好玩的东西——一个代码片段

#hello.s 
.data                    # 数据段声明
        msg : .string "Hello, world!\n" # 要输出的字符串
        len = . - msg                   # 字串长度
.text                    # 代码段声明
.global _start           # 指定入口函数
        
_start:                  # 在屏幕上显示一个字符串
        movl $len, %edx  # 参数三：字符串长度
        movl $msg, %ecx  # 参数二：要显示的字符串
        movl $1, %ebx    # 参数一：文件描述符(stdout) 
        movl $4, %eax    # 系统调用号(sys_write) 
        int  $0x80       # 调用内核功能
        
                         # 退出程序
        movl $0,%ebx     # 参数一：退出代码
        movl $1,%eax     # 系统调用号(sys_exit) 
        int  $0x80       # 调用内核功能

控制台的操作：

wangck@ubuntu:~$ as -o hello.o hello.s
wangck@ubuntu:~$ ld -s -o hello hello.o
wangck@ubuntu:~$ ./hello 
Hello, world!
wangck@ubuntu:~$ vim hello.o
wangck@ubuntu:~$ ld
ld: no input files
wangck@ubuntu:~$ objdump -s -d hello.o

hello.o:     file format elf64-x86-64

Contents of section .text:
 0000 ba100000 00b90000 0000bb01 000000b8  ................
 0010 04000000 cd80bb00 000000b8 01000000  ................
 0020 cd80                                 ..              
Contents of section .data:
 0000 48656c6c 6f2c2077 6f726c64 215c6e00  Hello, world!
.

Disassembly of section .text:

0000000000000000 <_start>:
   0:	ba 10 00 00 00       	mov    $0x10,%edx
   5:	b9 00 00 00 00       	mov    $0x0,%ecx
   a:	bb 01 00 00 00       	mov    $0x1,%ebx
   f:	b8 04 00 00 00       	mov    $0x4,%eax
  14:	cd 80                	int    $0x80
  16:	bb 00 00 00 00       	mov    $0x0,%ebx
  1b:	b8 01 00 00 00       	mov    $0x1,%eax
  20:	cd 80                	int    $0x80
wangck@ubuntu:~$