How do I fix this assembly code? It’s not giving the correct second permutation and the parity of the sum.

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Write a program in Assembly in Emu8086 language that allows you to enter a four-digit number from the keyboard, determines and displays on the screen: 1) the number formed by permuting the second and third digits of the four digit number inputted. For example, from the number 5084 to get 5804; 2) the number formed by permuting the first two and last two digits of the four digit inputted number. For example, from the number 1234 : 1.) the first permutation will be 1324; 2.) the second permutation will be 3412; and also output information about the parity of the sum of its digits, without calculating the sum itself

What I have tried:

ASM

.model small
.stack 100h

.data
msg_input db 'Enter a four-digit number: $'
msg_perm1 db 10, 13, 'First permutation: $'
msg_perm2 db 10, 13, 'Second permutation: $'
msg_even db 'Even sum of digits', 10, 13, '$'
msg_odd db 'Odd sum of digits', 10, 13, '$'
num_buf db '0000$'

.code
main proc
    mov ax, @data
    mov ds, ax

    ; Read the four-digit number
    mov dx, offset msg_input
    mov ah, 9
    int 21h

    ; Input first digit
    mov ah, 1
    int 21h
    sub al, 30h
    mov bh, al

    ; Input second digit
    int 21h
    sub al, 30h
    mov bl, al

    ; Input third digit
    int 21h
    sub al, 30h
    mov ch, al

    ; Input fourth digit
    int 21h
    sub al, 30h
    mov cl, al

    ; Display the first permutation
    mov dx, offset msg_perm1
    mov ah, 9
    int 21h

    ; Permute the second and third digits
    mov ah, bl
    mov bl, ch
    mov ch, ah

    add bh, 30h
    add bl, 30h
    add ch, 30h
    add cl, 30h

    mov dx, offset num_buf
    mov [num_buf], bh
    mov [num_buf+1], bl
    mov [num_buf+2], ch
    mov [num_buf+3], cl
    mov ah, 9
    int 21h

    ; Permute the first and fourth digits
    mov ah, bh
    mov bh, cl
    mov cl, ah

    ; Display the second permutation
    mov dx, offset msg_perm2
    mov ah, 9
    int 21h

    mov dx, offset num_buf
    mov [num_buf], bh
    mov [num_buf+1], bl
    mov [num_buf+2], ch
    mov [num_buf+3], cl
    mov ah, 9
    int 21h

    ; Terminate the program
    mov ah, 4ch
    int 21h
main endp
end main

Posted 16-Apr-23 13:28pm

Chidumebi Belonwu

Updated 16-Apr-23 19:10pm

Dave Kreskowiak

v2

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Richard MacCutchan 17-Apr-23 4:19am

Why are you subtractibng and adding 30h from each character? You only need to process the characters, not their binary values. Also, you should store each character in a memory array, it would then be a simple matter to display the various permutations.

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OriginalGriff · Answer 1 · 2023-04-16T19:10:00

Assembling without error does not mean your code is right! :laugh:
Think of the development process as writing an email: assembling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

ASM

multiply:
    imul eax, eax 
    ret ;

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.

If you don't know how to use the debugger, then google will help you for your specific assembler system (it's different for ASM and GDB for example).

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!