CPU Independant Timer (286+)
If you've even tried timing stuff on the PC it can become very fustrating.
The timer tied to IRQ#0 is great if you know how to adjust it but Windoze
and such will not allow you to do this. And using this timer for say
delays in your program when you need like a 1ms delay is too much overhead.
Using non-destructive reads from some IO ports can give you a long enough
delay but if your code moves to a faster CPU the delay may no longer be
long enough. Well as I was scrolling thru some old PC tech info I've found
the perfect anwser. It seems that there is a timer connected to port 61h
at bit 4. This bit toggles every 15.085 u sec (micro sec).
Here's an example of how to delay exactly 1 sec on any system.
in al,61h ;input
and al,10h ;mask bit 4
mov cx,0ffffh ;1 sec delay
jz cnt1 ;no change then loop back
This will cause a delay of exactly 1 sec on any system. It's completly CPU
independant. This timer is available only on 286 systems and up. There
is no IRQ for this timer but it's great for using delays when
programming timely critical devices (like the DMA, sound cards, etc.).
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