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The 80C188 has internal timers, DMA, &c. It provides much of
what the IBM BIOS requires.<br>
<br>
The 18.2hz timer is derived from the CPU crystal: at XX mhz, one
timer divides the CPU clock by XX to yield a 1mhz frequency;
pre-scaled by 4, then divided by 13731 (magic number), the resulting
clock is within 0.1% of the DOS tick; it is close enough to maintain
the BIOS time-of-day locations.<br>
<br>
Why 13731 works, and why it jives so closely with 14.318182mhz, /3 =
4.772727mhz, I don't know.<br>
<br>
--John<br>
<br>
<br>
<br>
On 04/26/2014 11:46 AM, Nikolay Dimitrov wrote:
<blockquote cite="mid:535BFEEF.8080104-***@public.gmane.org" type="cite">Gents,
<br>
<br>
I was wondering about the SBC-188 DOS compatibility. I don't see a
82C54 PIT on the schematic, and I still remember that there was
some interrupt fired periodically at 18.2 Hz, so maybe this timer
is required. Did you run DOS on this board, and is this PIT
required for DOS?
<br>
<br>
Regards,
<br>
Nikolay
<br>
<br>
</blockquote>
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