Reading Pascal 400kb Disk

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

"Michael J. Mahon" <mjmahon@aol.com> wrote in message
20030308145558.09900.00002271@mb-fc.aol.com">news:20030308145558.09900.00002271@mb-fc.aol.com...

> The issue of reading a single-sided 3.5" disk on a double-sided
> drive is only an issue of software.  It is only necessary to avoid
> reading sectors that are on the unused side of the disk, and then
> to interpret the single-side sectors with appropriately translated
> block numbers to correspond to its file system.
>
> As Wayne has pointed out, GSOS can interpret single-sided
> ProDOS 3.5" disks.  What you are looking for is a program that
> can interpret a single-sided Pascal file system.
>
> If all else fails, you can write such a program.  The job is made
> much simpler because the Pascal file system stores files as
> contiguous ranges of blocks--no index lists to worry about.
>
> I believe that someone posted here a while back that the physical
> sector ordering of a double-sided 3.5" disk was alternating sides
> by cylinder, so every N sectors (N/2 blocks) should be on the
> formatted side, and readable by any ProDOS block editor.
> (Don't use GSOS if GSOS can't "mount" the disk.)
>

Two things:

As I recall, the tech note for two 400k volumes on one 3.5 disk, uses the
first 800 blocks for volume 1 and the second 800 blocks for volume 2. They
do not use separate sides. Both volumes use both sides.
My memory is that GS/OS's Pascal FST did NOT recognize this format. My
recollection is that it didn't recognize all 3.5 format Pascal disks,
either. I seem to remember that a number of multi-os file copy programs did
not support Pascal on 3.5 disks even when they did support 5.25 Pascal
disks.

Years ago I had written a Pascal attach.driver for a 3.5 drive under Pascal
1.2 (Apple didn't support it until 1.3). I've never heard of a version 1.4.
The Pascal blocks are in the same order as ProDOS. I have a vague memory
that at some point they had boot blocks for Pascal that would work with
either 5.25 or 3.5 disks.

A minor difficulty here, if I remember correctly, is that the 3.5 drive
doesn't have the same number of blocks on each track. I don't remember the
exact layout, but the outer tracks had more blocks per track than the inner
ones. That's why we have 800k instead of 720k.

Assuming you can read the blocks on one side of the disk with a sector
editor, the easiest thing might be to write a disk copy program which copies
the 800 blocks on one side to the first 800 logical blocks on a standard
double sided disk. The disk might not be bootable, but any data on it might
be recognizable either with Pascal 1.3 or through one of the multi-o/s file
transfer programs.

Joel