Re: IIgs Floppy for the IIc ?

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

Louis Mazzei <kong@notrix.de> wrote:

> "David Empson" <dempson@actrix.gen.nz> wrote in message
> 1f4jd25.123a85b120lzxaN%dempson@actrix.gen.nz">news:1f4jd25.123a85b120lzxaN%dempson@actrix.gen.nz...
> 
> I believe that the IIgs' floppy drive formats disks at 2:1 interleave as
> opposed to the 4:1 of a UniDisk. This provides for faster read speed, IIRC...
> does anyone remember if this is correct?

The default interleave for the Apple 3.5 Drive on the IIgs is 2:1.  This
produces the best performance for a IIgs with this drive.

The optimal interleave for a UniDisk 3.5 is 4:1.  The UniDisk 3.5
transfers data to the host machine at about half the speed of the Apple
3.5 Drive, and using a 2:1 interleave with this drive would require
almost a complete revolution of the disk before being able to read the
next sector.  This means that accessing a 2:1 interleave disk in a
UniDisk 3.5 will be significantly slower than a 4:1 interleave disk.

The opposite also applies, to a lesser extent - accessing a 4:1
interleave disk in an Apple 3.5 Drive may result in a transfer rate
which is half that of a 2:1 interleave disk.

There are a few of other factors which influence this:

- If you are using an Apple 3.5 Drive or SuperDrive with a "SuperDrive
Card", it reads a track at a time and caches it in RAM.  This means the
the interleave is neutral for reading, but is still significant for
writing.  (Preformatted PC disks have 1:1 interleave, and read quickly
on a SuperDrive, but are written slowly since the computer can't write
sectors back to back.)

I don't know whether the SuperDrive card uses track caching with the
UniDisk 3.5, but it would still suffer from lower performance due to the
slower interface.

- The IIgs ROM 3 firmware and GS/OS drivers for the Apple 3.5 Drive also
use track caching, so interleave is neutral for read on a ROM 3 IIgs
under any operating system, or on a ROM 1 IIgs under GS/OS.  Writes are
still affected by the interleave.

- If an application delays between read requests with a non-caching
operating system and driver (e.g. ProDOS-8 on a ROM 1 IIgs) or write
requests under any operating system then it may get better performance
with a higher interleave, e.g. 4:1 on an Apple 3.5 Drive.

- All of the above assumes sequential access.  If it is necessary to
move to another track or side, then the interleave is insignificant.  If
sectors on the same track are accessed out of order then a caching
driver (SuperDrive card, ROM 3 or GS/OS) will perform equally well, but
it will be difficult to predict the performance of a non-caching driver.