## 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 EBP
  89                              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                                                                                                                      0/imm32           # copy 0 to EBX
  # set location to read character to
  89                              3/mod/direct    1/rm32/ECX                                  5/r32/EBP                                     # copy EBP to ECX
  # size = 1 character
  ba                                                                                                                      1/imm32           # copy 1 to EDX
  # syscall = read
  b8                                                                                                                      3/imm32           # copy 3 to EAX
  # call
  cd                                                                                                                      0x80/imm8         # int 80h

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

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

# vim:ft=subx:nowrap