response time

[Peter Hansen]

Peter Hansen wrote:
> 
> "John J. Lee" wrote:
> >
> > Peter Hansen <peter at engcorp.com> writes:
> > [...]
> > > Strange, but based on a relatively mundane thing: the frequency (14.31818MHz)
> > > of the NTSC color sub-carrier which was used when displaying computer output
> > > on a TV.  This clock was divided by 3 to produce the 4.77MHz clock for the
> > [...]
> > > in time-keeping, which then counted on every edge using a 16-bit counter
> > > which wrapped around every 65536 counts, producing one interrupt every
> > > 65536/(14.31818*1000000/12) or about 0.5492 ms, which is about 18.2 ticks
> > [...]
> >
> > That doesn't explain it AFAICS -- why not use a different (smaller)
> > divisor?  An eight bit counter would give about 0.2 ms resolution.
> 
> Can you imagine the overhead of the DOS timer interrupt executing over 500
> times a second?!  It would have crippled the system.  

Oops: 5000 times a second, even worse. :-)  I have a vague memory that 
the DOS timer interrupt could take well over a millisecond to execute
on the old machines, so it simply wasn't feasible in any case.