Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!uwmcsd1!marque!uunet!portal!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: ibm pc diskettes (LOTS OF DETAIL)
Keywords: ibmpc
Message-ID: <1133@atari.UUCP>
Date: 26 Aug 88 17:56:12 GMT
References: <5914@gryphon.CTS.COM>
Reply-To: apratt@atari.UUCP (Allan Pratt)
Organization: Atari (US) Corporation, Sunnyvale, California
Lines: 61

In article <5914@gryphon.CTS.COM> freuden@gryphon.CTS.COM 
(Ralph Freudenberger) writes:
> I am posting this article for a friend who is as ignorant about
> the atari machine as I am...
> 
> Is there software available for the st that will write ibm/pc compatible
> diskettes?
> 
> thanx in advance,
> 
> 	freuden@gryphon.cts.com

The ST already uses IBM PC compatible disks.  The one restriction is
that the disks must be formatted on a PC, not on an ST.  That
restriction is lifted for the new ROMs coming soon to an ST near you
(officially called TOS 1.4). 

Be careful when using PC disks, though: if you use one PC-formatted disk
on an ST, then pop it out and insert another one, the ST will NOT
recognize the fact that a new disk is in the drive.  The way to be sure
that the ST figures this out is to put an ST-formatted disk in the drive
and access it (get a directory or something) between the two
PC-formatted disks. 

If you want to know why this is, read on:

GEMDOS, like all good operating systems, keeps information about a disk
in its private memory.  This information includes things like the sizes
of the various parts of the disk, and some of the most-recently used
information.  When you change from one disk to another, the OS has to
realize that its memory is out of date -- this is called "media change."

The OS detects that the media MIGHT HAVE changed when you pop the disk
out.  If you put the same disk back in, the OS will realize that and
it won't invalidate its memory about that disk.

The trick used is to check the SERIAL NUMBER of the disk.  The serial
number is a random 3-byte number written to the disk when it was
formatted.  The assumption is that it is highly unlikely that two disks
will have the same serial number, or if they do they will not be used
one after the other. 

Unfortunately, the IBM PC doesn't use this same system.  Disks formatted
on a PC all seem to have the same serial number, so the OS can't tell
them apart.  When you switch from one to another, the OS will merrily
decide that the same disk is in the drive after all. 

This business about serial numbers has NOT changed in the new TOS. 
However, the desktop has been changed: it has always been true that a
window is open on a drive, you can hit ESCAPE to redisplay the
directory.  Now, however, when you hit ESCAPE the Desktop *forces*
the OS to forget what it knew about the disk in the drive and
re-read everything.  It *forces* a media change.

As I said before, disks formatted with TOS 1.4 are totally IBM compatible,
and have the added feature of random serial numbers, so these problems
should become more rare.

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt