Displaying Data for Debugging in MASM

• Microsoft Macro Assembler for MS-DOS, versions 5.x, 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 5.x, 6.0, 6.0a, and 6.0b

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.

          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.

          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

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.

          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.

          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

END Start