Question

Analyze this assembly program (emu 8086) line by line! and explain the purpose of the program!

PROGRAM:

.model small
.stack 100h
.data
   num1 db ?
   num2 db ?
   num3 db ?

   MSG1 DB 10,13,"ENTER FIRST NUMBER : $"
   MSG2 DB 10,13,"ENTER SECOND NUMBER: $"  
   MSG3 DB 10,13,"ENTER SECOND NUMBER: $"  
   MSG4 DB 10,13,"SMALLER NUMBER IS : $"
   avg db ?
ends
.code
.start
   main proc
       mov ax,data
       mov ds,ax
       lea dx,msg1

       mov ah,9
       int 21h
       mov ah,1
       int 21h
       mov num1,al
       lea dx,msg2
       mov ah,9
       int 21h
       mov num2,al
       lea dx,msg3
       mov ah,9
       int 21h

       mov al,num1
       mov al,num2
       mov al,num3
      
       mov ah,0
       mov dl,3
   `   div dl

       mov avg,al
       mov ah,4ch
       int 21h
   main endp
end main

Answer 1

The main objective of the program is

reading the three data from the command line by using three messages

after reading the data addition is performed then divide by 3 is done to find an average

finally display the average of three numbers

model small
.stack 100h
.data ; data definitions
   num1 db ? - num1 is temproray variable of type byte. db-means define byte
   num2 db ?    - num1 is temproray variable of type byte.  
   num3 db ?    - num1 is temproray variable of type byte.

   MSG1 DB 10,13,"ENTER FIRST NUMBER : $" -MSG1, MSG2,MSG3,MSG4 are messages or string of characters.
   MSG2 DB 10,13,"ENTER SECOND NUMBER: $"  
   MSG3 DB 10,13,"ENTER SECOND NUMBER: $"  
   MSG4 DB 10,13,"SMALLER NUMBER IS : $"
   avg db ? - avg is temproray variable of type byte.
ends
.code
.start
   main proc - procedure declaration
       mov ax,data -  set up data segment
       mov ds,ax - setup data segment
       lea dx,msg1 - msg1 will be loaded to dx register

mov ah,9 - to display the messgae 1
       int 21h
       mov ah,1
       int 21h
       mov num1,al data read externally or in commandline is moved to num1 -8 bit register   
       lea dx,msg2 msg2 will be loaded to dx register
       mov ah,9 - to display the messgae 2
       int 21h
       mov num2,al - data read externally or in commandline is moved to num2 -8 bit register
       lea dx,msg3   msg3 will be loaded to dx register

       mov ah,9 -to display the message

       int 21h   

       mov al,num1 - Data available in num1 transfered into al register
       mov al,num2 -Data available in num2 transfered into al register
       mov al,num3 -Data available in num3 transfered into al register
      
       mov ah,0 - zero value transferred to ah register to set ah register as zero
       mov dl,3 - to average data 3 is transferred to dl register
   `   div dl - divide AX register by dl register- this instruction used to take an average of three number

       mov avg,al - al register contain an average of three number is transferred to avg variable
       mov ah,4ch - terminate program
       int 21h
   main endp -terminate the procedure
end main

Note: some of the instructions in the program is missing

Reading of third message is missing

mov al, num1

mov al, num2

mov al, num3

the above instruction overwrites the al register

instead, add instruction should be used to add the three numbers and further div instruction performs average of three number

Display of message 4 is missing