## read a character from stdin, save it to a global, write it to stdout
#
# To run:
#   $ subx translate ex4.subx ex4
#   $ subx run ex4

== 0x08048074  # code segment, after leaving room for ELF header and segment headers
# opcode        ModR/M                    SIB                   displacement    immediate
# instruction   mod, reg, Reg/Mem bits    scale, index, base
# 1-3 bytes     0/1 byte                  0/1 byte              0/1/2/4 bytes   0/1/2/4 bytes

  ## read(stdin, x, 1)
  # fd = 0 (stdin)
  bb                                                                            0/imm32       # copy 0 to EBX
  # set location to write to
  b9                                                                            a7 90 04 08   # copy 0x080490a7 to ECX
  # size = 1 character
  ba                                                                            1/imm32       # copy 1 to EDX
  # syscall = read
  b8                                                                            3/imm32       # copy 3 to EAX
  # call
  cd                                                                            128/imm8      # int 80h

  ## write(stdout, x, 1)
  # fd = 1 (stdout)
  bb                                                                            1/imm32       # copy 1 to EBX
  # set location to write to
  b9                                                                            a7 90 04 08   # copy 0x080490a7 to ECX
  # size = 1 character
  ba                                                                            1/imm32       # copy 1 to EDX
  # syscall = write
  b8                                                                            4/imm32       # copy 3 to EAX
  # call
  cd                                                                            128/imm8      # int 80h

  ## exit(EBX)
  b8                                                                            1/imm32       # copy 1 to EAX
  cd                                                                            128/imm8      # int 80h

== 0x080490a7
00 00 00 00  # space for read() to write to

# vim:ft=subx:nowrap