Re: System disk specific to the IIc+?

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

Warren Ernst <wernst@gmail.com> wrote:

> Now, do you happen to know, just for the sake of discussion, how
> interleaving works with the SuperDrive and the SuperDrive Controller
> Card?

Mostly yes. The SuperDrive card works like the ROM 3 IIgs firmware in
terms of its capabilities. It can cope with any interleave factor for
reading from a SuperDrive (or Apple 3.5 Drive) because it reads an
entire track into the memory on the card in a single rotation, then
locates and decodes the sectors in the appropriate order.

For writing, the host computer and operating system are probably
significant. If used on a IIgs with GS/OS, the SuperDrive is able to get
best performance with 1:2 interleave. Writing to 1:1 is much slower, and
1:4 is about half speed.

With ProDOS-8, the operating system can only supply data to the card one
sector at a time. Despite this, as long as the writes are done as
quickly as possible, the card should be able to keep up with 1:2
interleave on write. I don't know if I tested this.

Upon further reflection, this might also be an issue with writing from
ProDOS-8 on a ROM 3 IIgs using the built-in disk controller and an Apple
3.5 Drive. I'd have to do some tests to confirm that can keep up.

With a UniDisk 3.5 attached to a SuperDrive card, it is no better off
than a directly connected drive. The overhead of the extra processor in
the UniDisk and the seqential transfer then encode then write sequence
means that 1:4 interleave is still the best option for a UniDisk 3.5.

-- 
David Empson
dempson@actrix.gen.nz