3.5 disk questions

["Joel" <joelbuckley54.nospam.@hotmail.com>]

Media Density
===========
The same disks (double-density) are used for 720k MS-DOS and 800k
ProDOS/HFS.
It's not that the IBM has more overhead.
The Apple drive uses an unsual variable speed drive to fit more sectors on
the outer tracks.
Think about it-with a constant rotational speed the inner tracks are
recorded at a much higher (linear) density than the outer tracks.
So they slow the rotational speed on the outer tracks, recording more data
at no greater linear density than the inner tracks.
It makes them unreadable in a standard IBM drive, but the media is the same.

You should NOT try to use double density disks (720k/800k) at high density
(1.4M) (you have to poke a hole in it to try.
Nor is it recommended to use HD (1.4M) disks at low density in a 3.5 drive.
While this is more likely to work than using HD 5.25 disks in an Apple drive
(which usually fails), it's better to use the disk optimised for that
capacity drive. Also if you stick a HD 3.5 disk that has been formatted at
800k on an Apple 3.5 drive into an Apple FDHD (1.4 M Superdrive), it will
just thinks its unformatted, rather than recognizing the 800k format.
It will properly recognize the 800k format on a DD disk.

INTERLEAVE
===========
Interleave refers to the sector ordering on each track.
A computer with a high speed processor can read and write sectors one right
after another-read a whole track in one rotation.
There is a certain amount of processor overhead associated with each sector
being accessed, so the slower processors  haven't finished by the time the
disk head is over the next sector. So to avoid having to wait one complete
rotation for that sector to come round again, the sectors are numerically
positioned 2 apart or 4 apart so that when the processor has finished its
work the next sector is coming up.

This was also a concern with 5.25 disks (see the posts under "Remove DOS
delay in disk image" under comp.emulators.apple2. DOS 3.3., Pascal, CP/M,
ProDOS, all used different interleaves to supposedly try to optimize it for
each operating system. DOS 3.3 did this wrong, which is why different
patches or DOS replacements {like DIversiDOS} could greatly improve on disk
performance.)
Old hard disk drives used to use interleave as well for the same reason.
Modern processors are so much faster than drives that interleave is not used
anymore.

Back to the 3.5 drives: With the Unidisk drive, the optimum interleave was
4:1.
 With the IIgs and Apple 3.5 drive, it was 2:1.
I believe the 1.4M Superdrive also was optimised at 2:1 on a IIgs (I'm not
sure if it's the same on a //e.)
I don't know if having an accelerator affects this either. I think most
accelerators slow down during disk access, but whether they are backup to
speed on the processing  between sectors, I'm not sure.
The Unidisk has its own (I believe 2MHz processor). The Superdrive interface
has its own higher speed processor.
I think a //c is 4:1. I'm not sure what the Apple //c+ was. Even though it
used a high speed procesor, it was more like an accelerator card add-in
rather than a complete high speed system..
Third party drives or controller may be different.

If you read a 4:1 disk on a drive that can do 2:1, it will run a little bit
slow.
If you read a 2:1 disk in a 4:1 drive, it will run much slower (ProDOS 8)
I believe this is less of a problem with GS/OS System 6, because the
operating system reads the entire track at a time (rather than sector by
sector) and sorts is out later.

So the rule of thumb is, if you may be using the disk on a //e or //c or
IIgs with a Unidisk, use 4:1.
If you are using it mostly on a IIgs with Apple 3.5 drive, use 2:1 (hi-speed
interleave).



"Simon Williams" <DON'Temail@luddite.ca> wrote in message
11282144.XLRKJRGA@news.telusplanet.net">news:11282144.XLRKJRGA@news.telusplanet.net...
> Hello,
>    I just picked up a box of 720KB double density 3.5" floppies which
> came formatted for IBM. So I reformatted them with CopyII+ and ended up
> with 800KB prodos...
>    I couldn't find a single point in which they differ *physically* from
> 800KB diskettes I have, are they in fact the same?
>    My guess is that they are the same thing but that IBM's formatting
> has more overhead than Apple's.
>    C2+ asked if I would like to use High Speed Interleave... I didn't.
> Does anyone think I should have??? I understand what this means, but not
> how it applies to Apple II's.
>
> SW