Re: Reading 5.25" Pascal 1.3 on a Mac

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

John Moreno <phenix@interpath.com> wrote:

> Sheldon Simms <sheldon@atlcom.net> wrote:
> 
> ] In article <199701010738012226980@roxboro-185.interpath.net>,
> ] phenix@interpath.com (John Moreno) wrote:
> ] 
> ] > For the last week or so I've been working on a program, similar
> ] > Apple File Exchange, that allow me to read Pascal 1.3 3.5" disk.
> ] > What I'd like to know is if anybody knows if something similar might
> ] > be possible with the 5.25" disk?
> ] > 
> ] > Is there anybody out there with a 5.25 drive hooked up to their mac
> ] > that would be willing to see if you can read the disk with a disk
> ] > editor?
> ] > 
> ] > What 5.25 drives is there for the Mac?
> ] 
> ] None.
> ] 
> ] Well actually, Apple did make a PC-compatible 5.25 drive for the Mac
> ] specifically for the purpose of reading MS-DOS disks, but that
> ] wouldn't help your project since the PC-style 5.25 drive couldn't read
> ] an Apple Pascal disk.
> 
> How sure are you of this?  That's what I thought at first about the 3.5
> disk, until I tried - now it's working quite well. What's the difference
> between a disk for the Apple II and a PC disk?

The low-level physical formatting method is completely different: the
Apple II uses GCR - "Group Code Recording", while the PC uses MFM -
"Modified Frequency Modulation".

A standard Apple II disk controller cannot read a PC disk, and a
standard PC disk controller cannot read an Apple II disk.

The only way the Mac PC-compatible 5.25" drive would be able to read an
Apple II 5.25" disk is if Apple had designed the controller card (and
possibly the drive as well) to support this, and the software supports
it.  I doubt this would be the case, but I've never used one of these
drives or heard of anyone trying this, so I cannot be certain.

By comparison, the Apple II and Macintosh use exactly the same physical
disk format for 800K 3.5" disks, so the only thing stopping you from
reading an Apple II Pascal 3.5" disk on a Mac is that the file system is
not recognised by the Macintosh.

> I've used 5.25 disk out of the same box on both.

The disks are physically identical, just formatted differently.

The differences between MFM and GCR go right down to the bit level.  A
standard MFM controller is pretty much hard-wired to deal with MFM (with
some variation in items such as the sector size).  A standard GCR
controller (such as the IWM chip used on the IIgs, IIc and early Macs)
cannot handle MFM data because it has different timing characteristics
and doesn't satisfy the minimum requirements of the IWM.

An enhanced GCR controller such as the SWIM chip (used on the SuperDrive
card, and in most Macs since the IIx) is able to support both MFM and
GCR data.  If the PC-compatible drive's controller used this chip (which
might not have existed at the time) then it might have sufficient
hardware to support the Apple II 5.25" drive and/or Apple II 5.25" disks
in a PC-compatible drive.

It is more likely that Apple used a standard MFM disk controller chip
and standard PC-compatible drive.

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand