printing output in assembly x86

Question

I have this code:

INCLUDE Irvine32.inc

.data
    arry BYTE ?
    prompt1 BYTE "Enter first hex number: ",0
    prompt2 BYTE "Enter second hex number: ",0
    prompt3 BYTE "The sum is ",0
    prompt4 BYTE "The sum is out of range ",0
    prompt5 BYTE "Convert again? [y/n]: ",0
    prompt6 BYTE "First number is invalid ",0
    prompt7 BYTE "Second number is invalid ",0

.code
main PROC
    ReadInput:
        L1:
            mov edx, OFFSET prompt1
            call writeString
            mov edx, 0
            call readHex
            call Crlf
            mov ecx, eax
            jmp L3
        L2:
            mov eax, 0
            mov edx, OFFSET prompt2
            call writeString
            mov edx, 0
            call readHex
            call Crlf
            mov ebx, eax
            mov eax, 0
            jmp L4
        L3:
                cmp ecx, 0FFFFh
                JA L5
                JBE L2
        L4:
                cmp ebx, 0FFFFh
                JA  L6
                JBE addInt
        L5:
                mov edx, OFFSET prompt6
                call writeString
                            mov edx, 0
                call Crlf
                jmp L1
        L6:
                mov edx, OFFSET prompt7
                call writeString
                mov edx, 0
                call Crlf
                jmp L2

    addInt:
        clc
        mov ax, cx
        add ax, bx
        JC printError
        jmp convert

; convert hex to string
    convert:
           mov ecx, 0
           mov esi, 0
            mov si, 4
            mov cx, 10h

    convertDigit:
            dec si
            mov dx , 0
            div cx
            cmp dx, 9h
            JA  convertLetter
            add dx, 30h
            jmp printSucess

    convertLetter:      
            add dx, 37h
            jmp printSucess

    printError:
            mov edx, OFFSET prompt4
            call writeString
            call Crlf

    printSucess:
            mov arry[si], dl
            cmp ax , 0
            JNE convertDigit
            mov edx, OFFSET arry
            add dx, si
            call writeString
            call Crlf


    exit
main ENDP
END main

when I tried to print output i get this

Enter first hex number: ff

Enter second hex number: ff

1FEer first hex number: Press any key to continue . . .

as you can see, there was a part of prompt1, er first hex number which stuck with the value of the sum 1FE, why did this happen

the program loop edx register 3 times first time the value is E, the second time is F, and the third time is 1, then it get to writeString to print output, which seems to be what the program suppose to be doing, at this point the edx value suddenly jump to 00405911, why did this happen?

thank in advance for the help

Answer 1

Add a NULL-terminator to the converted string. You could do that right before the convertDigit loop:

mov byte ptr [arry+4],0

By the way, you don't need div when you're dividing by a power of two (like 16). You can use the faster bitwise AND and shift instructions:

mov dx,ax
and dx,15   ; dx = ax & 15 (== ax % 16)

shr ax,4    ; ax = ax >> 4 (== ax / 16)