Date:	Thu, 6 Oct 1994 11:03:26 +0500
From:	hvb AT netrix DOT com
To:	hvb AT netrix DOT com, flaregun AT strauss DOT udel DOT edu
Subject:	Re: accuracy of clock()
Cc:	eliz AT is DOT elta DOT co DOT il, DJGPP AT sun DOT soe DOT clarkson DOT edu, victor AT euler DOT berkeley DOT edu
Reply-To:	hvb AT netrix DOT com

 
> 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).

==============================================================
Hung Bui                           Internet: hvb AT netrix DOT com
Netrix Corporation                    Phone: +1 703 793 1016
13595 Dulles Technology Drive           Fax: +1 703 713 3805
Herndon Va 22071
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