Microsoft KB Archive/35968

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CALL SetUEvent in Assembly Routine to Disable PRINT SCREEN

Article ID: 35968

Article Last Modified on 11/21/2006

This article was previously published under Q35968


This article provides an example of calling Basic's SetUEvent routine from an assembly-language interrupt routine to disable the screen dump performed by the PRINT SCREEN key (found on enhanced IBM keyboards) or the SHIFT+PRSCR or SHIFT+PRTSC key (found on earlier, nonenhanced IBM PC keyboards).

Special steps must be taken when triggering Basic's user-defined events from assembly-language interrupt routines. Primarily, the assembly routine needs to make sure that the DS register points to Basic's default data segment before making the call to SetUEvent. Also, the interrupt routine should be disabled before, and re-enabled after, a CHAIN or RUN statement. This information applies to Microsoft QuickBasic Versions 4.00b and 4.50, to Microsoft Basic Compiler Versions 6.00 and 6.00b for MS-DOS and MS OS/2, and to Microsoft Basic PDS Version 7.00 for MS-DOS and MS OS/2.


Note: The SetUEvent routine is not provided in QuickBasic Version 4.00 or earlier. SetUEvent was introduced in QuickBasic Version 4.00b and the Basic compiler Version 6.00 and later.

Basic user-defined interrupts are triggered by a call to SetUEvent. This routine can be called from an external assembly routine, usually an interrupt handler. However, in QuickBasic 4.00b, in the Basic compiler 6.00 and 6.00b, and in Basic PDS 7.00, SetUEvent assumes that DS is pointing to Basic's default data segment; if it isn't, the call to SetUEvent will not trigger the Basic interrupt routine. To ensure that DS is pointing to the correct place, save the value of DS in a location accessible to the interrupt routine when installing it. Then, in the interrupt routine, place that value in DS directly before making the call to SetUEvent. In QuickBasic Version 4.50, you don't have to set DS equal to the Basic Data Segment.

When CHAINing or RUNning other Basic programs that use the same interrupt routine, the first Basic program must deinstall the interrupt handler, CHAIN (or RUN) the second program, and reinstall the handler again. The sample program below demonstrates how to install, deinstall, and properly CALL SetUEvent in an interrupt handler situation.

The following is a Basic code example:

PRINT "Screen Dump interrupt has been disabled."
PRINT "Press PRINT SCREEN (or SHIFT+PRTSC) key to trigger event."
PRINT "Press any other key to end program."
WHILE i$ = ""
  i$ = INKEY$
PRINT "Screen Dump interrupt has been re-enabled."
  PRINT "Screen dumps are currently disabled."

The following is an assembly-language program that redefines the interrupt that occurs when you press the PRINT SCREEN (or SHIFT+PRTSC) key:

cr      equ     0dh
lf      equ     0ah
.model  medium
        extrn   SetUEvent:proc
        public  prscr

tsr_active      db      0
old_seg         dw      ?
old_off         dw      ?
bds             dw      ?

handler proc    far
      push      ax
      push      bx
      push      cx
      push      dx
      push      si
      push      di
      push      bp
      push      ds
      push      es                  ;save regs
        cmp     cs:tsr_active, byte ptr 0  ;are we already up?
        je      do_int                  ;no, go display message
        jmp     ex_int                  ;yes, get out!

do_int: mov     cs:tsr_active, byte ptr 1  ;okay, we're up now...
        mov     ax,bds
        mov     ds,ax                   ;set DS = Basic's data segment
        call    SetUEvent               ;trigger the user event
        mov     cs:tsr_active, 0        ;re-activate interrupt

ex_int: pop     es
        pop     ds
        pop     bp
        pop     di
        pop     si
        pop     dx
        pop     cx
        pop     bx
        pop     ax
        cli             ; CLI corresponds to STI at Handler beginning.
handler endp

prscr   proc    far
        push    bp
        mov     bp, sp
        push    ds
        push    es
        mov     cs:bds,ds               ;put Basic's DS here
        mov     ax, 3505h               ;get interrupt vector
        int     21h                     ;for PrScr
        mov     cs:old_seg,es
        mov     cs:old_off,bx           ;save it
        mov     dx, seg handler
        mov     ds,dx
        mov     dx, offset handler      ;reset interrupt 5
        mov     ax, 2505h               ;to point to handler
        int     21h
        pop     es
        pop     ds
        pop     bp
        ret                             ;back to Basic
prscr   endp

        public  deinst
deinst  proc    far                ;to de-install our Prscr interrupt
        push    bp
        mov     bp,sp
        push    ds
        mov     dx,cs:old_seg
        mov     ds, dx
        mov     dx,cs:old_off           ;get old INT 5 vector
        mov     ax, 2505h
        int     21h                     ;and restore it
        pop     ds
        pop     bp
deinst  endp

Additional query words: QuickBas BasicCom

Keywords: KB35968