CODEWITHSUNDEEP
assemblyprintingx86hex
Jeevesh Juneja
Jeevesh Juneja

Reputation: 212

What is wrong with this function (written in assembly) to print hexadecimal numbers?

I am only a beginner in assembly and was reading the OS book by Nick Blundell, when I came across the question of writing a function which could print hexadecimal number. But, despite verifying the logic multiple times , I can't seem to find why this code doesn't work. Please help, I would be grateful.


HEX_OUT:    db  '0x0000', 0
MASK:       dw  0b1111000000000000
COUNTER:    db  3

print_string :

    pusha                               ;SAVES ALL REGISTER VALUES TO BE RESTORED WHEN RETURNING.
    mov ah, 0x0e

    jmp print_loop                      ;NOT COMPULSORY

    print_loop :
        mov al, [bx]
        add bx, 1                       ;ADD 1, NOT 8, NOT 16.
        int 0x10
        cmp al, 0                       ;SETS A FLAG ACCORDING TO RESULT OF COMPARISON.
        jne print_loop                  ;CAUSES LOOP.
        jmp final_block                 ;CAN BE REPLACED BY THE STATEMENTS IN final_block, NO NEED FOR MAKING NEW LABEL.

    final_block :
        popa
        ret                             ;RETURNS TO THE POINT WHERE CALL HAPPENED.

print_hex :
    pusha

    mov bx, HEX_OUT
    add bx, 2

    alter_loop :                        ;LOOP TO ALTER HEX_OUT
        mov ax, [MASK]
        cmp ax, 0                       ;CONDITION TO END LOOP
        je after_loop

        mov ax, dx                      ;GETTING(INTO AX) THE DATA FOR N-TH POSITION 
        and ax, [MASK]

        mov cx, [COUNTER]

        shift_loop :
            cmp cx, 0
            je end_shift_loop
            shr ax, 4
            sub cx, 1
        end_shift_loop:

        cmp ax, 0x0009                       ;DO HEX->ALPHABET IF NUMBER IS GREATER THAN 9
        jle skip_hex_to_alphabet

        add ax, 39                       ;EQUIVALENT TO (sub ax, 48--- sub ax, 9 ---add ax, 96)

        skip_hex_to_alphabet :

        add ax, 48                      ;ADDING 48(ASCII OF 0), IS ALREADY SUBTRACTED IF N-TH NUMBER>9

        mov [bx], al                    ;STORING DATA IN LOCATION POINTED TO BY BX
        add bx, 1                       ;INCREMENT FOR LOOP
        mov ax, [MASK]                  ;CHANGING MASK
        shr ax, 4
        mov [MASK], ax

        mov ax, [COUNTER]               ;UPDATING COUNTER
        sub ax, 1
        mov [COUNTER], ax

        jmp alter_loop

    after_loop :
    mov bx, HEX_OUT
    call print_string    
    popa
    ret

Upon calling the function like :-

mov dx, 0x1fd6
call print_hex

It prints, 0xWGd0 instead of 0x1fd6 .

Upvotes: 0

Views: 235

Answers (1)

Margaret Bloom
Margaret Bloom

Reputation: 44146

You are missing a jump back to shift_loop and have wrongly declared the size of COUNTER.

Since you use mov cx, [COUNTER], COUNTER must be a word, fix it:

COUNTER:    dw  3

Finally, you are not correctly shifting the masked values. At the first iteration the and ax, [MASK] produces 0x1000 and in the shift_loop this is reduced to 0x0100 because it only iterates once.
Close the loop with a jump:

    shift_loop :
        cmp cx, 0
        je end_shift_loop
        shr ax, 4
        sub cx, 1
    jmp shift_loop

    end_shift_loop:

My two cents: I've been writing and reading assembly for more than two decades and your code managed to confuse me. I did not expect a hex printing routine to loop over a static mask and store the result in a static string. It's way too convoluted for the task given.
You can simply extract the nibbles with a variable shift counter decreased by four and a (constant) mask. You can then even use a 16 bytes lookup table to convert the nibble into a char, avoiding a branch.

Also, since you are programming for DOS, it is very worthwhile finding a copy of TASM online and use its debugger (td - Turbo Debugger). It's easy to use the wrong size for a variable and work with garbage, a debugger will immediately show you this.

If you like a concrete example, here's a simple implementation.

;AX = number to print in hex
hex:
  mov cx, 12              ;Shift counter, we start isolating the higher nibble (which starts at 12)
  mov bx, hexDigits       ;Lookup tables for the digitals
  mov dx, ax              ;We need a copy of the number and AX is used by the int10 service

.extract:
  mov si, dx              ;Make a copy of the original number so we don't lose it. Also we need it in SI for addressing purpose
  shr si, cl              ;Isolate a nibble by bringing it at the lower position
  and si, 0fh             ;Isolate the nibble by masking off any higher nibble

  mov al, [bx + si]       ;Transform the nibble into a digit (that's why we needed it in SI)
  mov ah, 0eh             ;You can also lift this out of the loop. It put it here for readability.
  int 10h                 ;Print it

  sub cx, 4               ;Next nibble is 4 bits apart
jnc .extract              ;Keep looping until we go from 0000h to 0fffch. This will set the CF

  ret

hexDigits db "0123456789abcdef"

Upvotes: 2

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