Q85068: Displaying Data for Debugging in MASM

Article: Q85068
Product(s): Microsoft Macro Assembler
Version(s): MS-DOS:5.x,6.0,6.0a,6.0b
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 5.x, 6.0, 6.0a, 6.0b 
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SUMMARY
=======

It is sometimes helpful to be able to find out the contents of a register or
memory location without using a debugger. The following two routines will print
the value of the AX register to the screen in hexadecimal. The first routine
accomplishes this by calling MS-DOS Interrupt 21h. The second routine
accomplishes the same thing by using BIOS Interrupt 10h. Depending on what the
program is doing, one of these routines may be more suitable than the other.

MORE INFORMATION
================

Sample Code 1
-------------

  ; Assemble options needed: none

  .MODEL small

  .STACK

  .CODE

  Start:
            mov ax, 0FFFFh     ; Load AX with something distinctive.
            call print_word    ; Print the contents of the AX register.

            mov ax, 4c00h      ; Prepare to exit.
            int 21h            ; Exit.

  print_word PROC              ; MS-DOS version.
            push dx            ; Save all registers that are used.
            push cx
            push bx
            push ax

            mov dx, 4          ; Loop will print out 4 hex characters.
  nexthex:
            push dx            ; Save the loop counter.
            mov cl, 4          ; Rotate register 4 bits.
            rol ax, cl
            push ax            ; Save current value in AX.

            and al, 0Fh        ; Mask off all but 4 lowest bits.
            cmp al, 10         ; Check to see if digit is 0-9.
            jl decimal         ; Digit is 0-9.
            add al, 7          ; Add 7 for Digits A-F.
  decimal:
            add al, 30h        ; Add 30h to get ASCII character.

            mov dl, al
            mov ah, 02h        ; Prepare for interrupt.
            int 21h            ; Do MS-DOS call to print out value.

            pop ax             ; Restore value to AX.
            pop dx             ; Restore the loop counter.
            dec dx             ; Decrement loop counter.
            jnz nexthex        ; Loop back if there is another character
                               ; to print.

            pop ax             ; Restore all registers that were used.
            pop bx
            pop cx
            pop dx
            ret
  print_word ENDP

  END Start

Sample Code 2
-------------

  ; Assemble options needed: none

  .MODEL small

  .STACK

  .CODE

  Start:
            mov ax, 0FFFFh     ; Load AX with something distinctive.
            call print_word    ; Print the contents of the AX register.

            mov ax, 4c00h      ; Prepare to exit.
            int 21h            ; Exit.

  print_word PROC              ; BIOS version.
            push dx            ; Save all registers that are used.
            push cx
            push bx
            push ax

            mov dx, 4          ; Loop will print out 4 hex characters.
  nexthex:
            mov cl, 4          ; Rotate register 4 bits.
            rol ax, cl
            push ax            ; Save current value in AX.

            and al, 0Fh        ; Mask off all but 4 lowest bits.
            cmp al, 10         ; Check to see if digit is 0-9.
            jl decimal         ; Digit is 0-9.
            add al, 7          ; Add 7 for Digits A-F.
  decimal:
            add al, 30h        ; Add 30h to get ASCII character.

            mov bx, 0003h      ; BH= Page 0, BL= Foreground Color.
            mov ah, 0Eh        ; Prepare for interrupt.
            int 10h            ; Do BIOS call to print out value.

            pop ax             ; Restore value to AX.
            dec dx             ; Decrement loop counter.
            jnz nexthex        ; Loop back if there is another character
                               ; to print.

            pop ax             ; Restore all registers that were used.
            pop bx
            pop cx
            pop dx
            ret
  print_word ENDP

  END Start

Additional query words: 5.00 5.10 5.10a 6.00 6.00a 6.00b

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM600a kbMASM600b
Version           : MS-DOS:5.x,6.0,6.0a,6.0b

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