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