Re: Advant/Disad of Zip8MHz

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

In article <3scbgb$leb@styx.uwa.edu.au>,
Joseph Turner <jturner@tartarus.uwa.edu.au> wrote:
>    Hi, I amy be getting myself a zip 8MHz for my 2c with z-ram, I am just 
> wondering what the advantages are and disadvantages.

Obvious advantage: most programs will run faster.  Disadvantage:
timing critical software (paritcularly games) may become unusable.
You can disable the ZIP when running these programs, so it isn't a
major problem.

> One question I do  have in mind is, will my programs be 8 times
> faster?

No.  Under ideal conditions, they might be able to run 8 times faster,
but this won't happen very often.

To be running at full speed requires the following conditions:

- Only read accesses are being made to memory.
- All memory accesses are in a smallish area of memory that has already
  been read into the ZIP's on-chip cache.
- No I/O reads are made.

In practise, most programs will be performing memory writes quite
often (though there will always be a lot more reads than writes).  All
write operations run at the normal speed of the machine (1 MHz).

In many cases, the program is not very "localised" - it is accessing
memory in a semi-random way, requiring new locations to be read into
the ZIP's cache and old locations to be discarded.  Any access to a
non-cached location requires a standard speed memory access.

I/O accesses are always at 1 MHz.  In addition, some I/O accesses will
force the ZIP to run at 1 MHz for a short time afterwards
(particularly disk drive accesses, which are timing critical).  This
will completely remove any speed benefit.


So, in practise, you should see results something like this:

- I/O operations (disk accesses) will be exactly the same speed as an
  unaccelerated machine.

- Programs that do large amounts of memory writing, e.g. highly animated
  graphics, will be somewhat faster than on an unaccelerated machine.
  I would expect this type of program to run about two or three times
  as fast.

- Programs that do tight loops and lots of reads and/or calculations will
  be noticeably faster.  For example, AppleWorks spreadsheet calculations
  could be accelerated three or four times over a standard machine.

- You will rarely find anything that runs more than four to six times
  faster than a standard machine.  Selected operations in some programs
  may run faster, but there will be a less noticeable effect when the
  entire program is taken into consideration.

For a 4 MHz ZIP, halve all the speed factors listed above.

A 3.6 MHz TransWarp in a IIe will be faster than a 4 MHz ZIP under
most conditions, because it has more memory available (the TransWarp
caches all of the Apple II memory).  An 8 MHz ZIP will probably beat a
TransWarp in special cases.

[This is all theory, of course - if anyone has any measured speed
ratios, could they post them?]

>   Also, I have a friend who used to have an apple and he got a few Zip 
> Chips but he told me that he had a problem with them blowing up. Is this 
> a real problem in most situations?

I haven't heard of any such problem, but I haven't paid much attention
to the 8-bit version of the ZIP.  (I use a ZIP GS in my IIgs.)
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand