path: root/subx/examples/ex5.subx

## read a character from stdin, save it to a local on the stack, write it to stdout
#
# To run (from the subx directory):
#   $ subx translate examples/ex5.subx -o examples/ex5
#   $ subx run examples/ex5

== 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:
  # allocate x on the stack
  81          5/subop/subtract    3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # subtract from ESP

  # read(stdin, x, 1)
    # fd = 0 (stdin)
  bb/copy-to-EBX  0/imm32
    # initialize x (location to write result to)
  8d/copy-address                 1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              1/r32/ECX   4/disp8         .                 # copy ESP+4 to ECX
    # size = 1 character
  ba/copy-to-EDX  1/imm32
    # read(fd, x, size)
  b8/copy-to-EAX  3/imm32/read
  cd/syscall  0x80/imm8

  # write(stdout, x, 1)
    # fd = 1 (stdout)
  bb/copy-to-EBX  1/imm32
    # initialize x (location to read from)
  8d/copy-address                 1/mod/*+disp8   4/rm32/sib    4/base/ESP  4/index/none              1/r32/ECX   4/disp8         .                 # copy ESP+4 to ECX
    # size = 1 character
  ba/copy-to-EDX  1/imm32
    # write(fd, x, size)
  b8/copy-to-EAX  4/imm32/write
  cd/syscall  0x80/imm8

  # exit(EBX)
  b8/copy-to-EAX  1/imm32/exit
  cd/syscall  0x80/imm8

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