Hi.

  One of the basic PC components is a "timer chip".  It has
several
"pulse" outputs, and the frequency of each is programmable.
[One of
the outputs goes to the speaker built into the PC case, and
this
provides the "beep" sound and a very crude musical
capability.]
  The most important output from this chip is connected to
IRQ 0, and
set to pulse every 55 milliseconds -- about 18.2
times/second.  [I
may be rusty, but I believe that's awfully close to 64K
ticks/hour.]

Could be seen this way, but the main reason for these odd
numbers was that the first PC had to be CHEAP. So instead of
putting a 15 MHz quartz (then expensive), that divided by 3
(8284 chip) would give 5 MHz, just for a 5 MHz 8088, they
chose the quartz found in video cameras and other devices
with four times the NTSC subcarrier, which divided by 3
gives moreless 4.78 MHz. And this quartz, for compatibility,
has been inherited, processor clock synthesized from it,
etc.
The NTSC frequency was also divided on the 8254 timer by 12,
and this by 65536, giving IRQ0 every 54.925 ms, moreless.
************************************
Javier Vizcaino. Ability Electronics. [log in to unmask]
http://ability53.hypermart.net
  Starting point:        (-1)^(-1) = -1
  Applying logarithms: (-1)*ln(-1) = ln(-1)
  Since ln(-1) <> 0, dividing:  -1 = 1     (ln(-1) is
complex, but exists)
-----Mensaje original-----
De: David Gillett <[log in to unmask]>

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