment/imm32
    89/copy                         1/mod/*+disp8   3/rm32/EBX    .           .             .           0/r32/EAX   4/disp8         .                 # copy EAX to *(EBX+4)
    # mmap(_mmap-new-segment)
    bb/copy-to-EBX  _mmap-new-segment/imm32
    b8/copy-to-EAX  0x5a/imm32/mmap
    cd/syscall  0x80/imm8
$new-segment:end:
    # . epilog
    5b/pop-to-EBX
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

== data

# various constants used here were found in the Linux sources (search for file mman-common.h)
_mmap-new-segment:  # type mmap_arg_struct
    # addr
    0/imm32
    # len
    0/imm32
    # protection flags
    3/imm32  # PROT_READ | PROT_WRITE
    # sharing flags
    0x22/imm32  # MAP_PRIVATE | MAP_ANONYMOUS
    # fd
    -1/imm32  # since MAP_ANONYMOUS is specified
    # offset
    0/imm32  # since MAP_ANONYMOUS is specified

# . . vim:nowrap:textwidth=0