Re: Reading 5.25" Pascal 1.3 on a Mac

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

THE MARVELOUS MERV <u1002942@warwick.net> wrote:

> Sure, a PC disk drive can read an Apple format disk -- but only if you
> are a very serious drive mechanism programmer type, and can tell the
> computer to ignore all the data coming in from the top head, and somehow
> coerce the drive into reading directory information from just the bottom
> head . . . . 

Sorry, that is completely wrong.

It is not possible to read an Apple II disk in a standard PC drive, with
a standard PC disk controller, even if you went to the lowest level
offered by the hardware.

The fundamental problem is that the Apple II and PC use different
methods to record the individual bits on the disk.  The method used by
the Apple II is unreadable by the standard PC controller, and the
opposite is also true.

Beyond the bit-level problems, there are also significant differences in
the organisation of data into sectors, and in the file system.

Given a special PC disk controller (which knows how to handle the Apple
II bit-level and sector formatting), all you need is software which can
interpret the file system used.  In theory, you could use the standard
PC 5.25" drive, though I expect a high density 5.25" drive wouldn't work
too well, and a PC drive would not be able to cope with protected disks
that use half-tracks (PC drives have fixed track positioning, unlike
Apple II 5.25" drives).

The issue of one vs two heads is a red herring.  Even when the floppy
drive has two heads, the controller only pays attention to one of them
at a time.  There is usually a "head select" signal which the controller
uses to tell the drive which head to activate.

(By comparison, some hard drives can read data from multiple heads
simultaneously, particularly in the case of fast RAID drives that write
individual data bits onto separate surfaces.)

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