On Wednesday, October 14, 1998 at 3:00:00 AM UTC-4, Oliver Kreylos wrote:
> device wrote:
> >
> > I think there are ways to reprogram the clock to generate more higher
> > resolution tics.
> >
> > Dave
> Yes, there are. I am using a custom timer resolution in my game, here's
> what you have to do to
> a) set the timer to 1ms resolution
> b) keep the DOS clock going correctly nonetheless
> Just reprogramming the timer chip without some book-keeping would let
> the DOS clock run 18 times as fast. We don't want that...
> --8<------------------------------------------------------------
> typedef void (__interrupt __far* INTFUNCPTR)(void);
> INTFUNCPTR oldTimerInterrupt; // Original interrupt handler
> volatile long int milliseconds; // Elapsed time in milliseconds
> void __interrupt __far timerHandler(void)
> {
> static unsigned long count=0; // To keep track of original timer ticks
> ++milliseconds;
> count+=1103;
> if(count>=65536) // It is now time to call the original handler
> {
> count-=65536;
> _chain_intr(oldTimerInterrupt);
> }
> else
> outp(0x20,0x20); // Acknowledge interrupt
> }
> void installInterrupt(void)
> {
> union REGS r;
> struct SREGS s;
> _disable();
> segread(&s);
> /* Save old interrupt vector: */
> r.h.al=0x08;
> r.h.ah=0x35;
> int386x(0x21,&r,&r,&s);
> oldTimerInterrupt=(INTFUNCPTR)MK_FP(s.es,r.x.ebx);
> /* Install new interrupt handler: */
> milliseconds=0;
> r.h.al=0x08;
> r.h.ah=0x25;
> s.ds=FP_SEG(timerHandler);
> r.x.edx=FP_OFF(timerHandler);
> int386x(0x21,&r,&r,&s);
> /* Set resolution of timer chip to 1ms: */
> outp(0x43,0x36);
> outp(0x40,(unsigned char)(1103&0xff));
> outp(0x40,(unsigned char)((1103>>8)&0xff));
> _enable();
> }
> void removeInterrupt(void)
> {
> union REGS r;
> struct SREGS s;
> _disable();
> segread(&s);
> /* Re-install original interrupt handler: */
> r.h.al=0x08;
> r.h.ah=0x25;
> s.ds=FP_SEG(oldTimerInterrupt);
> r.x.edx=FP_OFF(oldTimerInterrupt);
> int386x(0x21,&r,&r,&s);
> /* Reset timer chip resolution to 18.2...ms: */
> outp(0x43,0x36);
> outp(0x40,0x00);
> outp(0x40,0x00);
> _enable();
> }
> --8<------------------------------------------------------------
> To query the elapsed time in ms, just read the variable milliseconds.
> A note: This program is for Watcom C. For other compilers, some
> structures and functions (like _enable) might have different names.
> Refer to your compiler's manual.
> A second note: This program is for Protected Mode using DOS/4G. To
> compile it for real mode, just replace all int386 by int86, and all wide
> registers (like edx) with their short counterparts (like dx). The
> program should work fine then.
> I hope this helps a bit,
> Oliver