echo_partial.s

/*** echo.s ***/

// Description
//   echo - print arguments to stdout
// Synopsis
//   echo [STRING]...
// Build
//   $ as --32 -o echo.o echo.s
//   $ ld -m elf_i386 -o echo echo.o

# ************************************
# * print macro and strlen function
# * are omitted.
# ************************************

  .section .rodata

newline_char:
  .ascii "\n"
space_char:
  .ascii " "

  .section .text

# ************************************
# * echo [STRING]... 
# * Prints the string(s) to stdout.
# ************************************
  .globl _start
_start:
  movl  %esp, %ebp
  # Local variables (offset from %ebp)
  .equ  index, -4           # Stack index being operated on
  .equ  address, -8         # Current arg address
  .equ  length, -12         # Current arg length
  subl  $12, %esp
  # Start at index 1 (this skips argc)
  movl  $1, index(%ebp)
echo_loop:
  # Set address local variable
  movl  index(%ebp), %ecx
  movl  4(%ebp, %ecx, 4), %eax
  movl  %eax, address(%ebp)
  # Check if we're done (reached a NULL pointer)
  cmpl  $0, address(%ebp)
  je    echo_loop_end
  # Calculate length of string
  pushl address(%ebp)
  call  strlen
  addl  $4, %esp
  # Set length local variable
  movl  %eax, length(%ebp)
  # Print leading space if index > 1
  cmpl  $1, index(%ebp)
  jle   leading_space_end
  movl  $space_char, %ecx
  movl  $1, %edx
  print
leading_space_end:
  # Print current argument
  movl  address(%ebp), %ecx
  movl  length(%ebp), %edx
  print
  incl  index(%ebp)
  jmp   echo_loop
echo_loop_end:
  # Print newline char
  movl  $newline_char, %ecx
  movl  $1, %edx
  print
  # Exit
  movl  $1, %eax
  movl  $0, %ebx
  int   $0x80