Assembly operations with inputed number

Question

I'm learning Assembly in University and even though I just started I decided to dig deeper and I made some cool programs so far. My latest one was an Encrypter/Decrypter. It works by reversing the string you want to encrypt and then increase every character by a given value, like Caesar Cipher, only reversed.

My problem occurs when I ask the user for the value to add to the characters. The user inputs the value but it gets stored as hexadecimal and I cannot do the arithmetic operations correctly.

I'm using Emu8086 because that's what we use in classes.

Any help will be very apreciated. Below is the full program, feel free to suggest tips on how to improve it since I'm only a beginner.

#make_COM#

org 100h

.data

    EorD db 'Choose what you wish to do (E)ncrypt/(D)ecrypt: $'

    Einput db 'Enter string to encrypt: $'
    Dinput db 'Enter string to decrypt: $'

    value db 'Enter value: $'

    output db 'Your original string is: $'

    Eoutput db 'Your encrypted string is: $'
    Doutput db 'Your decrypted string is: $'

    copying db 'Copying string...$'
    encrypting db 'Encrypting string...$'
    decrypting db 'Decrypting string...$'

    done db 'done$'

    line db 13, 10, '$'

    str db 80 dup(?)
    newStr db 80 dup(?)

.code

    lea si, str
    lea bp, str
    lea di, newStr

    mov ah, 9
    lea dx, EorD
    int 21h

    mov ah, 1

    begining:

        int 21h

        cmp al, 'E'
        jz encryptor

        cmp al, 'e'
        jz encryptor

        cmp al, 'D'
        jz decryptor

        cmp al, 'd'
        jz decryptor

        call delete

        jmp begining

    delete:

        mov ah, 2
        mov dx, 8   ;backspace
        int 21h

        mov dx, 32  ;space
        int 21h

        mov ah, 2
        mov dx, 8   ;backspace
        int 21h

        mov ah, 1

        ret

    encryptor:

        mov ah, 9
        lea dx, line
        int 21h
        int 21h

        lea dx, Einput
        int 21h

        call inputStr

        call getValue

        lea dx, encrypting
        int 21h

        Estart:

            cmp [di], '$'
            jz Edone

            add [di], bx

            inc di

            jmp Estart

        Edone:

            lea dx, done
            int 21h

            lea dx, line
            int 21h
            int 21h

            lea dx, output
            int 21h

            lea dx, str
            int 21h

            lea dx, line
            int 21h
            int 21h

            lea dx, Eoutput
            int 21h

            lea dx, newStr
            int 21h

            jmp finish

    decryptor:

        mov ah, 9
        lea dx, line
        int 21h
        int 21h

        lea dx, Dinput
        int 21h

        call inputStr

        call getValue

        lea dx, decrypting
        int 21h

        Dstart:

            cmp [di], '$'
            jz Ddone

            sub [di], bl

            inc di

            jmp Dstart

        Ddone:

            lea dx, done
            int 21h

            lea dx, line
            int 21h
            int 21h

            lea dx, Eoutput
            int 21h

            lea dx, str
            int 21h

            lea dx, line
            int 21h
            int 21h

            lea dx, Doutput
            int 21h

            lea dx, newStr
            int 21h

            jmp finish

    inputStr:

        mov ah, 1

        getChar:

            int 21h

            cmp al, 13
            jz endStr

            mov [si], al
            inc si

            jmp getChar

        endStr:

            mov [si], '$'

            dec si    ;getting ready to reverse  

            ret

    getValue:

        mov ah, 9
        lea dx, line
        int 21h
        int 21h

        lea dx, value
        int 21h

        mov ah, 1

        checkNum:

            int 21h

            cmp al, 0x30
            jbe deleteNum

            cmp al, 0x39
            ja deleteNum

            jmp isNum

        deleteNum:

            call delete

            jmp checkNum

        isNum:

            mov ah, 0 
            int 16h

            mov bl, al

            ;sub bx, 30h

            mov ah, 9
            lea dx, line
            int 21h
            int 21h

            lea dx, copying
            int 21h

            call copyFunc

            lea dx, line
            int 21h

            ret

    copyFunc:     ;automatically reverses while copying

        cmp [bp], '$'
        jz endCopy

        mov al, [si]
        mov [di], al

        dec si
        inc bp
        inc di

        jmp copyFunc

        endCopy:

            mov [di], '$'
            lea di, newStr

            lea dx, done
            int 21h

            ret

    finish:

        mov ah, 4ch
        int 21h 

Answer 1

A simple algorithm would look like this (pseudo code):

readText()
value = 0

loop:
value = value * 10
value = value + (getNextCharacter() - 0x30)
if (charactersLeft() > 0) goto loop

You either need to know how long the input text is or you need to terminate it with a special character (like the NULL-byte, as is usually done in C).

The "magic" is that the digits 0, 1, …, 9 are represented by the byte values 0x30, 0x31, …, 0x39 in ASCII. So simply calculating byteValue - 0x30 gives you the represented digit.

You also might want to check whether the character you're processing is within the range of 0x30 (0) and 0x39 (9). If not, abort.