Re: Apple II GS Question

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

<pubpc1@library.ucla.edu> wrote:

> [slot speeds]
>
> On the Zip, you are given a choice.  You can set each slot to so that
> the CPU does NOT slow down to 1MHz when accessing it.  Doing this
> allows the Zip to maintain its fastest CPU speed (say, 15MHz) while
> accessing the slots.  As you can imagine, setting a 5.25" floppy
> drive's slot to accelerated speed makes it unusable, but setting a
> hard drive controller's slot to fast makes the hard disk run faster.

Sorry, Scott.  You are mistaken.

There is no way that the ZIP can bypass the 1 MHz bandwidth limit on the
slots, as it is controlled by the FPI (or CYA) and Mega II (both on the
motherboard), which the ZIP must cooperate with.

The slot speed setting for the ZIP determines whether the ZIP enables a
delay when any I/O location in the slot is accessed.  (Probably also
firmware locations - I haven't experimented, and the technical
documentation is vague.)

If the slot speed is set to "fast", the cycle in which the slot is
accessed will be at 1 MHz, but subsequent cycles can resume fully
accelerated operation (assuming no other delay is active, and the
accelerator is enabled).

If the slot speed is set to "slow", the cycle in which the slot is
accessed will be at 1 MHz, and the ZIP is disabled for the next 50
milliseconds, causing subsequent cycles to run at 1 MHz, 2.6 MHz or 2.8
MHz depending on the memory area being accessed.  If another access to
the slot occurs during this time, the delay is retriggered.


The reason that there is a problem with 5.25" drives (and some other
hardware) is that software which is accessing the I/O locations is
performing timing-critical operations, relying on the speed at which the
CPU executes the instructons.  The accelerator would throw this
completely out of kilter, so it is necessary to temporarily disable the
accelerator whenever the appropriate slots are accessed.

Some I/O devices are not timing-critical, so there is no need to slow
down the software (e.g. most parallel cards should work fine with the
slot set to "fast").

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