Re: Reading 5.25" Pascal 1.3 on a Mac

[nathan@visi.com (Nathan Mates)]

In article <199701020738517426609@roxboro-174.interpath.net>,
John Moreno <phenix@interpath.com> wrote:
>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?  I've used 5.25 disk out
>of the same box on both.  

   This is most definitely a 'frequently asked question' around here;
it's answered in both sections 5.4 and 5.10 of the comp.sys.apple2
Frequently Asked Questions (FAQ) at
http://www.visi.com/~nathan/a2/faq/csa2.html. Although the drives may
both use 5.25" Double Density (DD) disks, they write data to the disk
in ways different at the *hardware* level-- PCs use MFM endocing for
the mitstream, Apples use GCR. Without (rare) addon hardware, you
cannot read a disk formatted for the other system.

   For 3.5" drives, similar deal. Apple's 800K DSDD (and 400K SSDD)
disks use GCR and vary the disk's rotational velocity to pack more
sectors per track towards the owter part of the disk. PCs with 720K
and 1.44MB disks use MFM again, and use a constant rotational
velocity. [I think they may also rotate the disks in opposite
directions, though I may be wrong.] Apple surrendered to market
pressures for compatability reasons, and built in *hardware* support
for both 800K GCR and 1.44MB MFM disks in their Macs. [And more recent
ones have had lots of reports of only really supporting 1.44MB disks,
not 800K ones.]
 
   In short: because the 5.25" disk controller on a stock Apple II or
PC [mis]interprets the wrong type of bitstream, the software cannot
reliably get the bits off the disk. You can't get sectors off the disk,
you can't get a disk listing or files. You need (rare) addon hardware.
End of story.

Nathan Mates
--
<*> Nathan Mates http://www.visi.com/~nathan/      <*>
# What are the facts? Again and again and again-- what are the _facts_?
# Shun wishful thinking, avoid opinion, care not what the neighbors
# think-- what are the facts, and to how many decimal places?  -R.A. Heinlein