Assembly Language

Assembly language is a symbolic representation of machine language. It is therefore very 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 written for an x86_64 Linux system using the GNU Assembler (gas/as) syntax (which is the main assembler used in open source projects such as the Linux kernel, as well as the SPO600 course), using Linux Syscalls:

.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

Here is a similar program for a 32-bit x86 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

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 using Linux Syscalls:

.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

ARM (64-bit) - AArch64

This is also in GNU Assembler (gas/as) syntax using Linux Syscalls:

.text
.globl _start
_start:

	mov     x0, 1           /* file descriptor: 1 is stdout */
	adr     x1, msg   	/* message location (memory address) */
 	mov     x2, len   	/* message length (bytes) */
 
 	mov     x8, 64     	/* write is syscall #64 */
	svc     0          	/* invoke syscall */

	mov     x0, 0     	/* status -> 0 */
	mov     x8, 93    	/* exit is syscall #93 */
	svc     0          	/* invoke syscall */

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

6502

Here is the same "Hello World" program in 6502 assembler as used in the 6502 Emulator, using the ROM routines for output:

define SCINIT $ff81 ; initialize/clear screen
define CHROUT $ffd2 ; output character to screen

	JSR SCINIT	; clear screen
	LDY #$00	; set Y index to zero

loop:	LDA msg,Y	; get a character
	BEQ done	; quit if character is null
	JSR CHROUT	; output the character
	INY		; increment index
	JMP loop	; get next character

done:	BRK		; break (stop program)

msg:	
	DCB "H","e","l","l","o",$2C,$20
	DCB "W","o","r","l","d","!",$0d, $00

Resources

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