Re: Zip GS questions

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

Sheldon Simms <sheldon@atlcom.net> wrote:

> In article <32AA91D6.1857@unixg.ubc.ca>, benlai@unixg.ubc.ca wrote:
> > Yet, at Alltech they are selling 7mhz Zip Chips for a lot cheaper than
> > the ones by EGO System ($140 versus $175), what is the difference
> > between the two?  
> 
> Zip chip or Zip card? A Zip chip is for IIe's. The GS accelerator is
> a card.

There is no 7 MHz "ZIP Chip", therefore it must be a typo.

> I have never had the opportunity to compare differently zipped
> GSes, so I don't speak from personal experience, but I think I would
> rather have another 16kb of cache than another 1Mhz speed. That's why
> I bought a Zip 7/32 card a few years back for my GS. Of course, having
> both the cache and the clock is nice too...

An increase in the size of the cache does not generally produce as much
of a noticeable speed improvement as an increase in the CPU speed.  It
depends on what the software is doing.

I have had personal experience of 8/16 (my current card), 8/64 (my
original card, now dead), and I've also used a 9/64 card.  I didn't
notice a significant drop in the apparent speed between the 8/64 and
8/16 cards.  The 9/64 card was noticeably faster than either of the
others.


Let us consider an example of three ZIP cards: a 7/16, 7/32 and 8/16.

The peak execution speed is given by the CPU speed in each case, thus
the 8 MHz card will be able to execute fully cached code faster than
either of the other cards.  This means that tight loops will be
noticeably faster (about 14%) compared to the 7 MHz cards.  (The average
speed of the computer will not increase as much.)

An increase in the cache size doesn't help with tight loops, but allows
the average speed of more complicated (or widely distributed) code to
increase, because more of it can be stored in the cache.

Considering that operations like moving memory and graphics operations
involve tight loops, the higher clock speed produces a more visible
increase in speed.

I think you would find that an 8/16 card will be faster on average than
a 7/32 card.  If you can't afford both higher speed and larger cache,
then go for the speed.

The same argument applies for higher clock speeds, but you get a
steadily smaller relative increase in peak and average speed for each 1
MHz increase in CPU speed.  At some point, you would have more to gain
by increasing the cache size than by adding another 1 MHz.

(In addition, there is a limit to how much you can increase the clock
speed, which depends on the particular machine and accelerator.)

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand