Assembly Language

Assembly language is a symbolic representation of machine language. It is therefore architecture-specific.

Each instruction is represented by a short mnemonic word such as "LDR" for Load Register, "MOV" for move, or "MUL" for multiply, followed by (optional) arguments. The addressing mode is implied by the format of the arguments. Different assemblers use slightly different syntax.

Examples

x86

Here is a "Hello, World!" program in x86 assembler for a Linux system, using the Nasm syntax:

section    .text
global    _start

_start:
    mov    edx,len          ; message length (bytes)
    mov    ecx,msg          ; message location (memory address)
    mov    ebx,1            ; file descriptor: 1 is stdout
    mov    eax,4            ; kernel syscall number: 4 is sys_write
    int    0x80             ; invoke syscall

    mov    ebx,0            ; exit status: 0 (good)
    mov    eax,1            ; kernel syscall number: 1 is sys_exit
    int    0x80             ; invoke syscall

section    .rodata

msg    db    'Hello, world!\n'
len    equ    $ - msg

Here is the same program with GNU Assembler (gas/as) syntax:

.text
.globl _start
_start:
    mov $len, %edx     /* file descriptor: 1 is stdout */ 
    mov $msg, %ecx     /* message location (memory address) */
    mov $1, %ebx       /* message length (bytes) */
    mov $4, %eax       /* write is syscall #4 */
    int $0x80          /* invoke syscall */

    mov $0, %ebx       /* exit status: 0 (good) */
    mov $1, %eax       /* kernel syscall number: 1 is sys_exit */
    int $0x80          /* invoke syscall */

.data
msg:
    .ascii "Hello, World!\n"
    len = . - msg

Notice that the order of the arguments in some lines is reversed between the two assemblers, and the prefixes to symbols and values also change.

ARM (32-bit)

This is written in GNU Assembler (gas/as) syntax:

.text
.globl _start
_start:

     mov     %r0, $1     /* file descriptor: 1 is stdout */
     ldr     %r1, =msg   /* message location (memory address) */
     ldr     %r2, =len   /* message length (bytes) */
     mov     %r7, $4     /* write is syscall #4 */
     swi     $0          /* invoke syscall */

     mov     %r0, $0     /* exit status: 0 (good) */
     mov     %r7, $1     /* kernel syscall number: 1 is sys_exit */
     swi     $0          /* invoke syscall */

.data
msg:
     .ascii      "Hello, world!\n"
     len = . - msg

Resources

• Assembly Basics
• "Hello World" in many different types of assembler
• x86_64 Register and Instruction Quick Start
• aarch64 Register and Instruction Quick Start