An Assembly program to exchange two memory variables using XCHG instruction

DATA SEGMENT
     NUM1 DB 9H
     NUM2 DB 7H
ENDS
CODE SEGMENT 
    ASSUME DS:DATA CS:CODE
START:
      MOV AX,DATA
      MOV DS,AX
     
      MOV AL,NUM1
      MOV BL,NUM2
        
      XCHG AL,NUM2
      XCHG BL,NUM1 
     
      MOV AH,4CH
      INT 21H     
ENDS
END START

Explanation : 
In this Assembly Language Programming, A single program is divided into four Segments which are 1. Data Segment, 2. Code Segment, 3. Stack Segment, and 4. Extra  Segment. Now, from these one is compulsory i.e. Code Segment if at all you don’t need variable(s) for your program.if you need variable(s) for your program you will need two Segments i.e. Code Segment and Data Segment.

CODE SEGMENT

CODE SEGMENT is the starting point of the Code Segment in a Program and CODE is the name given to this segment and SEGMENT is the keyword for defining Segments, Where we can write the coding of the program.

ASSUME DS:DATA CS:CODE
In this Assembly Language Programming, their are Different Registers present for Different Purpose So we have to assume DATA is the name given to Data Segment register and CODE is the name given to Code Segment register (SS,ES are used in the same way as CS,DS )

START:

START is the label used to show the starting point of the code which is written in the Code Segment. : is used to define a label as in C programming.

Next Line – MOV AX,DATA
MOV DS,AX


After Assuming DATA and CODE Segment, Still it is compulsory to initialize Data Segment to DS register.  MOV is a keyword to move the second element into the first element. But we cannot move DATA Directly to DS due to MOV commands restriction, Hence we move DATA to AX and then from AX to DS. AX is the first and most important register in the ALU unit. This part is also called INITIALIZATION OF DATA SEGMENT and It is important so that the Data elements or variables in the DATA Segment are made accessable. Other Segments are not needed to be initialized, Only assuming is enhalf.

MOV AL,NUM1
MOV BL,NUM2

The above two line code is used to move the two variables i.e. NUM1 and NUM2 to AL and BL registers respectively.

XCHG AL,NUM2
XCHG BL,NUM1

The above two line code is used to Exchange NUM2 variable to AL registers and to Exchange NUM1 variable to BL registers respectively.
Now, we have understood part of it to Exchange to number we can write XCHG NUM1, NUM2, But there is no permutation for XCHG memory, memory, Hence we have to send one memory variable to AL or AX depending on DB or DW and send another one memory variable to BL or BX depending on DB or DW. Now we are taking DB, So we have t0 instruction XCHG AL,NUM2 Exchange NUM2 variable value to AL Register in which value of NUM1 variable is already present XCHG BL,NUM1 Exchange NUM1 variable value to BL Register in which value of NUM2 variable is already present. Finally we have Exchanged NUM1 with NUM2 memory variables.

MOV AH,4CH
INT 21H

The above two line code is used to exit to dos or exit to operating system. Standard Input and Standard Output related Interupts are found in INT 21H which is also called as DOS interrupt. It works with the value of AH register, If the Value is 4ch, That means Return to Operating System or DOS which is the End of the program.

CODE ENDS


CODE ENDS is the End point of the Code Segment in a Program. We can write just ENDS But to differentiate the end of which segment it is of which we have to write the same name given to the Code Segment.

END START


END START is the end of the label used to show the ending point of the code which is written in the Code Segment.