Re: Apple IIc+ @8Mhz

["Michael J. Mahon" <mjmahon@aol.com>]

Hotte wrote:
> Thanks Michael,
> your answer has helped a lot.
>
> I will do some experiments to figure out which is the speed bummer.
> If it is the ASIC or any other customized circuit, well thats the end.
>
> I have looked at the ZIP chip patent last night and I hope I have
> enough understanding to trace it back on the IIc+.
>
> It will take a while but I hope I can post some findings when I am
> done.

That's great!

I'm just doing a few "speed trials" myself.

I'm particularly interested in whether the "slowdown" interval
(caused by referencing a "slow" /DEVSEL address) decreases as
the clock frequency increases.  I expect that it does, which
could also cause problems if the slowdown interval is marginal
for some cases.

IIRC, it's supposed to be 38ms for a slow disk slot access.

Since it's unlikely that the ASIC existed in multiple versions
for the 4MHz and 8MHz Zips, I'm guessing that one of the ASIC
pins is strapable for another 2x division of the clock for the
"slowdown" timer register.

I never had any version of the Zip Utilities that had other than
4MHz as the maximum speed, so if you had an 8MHz chip, you just
had to know that all the actual frequencies were twice what was
shown by the utility.

I've confirmed that the Zip Utilities (including diagnostics)
work fine with the stock IIc+, though the meaning of the "slot"
speeds is a little different.

The IIc+ defaults are slots 1 & 2 normal, slots 3 & 4 fast,
slots 5 & 6 normal, and slot 7 fast.  The language card RAM
is cacheable and the paddles and speaker are normal speed.

I don't seem to be able to set the paddles to fast, but the
speaker can be set to fast, and the built-in beep is fast if
slot 1 is also set to fast.  Slots 5 & 6 should be left normal
for proper disk operation.

I've never actually used the utility to slow down my Zip Chip
in my //e, but it slows down the IIc+ just fine--except that
1MHz speed is about 80% faster than "chip off" (either set by
the utility or by pressing ESC during reset).

-michael

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