Mail Archives: djgpp/1994/10/06/17:44:01
> On Wed, 5 Oct 1994 hvb AT netrix DOT com wrote:
>
> >
> > >
> > > > We had to reprogram the timer chip (8253) to have a faster interrupt rate. You
> > > > still need to call the original interrupt to have the DOS clock up to date.
> > > > On some hardware platform, this is not achievable as the disk drive actually
> > > > uses the chip timer output for some of its own timing!!!
> > >
> > > AFAIK the timer chip has 3 independent timers/counters, each of which
> > > can be programmed individually. The first 2 are used, but the third
> > > isn't. So, unless those disks you mention use that third timer, it
> > > can be reprogrammed without affecting system operation in any way
> > > whatsoever, including the luxury to not pass the interrupts to the
> > > original Int 08 handler which maintains DOS time. Am I right?
> > >
> > The truth is:
> >
> > Timer 0 is the one that DOS uses for its real time clock interrupt. I've
> > read somewhere that some disk hardware uses this timer 0 output to time
> > some of its internal operation, so if we reprogram this timer, on some
> > hardware you may have to restore it back to its original state before
> > certain disk operation. We did not need to do this on our PC based hardware.
> > If we had to we probably have to work on some other solutions as it
> > is not acceptable to do so.
> >
> > Timer 1: is used for the RAM refresh. So it is a no no.
> >
> > Timer 2: is used to generate sound for the speaker. This output does not
> > generate any interrupt to the CPU. If you just want to sample the clock timer
> > then it is ok.
> >
>
> Anyone got docs on how to program the timers? If this is correct and
> you can use timer 2 without worrying about disk drives/system stuff, then
> it seems that we could make some decent use of it for DJGPP.
>
I used:
1. Intel Microprocessor and Peripheral Handbook, volume II-peripheral
8253/8253-5 Programmable Time Interval
(some hardware uses the 8254 Programmable Time Interval)
2. Robert Jourdain, Programmer's problem solver for the IBM PC, XT & AT
(I am sure any PC reference book would contain similar info).
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Hung Bui Internet: hvb AT netrix DOT com
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