path: root/subx/ex5.subx

## read a character from stdin, save it to a local on the stack, write it to stdout
#
# To run:
#   $ subx translate ex5.subx ex5
#   $ subx run ex5

== 0x08048054  # code segment, after leaving room for ELF header and segment headers
# 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

## function main
  # prolog
  55/push                                                                                                                                           # push EBP
  89/copy                         3/mod/direct    5/rm32/EBP                                          4/r32/ESP                                     # copy ESP to EBP
  # allocate x on the stack
  81          5/subop/subtract    3/mod/direct    4/rm32/ESP                                                                      4/imm32           # subtract 4 bytes from ESP

  ## read(stdin, x, 1)
  # fd = 0 (stdin)
  bb/copy                                                                                                                         0/imm32           # copy 0 to EBX
  # initialize x (location to write result to)
  89/copy                         3/mod/direct    1/rm32/ECX                                          5/r32/EBP                                     # copy EBP to ECX
  # size = 1 character
  ba/copy                                                                                                                         1/imm32           # copy 1 to EDX
  # read(fd, x, size)
  b8/copy                                                                                                                         3/imm32           # copy 3 to EAX
  cd/syscall                                                                                                                      0x80/imm8         # int 80h

  ## write(stdout, x, 1)
  # fd = 1 (stdout)
  bb/copy                                                                                                                         1/imm32           # copy 1 to EBX
  # initialize x (location to read from)
  89/copy                         3/mod/direct    1/rm32/ECX                                          5/r32/EBP                                     # copy EBP to ECX
  # size = 1 character
  ba/copy                                                                                                                         1/imm32           # copy 1 to EDX
  # write(fd, x, size)
  b8/copy                                                                                                                         4/imm32           # copy 4 to EAX
  cd/syscall                                                                                                                      0x80/imm8         # int 80h

  ## exit(EBX)
  b8/copy                                                                                                                         1/imm32           # copy 1 to EAX
  cd/syscall                                                                                                                      0x80/imm8         # int 80h

# vim:ft=subx:nowrap