path: root/subx/053new_segment.subx

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

== code

# instruction                     effective address                                                   operand     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

# main:  (manual test if this is the last file loaded)
  # EAX = new-segment(0x1000)
    # push arg
  68/push  0x1000/imm32
    # call
  e8/call  new-segment/disp32
    # discard arg
  81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32                 # add to ESP

  # store to *EAX
  c7/copy                         0/mod/direct    0/rm32/EAX    .           .             .           .           .               0x34/imm32              # copy to *EAX

  # 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   4/rm32/sib    5/base/EBP  4/index/none  .           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
  # 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
  00 00 00 00  # null
  # len
  00 00 00 00  # 0x1000
  # protection flags
  03 00 00 00  # PROT_READ | PROT_WRITE
  # sharing flags
  22 00 00 00  # MAP_PRIVATE | MAP_ANONYMOUS
  # fd
  ff ff ff ff  # -1 since MAP_ANONYMOUS is specified
  # offset
  00 00 00 00  # 0 since MAP_ANONYMOUS is specified

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