Re: 800k floppy on a Mac Classic II to transfer files to IIgs

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

Tristan Mumford <xtristan.xmumford@xgmail.xcom> wrote:

> The Classic seems to do a fair job of making floppies. I can't vouch for the
> classic II though. As it has been said though, formatting on the IIgs first
> does make a difference.

Regarding the formatting issue: there are two potential problems.

If you use a Mac which is "too new" to format the disk, its drive is not
a real SuperDrive. Later Macs switched from the Sony mechanism used in
the original SuperDrive to a cheaper one (made by Mitsubishi?). The way
to tell them apart is whether the drive will "grab" a disk and pull it
in when you insert it. If you have to manually push it all the way in,
it is the newer/cheaper type, not a Sony SuperDrive.

The newer drive is not as accurate as the old one for handling the
variable rotation speed used on the 400K and 800K GCR formats, so if you
a format a 400K/800K disk in one of these drives, it may not work
reliably. No problem for 720K or 1440K MFM formats, which use fixed
rotation speed, but they need a SuperDrive and SuperDrive card on the
IIgs. The newer drives can read 400K/800K well enough, and seem to be OK
for writing as long as the disk was formatted in an older drive.

The second potential problem is interleaving. A disk formatted in a
SuperDrive (any type) on a Mac may be 1:1 interleave by default. This
will result in very slow performance when the disk is written to by a
IIgs, and also for reading in some cases. (Apple might have deliberately
used 2:1 interleave when formatting 400K or 800K disks on later drives,
in which case this is a red herring.)

A SuperDrive card can buffer the entire track in a single read pass
(with an Apple 3.5 Drive or SuperDrive), so it doesn't have a read
performance penalty for 1:1 interleave, but it is still slow for
writing.

For an Apple 3.5 Drive on the IIgs disk port, the ROM 3 IIgs firmware
driver (used in ProDOS-8 or ProDOS-16) and the GS/OS firmware (for both
ROM 01 and ROM 3) have better performance than the original ROM 00/01
firmware driver, allowing them to keep up with 2:1 interleave for read.
I'm not sure whether they are able to keep up with 1:1. To do that, they
would have to buffer an entire track and then pick sectors out of it.

For a UniDisk 3.5, or a ROM 00/01 IIgs in ProDOS-8 or ProDOS-16, you
have to stick to 4:1 interleave for best performance.

If you format the disk on the IIgs, it will have 2:1 or 4:1 interleave
(depending on what software and computer you used). This will result in
somewhat slower performance on the Mac when copying data to the disk
(half or quarter speed for writing, possibly no difference for reading),
but the IIgs will operate normally.

-- 
David Empson
dempson@actrix.gen.nz