Re: ZipGS Upgrades

[dempson@swell.actrix.gen.nz (David Empson)]

In article <1994Jan24.124604.23915@csc.canberra.edu.au>
u901841@student.canberra.edu.au (Hickey / Michael Gerard (ISE)) writes:
> 
> I notice two different SRAMS in the cache: 
> a MOSEL MS6264L - 10PC (tag) and a MS6264AL - 70PC (data) 
> 
> Q: Are they _both_ 8k * 8bit SRAMS? What's the difference between them? 

Yep - "6264" is a standard number for 8Kx8 static RAMs.  My guess is
that the former is rated at 100 nanoseconds, and the latter is 70
nanoseconds.

I'm surprised that they are this way around.  I thought the tag RAM
had to be reasonably fast, but the data RAM wasn't so critical.

> Q: Can I replace them with two identical 32k * 8bit SRAMS to get a 32k
> cache? (Presumably 4 SRAMS will give me a 64k cache)

Correct (don't forget to change the option switches accordingly).

Picking the correct speed for the RAMs can be tricky.  I think ZIP
hand-pick their CPUs and RAMs anyway, so two ZIPs may have completely
different speed ratings on these chips.

My 8 MHz ZIP was supplied with 45 ns RAMs.  By comparison, a friend's
9 MHz ZIP had 70 ns RAMs.  Guess whose worked more reliably.  :-(

Bang goes my theory, but here it is anyway:

For an 8 MHz ZIP, I'd recommend 70 ns RAM (or faster).  For 9 MHz, go
for 60 ns or faster.  For 10 MHz, go for 50 ns or faster.

> Q: Is there an optimal amount of cache for a given speed? Specifically,,
> for a 7MHz ZipGS, is a 64k (or 32k for that matter) cache too much of a
> good thing?

No.  Increasing the cache size will help at any clock speed.  The
theory is that with a larger cache, then more locations can be stored
in the cache, and the CPU will spend less time accessing main memory.

A larger cache will therefore allow the ZIP to run closer to its rated
clock speed (on average), but the effectiveness will vary depending on
the behaviour of the computer.

You will generally get a more obvious speed increase by boosting the
crystal speed (and replacing the processor and cache RAM if necessary).

If you have to increase the RAM speed anyway, you may as well get as
much cache RAM as you can.

> My ZipGS CPU is marked W65C816PL-6. I am very happy with the performance
> and operation of my ZipGS but I am curious... 
> 
> Q: Does the -6 mean it is rated at _only_ 6Mhz?

It means the manufacturer has tested it and certified that it works
reliably at 6 MHz.  If ZIP is using it in a card rated at a higher
speed, it means they have tested it and verified that it works at the
higher speed.

I have mixed feelings about this practise.  CPUs are normally made in
batches, then tested individually to determine a speed rating.  Some
will be able to run at higher speeds than others, and they will be
rated accordingly.  If the CPU was only rated at 6 MHz, there must be
some reason why it wouldn't work at a higher speed.

My 8 MHz ZIP had a CPU rated at 7 MHz, as did the aforementioned
friend's 9 MHz ZIP.  I don't know what caused mine to be so
unreliable, but it could have been the CPU misbehaving because it
couldn't quite cope with the higher clock speed.

> Q: How much faster might I be able to make it go without buying a fast
> 65C816? 

I wouldn't want to push a 6 MHz CPU any faster than 7 MHz.  You could
try 8 MHz, but I'd recommend a doing of very intensive testing to
prove that it works reliably.  It is only the 14 MHz 65816 which is
very expensive - you should be able to get a processor rated at 7 MHz
to 10 MHz for much less.


The problem with my ZIP was that it was just very flakey.  It worked
fine for a while, but once the temperature of the computer rose to a
certain point, it starting crashing at random.

It used to be OK - the behaviour got progressively worse over a couple
of years.

I don't use it any more - I cannot be bothered dealing with
unpredictable crashes and data corruption.  One of these days, when I
have some spare money (ha ha) I will look into getting this one fixed
or buying another one.

One idea I'm playing with is to get a 14 MHz CPU and try beefing my
current card up to 12 MHz (replacing the RAM as well).  This may
eliminate the problem.  Here's hoping...
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand