Re: Overclocking an Apple ][?

[mjmahon@aol.com (Michael J. Mahon)]

Wayne wrote:

>There's also the problem that if you speed up the Apple II most
>cards won't work in the slots. Accellerators are made so they will
>slow down when accessing some slots. Disk II drives are one of the
>devices that need to be accessed at 1 mhz.

Excellent point.

Then there's the question of whether a "multisync" monitor will
actually sync _vertically_ at a frequency much higher than 75Hz...

And, finally, changing the main crystal requires that _all_ the
logic operate at the higher speed.  A good design should have
at least 20%-30% timing headroom, but beyond that, I wouldn't
expect much.

"Overclocking", as applied in the PC world, usually means that
only a very small part of the logic is actually run faster, with the
processor core itself being the most common element.  The PC
has multiple timing domains, phase-locked or asynchronously
connected.  The PCI bus domain may also be overclocked
somewhat, but usually only enough to accomodate a higher
multiplier for the phase-locked processor domain.

In contrast, the Apple II is a single timing domain.  Everything in
the machine is timed from the single clock, which is precisely
set to maintain compatibility with NTSC color video.

Apple II accelerators operate by adding another, faster
processor and a cache or replicated memory, operating
in a higher-speed clock domain that is phase locked to the
standard clock.

The Apple IIc+ is such a two-domain system, and it alone
offers the possibility of easy overclocking of its "fast"
processor/cache domain.

-michael

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