Re: How fast is a IIgs?

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

In article <3ut7ag$cbm@sun.lclark.edu>,
Alfred Barter <barter@sun.lclark.edu> wrote:
> What is the clock speed of an Apple IIGs?

Variable.  The maximum rate, achievable when accessing ROM, is 2.8
MHz.  "Fast" RAM is accessed at about 2.5 MHz on average (2.8 MHz with
regular refresh cycles).  "Slow" RAM and I/O is accessed at 1.023 MHz,
the same speed as an Apple ][, ][+, IIe or IIc.

> I opened the sucker up and saw a crystal rated at about 3.6 MHz.

Wrong crystal.  That is the crystal that controls the speed of the
serial port (3.6864 MHz, divided by various amounts, produces 57600
and lower baud rates).

The main crystal in the IIgs is under the power supply.  It is rated
at 28.63636 MHz.  This is divided down to produce the internal clock
speeds such as 14.31818 MHz, 7.15909 MHz, etc. which are used in the
video circuitry.

> If I ripped that puppy out and put in say, oh...a 6.54 MHz crystal
> would that speed up my IIgs?

No.  If you tried to change the main crystal in the IIgs, the entire
machine would stop working.  Nearly every part of the machine is
timing critical.  The main component is video generation, which is all
based on the 14.31818 MHz crystal speed.  Everything on the "slow"
side of the machine is tightly synchronized to video generation, with
half of each 1 MHz cycle used for video generation and the other half
for CPU accesses to slow memory.

The "fast" side of the machine is less speed critical, but the chips
that interface between the two sides of the machine (particularly the
"Fast Processor Interface") are designed around the crystal speed.
Synchronization would be completely thrown out if the speed was changed.

This is ignoring any issues such as component speeds - they may
already ben near their limits.

So, in summary, you cannot increase the speed of the machine by
changing the crystal speed.

The only way that the IIgs can be accelerated is by using a technique
similar to that used by the ZIP GS or TransWarp GS: cache memory and a
cache controller is added to the CPU, which runs at much higher speeds
(7 MHz to 16 MHz, depending on the accelerator).  When accessing
non-cached locations, the accelerator slows down and synchronizes with
the motherboard.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand