HelloWorld.asm

 section .data
section .bss
str resb 200
section .text
global _start
_start:
; When you accept string, length automatically returns in RAX register
mov rax , 0
mov rdi , 0
mov rsi , str
mov rdx , 200
syscall
;str
;welcome
;rax  0000000000000008
; Display rax
mov rbx,rax ;store number in rbx
mov rdi , result ;point rdi to result variable
mov cx,16
;load count of rotation in cl
up1:
rol rbx,04 ;rotate number left by four bits
mov al,bl ;move lower byte in dl
and al,0fh
; get only LSB
cmp al,09h ;compare with 39h
jg add_37 ;if grater than 39h skip add 37
add al,30h
jmp skip ;else add 30
add_37 : add al,37h
skip: 
mov [rdi],al ;store ascii code in result variable
inc rdi ;point to next byte
dec cx
;decrement the count of digits to display
jnz up1 ;if not zero jump to repeat
dispmsg result,16 ;call to macro
ret

created 3 years ago by Mokshada Potdar

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About Assembly

Assembly language(asm) is a low-level programming language, where the language instructions will be more similar to machine code instructions.

Every assembler may have it's own assembly language designed for a specific computers or an operating system.

Assembly language requires less execution time and memory. It is more helful for direct hardware manipulation, real-time critical applications. It is used in device drivers, low-level embedded systems etc.

Syntax help

Assembly language usually consists of three sections,

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

Syntax

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 is a sub-routine which contains set of statements. Usually procedures are written when multiple calls are required to same set of statements which increases re-usuability and modularity.

procedure_name:
   ;procedure body
   ret