Re: How does the ZIP chip work?

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

Eric wrote:
> I've been wondoring how the ZIP chip can speed up an Apple IIe. I
> understand it has a 4 Mhz 65C02 and it plugs into the original 6502
> socket.
>
> But, the original 6502 only has half of each 1 Mhz interval (I'm
> rounding to 1 Mhz to make it easier to discuss) to access memory
> because the video refresh circuit uses the other half. So, the orig
> 6502 has a 500ns cycle to access memory.
>
> If the ZIP chip does not have it's own memory (does it?), then it can
> only speed up that part of the 1 Mhz cycle that is not used for video
> refresh. Assuming main memory could run 4X faster (which it can't),
> then it could bring that 500ns down to maybe 125ns. This means the
> overall processing (including the obligatory video cycles) would go
> from 1000ns to about 625ns, which is less that a 50% speedup.
>
> But main memory can't run any faster, so the zip chip must have some
> kind of cache? What am I missing here?

The Zip Chip is a processor-cache-bus interface multi-chip module.

It runs at its own speed until either a cache miss occurs on a
memory read or a memory write occurs.  For those activities it
must synchronize with the Apple clock and wiggle the processor
wires appropriately.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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