HelloWorld.asm

 DATA SEGMENT
    MSG1 DB 10,13,'ENTER ANY STRING :- $'
    MSG2 DB 10,13,'LENGTH OF STRING IS :- $'
    MSG3 DB 10,13,'REVERSE OF ENTERED STRING IS :- $' 
   
    P1 LABEL BYTE
    M1 DB 0FFH
    L1 DB ?
    P11 DB 0FFH DUP ('$')
    P22 DB 0FFH DUP ('$')
   
DATA ENDS

CODE SEGMENT
    ASSUME CS:CODE,DS:DATA
START:
    MOV AX,DATA
    MOV DS,AX                

    ; Display message to enter a string
    MOV AH,9
    LEA DX,MSG1
    INT 21H

    ; Input a string
    LEA DX,P1
    MOV AH,0AH    
    INT 21H

    ; Display message for the length of the string
    MOV AH,9
    LEA DX,MSG2
    INT 21H

    ; Display the length of the string
    MOV DL,L1
    ADD DL,30H
    MOV AH,2
    INT 21H

    ; Display message for the reversed string
    MOV AH,9
    LEA DX,MSG3
    INT 21H

    ; Reverse the string
    LEA SI,P11
    LEA DI,P22

    MOV DL,L1
    DEC DL
    MOV DH,0
    ADD SI,DX
    MOV CL,L1
    MOV CH,0

REVERSE:
    MOV AL,[SI]
    MOV [DI],AL
    INC DI
    DEC SI
    LOOP REVERSE

    ; Display the reversed string
    LEA SI,P22
    MOV AH,9
    INT 21H

    ; Exit the program
    MOV AH,4CH
    INT 21H

CODE ENDS
END START

created 11 months ago

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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