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MASM convert string to integer: processing invalid input

I have a program that converts a string to integers. The program works well as long as the user inputs valid numbers. However, once I added some error checking to ensure that the values entered are actually numbers, I ran into some hitches. Initially, I had the tryAgain tag as the first line of code in the getData procedure. However, the error checking meant that following an invalid input when valid numbers are entered during the next iteration, the program kept returning the message "invalid input". Putting the tryAgain tag in the first line of code causes the program to keep returning the invalid input message even when valid input follows invalid input. First, I attempted to put the tryAgain tag in its current position, but I think it's running on an infinite loop. Second, I attempted to reset variables if the program jumps to invalidInput, but that hasn't fixed it (I tried this with tryAgain in its original position and current position).

This is MASM for x86 processors. Thanks in advance for your suggestions.

Here's the code:

.data
result  DWORD   ?
temp        BYTE        21 DUP(0)
answer  DWORD   ?

.code
main    PROC

(some preliminary procedure calls)

push OFFSET temp        ;ebp+16
push OFFSET answer      ;ebp+12
push answerSize     ;ebp+8
call    getData

(more procedure calls)

exit        ; exit to operating system
main ENDP


;*************************************************
; prompts / gets the user’s answer.
; receives: the OFFSET of answer and temp and value of answerSize
; returns: none
; preconditions: none
; registers changed:  eax, ebx, ecx, edx, ebp, esi, esp
;*************************************************
getData PROC
push        ebp
mov     ebp,esp

tryAgain:
mWriteStr   prompt_1
mov     edx, [ebp+16]       ;move OFFSET of temp to receive string of integers
mov     ecx, 12
call        ReadString
cmp     eax, 10
jg      invalidInput

mov     ecx, eax        ;loop for each char in string
mov     esi,[ebp+16]    ;point at char in string

pushad
loopString:             ;loop looks at each char in string
    mov     ebx,[ebp+12]
    mov     eax,[ebx]   ;move address of answer into eax
    mov     ebx,10d     
    mul     ebx         ;multiply answer by 10
    mov     ebx,[ebp+12]    ;move address of answer into ebx
    mov     [ebx],eax       ;add product to answer
    mov     al,[esi]        ;move value of char into al register
    inc     esi         ;point to next char
    sub     al,48d      ;subtract 48 from ASCII value of char to get integer  

    cmp     al,0            ;error checking to ensure values are digits 0-9
    jl      invalidInput
    cmp     al,9
    jg      invalidInput

    mov     ebx,[ebp+12]    ;move address of answer into ebx
    add     [ebx],al        ;add int to value in answer

    loop        loopString  
popad
jmp     moveOn
invalidInput:               ;reset registers and variables to 0
    mov     al,0
    mov     eax,0
    mov     ebx,[ebp+12]
    mov     [ebx],eax
    mov     ebx,[ebp+16]
    mov     [ebx],eax       
    mWriteStr   error
    jmp     tryAgain
moveOn:
    pop     ebp
    ret     12
getData ENDP

Just so you have a sense of where I'm going with this, here's my pseudocode:

  1. Start at the beginning of the string

  2. Multiply the value of answer by 10.

  3. Split each character off the string and subtract by 48d to get the integer. Ex. student enters 156. 49 is stored as the first char in the variable temp. Subtract 48 from 49. The integer is 1.

  4. Add integer to value of answer.

  5. Inc esi (move one character right).

  6. Loop.

Solution

  • Get rid of the pushad and popad. Every time and invalid character is inputed, it jumps back to tryAgain then another pushad gets executed - each time you jump back to tryAgain.

    If you need to save any registers, do it in the prologue and restore them in the epilogue.