assembly.md

title	Assembly language
description	Assembly language cheatsheet gives you a quick reference to code syntax with examples makes it handy while coding.
created	2020-07-06
updated	2020-07-06

Sample program

section .data
	hello:     db 'Hello world!',10    ; 'Hello world!' plus a linefeed character
	helloLen:  equ $-hello             ; Length of the 'Hello world!' string

section .text
	global _start

_start:
	mov eax,4            ; The system call for write (sys_write)
	mov ebx,1            ; File descriptor 1 - standard output
	mov ecx,hello        ; Put the offset of hello in ecx
	mov edx,helloLen     ; helloLen is a constant, so we don't need to say
	                     ;  mov edx,[helloLen] to get it's actual value
	int 80h              ; Call the kernel
	mov eax,1            ; The system call for exit (sys_exit)
	mov ebx,0            ; Exit with return "code" of 0 (no error)
	int 80h;

Assembly language usually consists of three sections,

1. Data section

   To initialize variables and constants, buffer size these values doesn't change at runtime.

2. bss section

   To declare variables

3. text section

   _start specifies the starting of this section where the actually code is written.

Variables

There are various define directives to allocate space for variables for both initialized and uninitialized data.

1. To allocate storage space to Initialized data

variable-name    define-directive    initial-value 

Define Directive	Description	Allocated Space
DB	Define Byte	1 byte
DW	Define Word	2 bytes
DD	Define Doubleword	4 bytes
DQ	Define Quadword	8 bytes
DT	Define Ten Bytes	10 bytes

2. To allocate storage space to un-initialized data

Define Directive	Description
RESB	Reserve a Byte
RESW	Reserve a Word
RESD	Reserve a Doubleword
RESQ	Reserve a Quadword
REST	Reserve a Ten Bytes

Constants

Constants can be defined using

1. equ

• To define numeric constants

CONSTANT_NAME EQU regular-exp or value

2. %assign

• To define numeric constants.

%assign constant_name value

3. %define

• To define numeric or string constants.

%define constant_name value

Loops

Loops are used to iterate a set of statements for a specific number of times.

mov ECX,n
L1:
;<loop body>
loop L1

where n specifies the no of times loops should iterate.

Procedures

procedure_name:
   ;procedure body
   ret