Question

I have the following assignment:

Write a complete 8086 program to perform the calculator functions: ADD/SUB/DIV/MUL.
When the user presses “=” your program should display the result. Only the numbers from 0-9 are input.

    ones  db ?
    tens  db ?

    mov   ah,1
    int   21h
    add   al,30H
    mov   tens, al
    mov   ah,1       
    int   21H 
    mov   dl,al
    cmp   dl, '+'
    je    addition

addition:
    mov   ah,1    
    int   21h
    mov   bl,al   
    mov   ones,al

I have to do it by adding 30h to each number then subtracting it. Can someone explain how I can do this?

Answer 1

0x30 is the 0 sign in ASCII so if you want to print a number between 0-9 you should add 0x30 to the value to make it an ‘0’ character. If you read from the terminal you would read 0x30 instead of 0 so if you want to make calculation with it you would need to subtract 0x30 to convert an ‘0’ character to the value 0

http://en.wikipedia.org/wiki/ASCII