参考 x86assembly, 汇编语言格式有 NASM、MASM、gas 等。

安装

$ sudo apt install nasm
$ sudo apt install gcc

gas

gcc

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-linux-gnu/9/lto-wrapper
Target: i686-linux-gnu
Thread model: posix
gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)

参考 gas example

.global _main
.text
_main:
        # write(1, message, 13)
        mov     $1, %eax                # system call 1 is write
        mov     $1, %edi                # file handle 1 is stdout
        mov     $message, %esi          # address of string to output
        mov     $13, %edx               # number of bytes
        syscall                         # invoke operating system to do the write

        # exit(0)
        mov     $60, %eax               # system call 60 is exit
        xor     %edi, %edi              # we want return code 0
        syscall                         # invoke operating system to exit
message:
        .ascii  "Hello, world\n"

NASM

https://www.tutorialspoint.com/assembly_programming/assembly_basic_syntax.htm

Hello

新建 hello.asm

section	.text
   global _start     ;must be declared for linker (ld)

_start:	            ;tells linker entry point
   mov	edx,len     ;message length
   mov	ecx,msg     ;message to write
   mov	ebx,1       ;file descriptor (stdout)
   mov	eax,4       ;system call number (sys_write)
   int	0x80        ;call kernel

   mov	eax,1       ;system call number (sys_exit)
   int	0x80        ;call kernel

section	.data
msg db 'Hello, world!', 0xa  ;string to be printed
len equ $ - msg     ;length of the string

# 编译
$ nasm -f elf hello.asm

# 连接
$ ld hello.o
# $ ld -m elf_i386 -s -o dt dt.o

# 运行
$ ./a.out

DT

section	.text
	global _start
_start:
	SGDT	[gdtcopy]
	SLDT	[ldtcopy]

section	.data
gdtcopy times 16 db 'x'
ldtcopy times 16 db 'x'

$ nasm -f elf dt.asm
$ ld dt.o
$ gdb ./a.out


(gdb) layout asm
(gdb) starti
(gdb) si
(gdb) x/32x 0x804a000

0xff    0x00    0x00    0x40    0x7c    0xf6

References

• https://cs.lmu.edu/~ray/notes/x86assembly/