this_is_hard.S

#include <sys/syscall.h>

#define STRING "Hello, World!\n"

.globl  main
.type   main, @function

main:
  jmp     call_addr    /* jmp to call addr to get the address of STRING */

pop_addr:
  /*
      Objective:
        int fd = open("txt", 0, 0);
        size_t size = read(fd, buffer, 256);
        write(1, buffer, size);

      System call for Linux with x86 architecture gets syscall number from eax,
      and its 1st argument from ebx,
      2nd argument from ecx,
      3rd argument from edx,
      4th argument from edi,
      5th argument from esi, ...
  */

  // int fd = open("txt", 0, 0);

  // build a string "txt" using push!
  push $0x00747874
  // move that string to ebx
  mov %esp, %ebx
  // make ecx zero
  mov $0, %ecx
  // make edx zero
  mov $0, %ecx
  // move $SYS_open to eax
  mov $SYS_open, %eax
  // syscall!
  int $0x80



  // size_t size = read(fd, buffer, 256);

  // move eax to ebx (to set fd!)
  mov %eax, %ebx
  // set ecx as esp (stack buffer)
  mov %esp, %ecx
  // set edx as 256
  mov $256, %edx
  // set eax as $SYS_read
  mov $SYS_read, %eax
  // syscall!
  int $0x80

  // write(1, buffer, size);

  // set edx as eax, to set it as size read
  mov %eax, %edx
  // set ecx as esp (stack buffer)
  mov %esp, %ecx
  // set ebx as 1 (stdout!)
  mov $1, %ebx
  // set eax as $SYS_write
  mov $SYS_write, %eax
  // syscall!
  int $0x80


  // Do you want to exit at here?
  // set eax to be 1 ($SYS_exit)
  mov $1, %eax
  // syscall!
  int $0x80


  int   $0x80 /* syscall */

call_addr:
  call    pop_addr     /*  call popaddr, at the stack, it will store
                          the address of (call popaddr) + 5, which is the
                          address of STRING */
  .string STRING