Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!rutgers!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.atari.st,comp.sys.mac
Subject: Re: Disk interchanges between Atari ST and Macintosh
Message-ID: <2167@cbmvax.UUCP>
Date: Wed, 29-Jul-87 16:22:26 EDT
Article-I.D.: cbmvax.2167
Posted: Wed Jul 29 16:22:26 1987
Date-Received: Fri, 31-Jul-87 05:46:30 EDT
References: <17421ZSR@PSUVM>
Organization: Commodore Technology, West Chester, PA
Lines: 77
Xref: mnetor comp.sys.atari.st:4638 comp.sys.mac:5057

in article <17421ZSR@PSUVM>, ZSR@PSUVM.BITNET says:
> Xref: cbmvax comp.sys.atari.st:4520 comp.sys.mac:4957
> 
> Therefore, is there any way
> that I can get a Macintosh to read files written by my ST?  I know that the
> floppy chips are similar, but not exactly the same (or is it the drives?)
> I also know that the ST can be kluged to write 800K disks (I read it on the
> net somewhere).  My question is, can the ST write files that are readable
> by the Mac?  

Maybe.  First of all, the floppy controller chips on the two machines are
very different.  The ST uses a more traditional controller chip, much the
same as the IBM PC world, which is why the ST format is essentially in the
"standard" format (kind of funny, since most 3.5" floppies are in Mac format
today, as a whole.  Though most floppies in general are in IBM format, of
which the ST format is basically an 80 track extension of).

IBMs and Ataris do things on their floppies pretty straightforward.  They
format a standard 9 sectors per track for 80 tracks, double sided (though
some STs still do single sided), and this can be read with just about any
floppy controller.  This is all in MFM encoding, which dates back to the
CP/M world (at least).

The Mac is a bit stranger.  It uses a Apple-specific version of an encoding
scheme know as GCR, with some additional strangeness.  When you think about
storing data on a floppy, you should consider the fact that you're dealing
with a circular medium.  This medium has a maxium bit density that can be
written reliably.  The thing is that, at a constant angular velocity and
data rate, this maximum density is only achieved on the inner track of a
floppy, which is the smallest linear space written to.  The older Macs
realize that, on the outer tracks, there's much more space available, but
to write to this, you've got to change something.  These Macs change their
drive speed as they get to the outer tracks.  So with a Mac, you get more
tracks written on the outer tracks than the inner tracks.   The newer
ones (Mac SE, Mac II) do this electronically, not mechanically.  But this
causes problems for anyone attempting to read this with a non-Mac machine.
In passing, I should mention that the 8-bit Commodore disk drives, which
also use a form of GCR encoding for their native format, have used the
electronic method mentioned since they were originally designed.  

I should also mention the Amiga here, 'cause it does thing a different
way.  Basically, on a normal PC style drive, you get a request to read/write
a sector, then you read/write it.  This requires sectors to be spaced apart
from one another, to compensate for inaccuracies in the positioning 
mechanisms of the floppy drives.  The Amiga gets it's 880K per floppy
by read/writing an entire track at a time.  So there's no need for any
spacing between tracks.  So an Amiga can fit 11 sectors per track, versus
the 9 per track on a PC or an ST.  The Amiga also uses it's Blitter to
decode disk data, versus a disk controller chip.  So it can easily read
other MFM formats, like the PC or ST format, and can even be programmed to
decode stranger formats, like the 8-bit Commodore GCR format.  There are
commercial programs for the Amiga that let it read both of these, and
Apple II as well (which is a very low density format similar in other 
respects to the Mac format).  There's some work being done on getting the
Amiga to read Mac format; it can certainly read parts of the Mac disk,
but there's some debate as to whether it will be able to read the whole
thing or not.

> Does anyone in either plane of existance have any ideas on a possible
> compromise format?  

You're probably best sticking with the native ST format, believe it or
not.  Since that's, at the low levels at least, exactly the same as the
PC format, others will come to you, rather than you coming to them.  There's
no way an ST will read a Mac disk without duplicating the Mac hardware and
attaching it to the Mac.  I'm not sure how flexible the Mac disk controller
is, but I'd imagine that it won't be too long before there's some way for a
Mac to read PC, and thus ST, disks.  In the same time frame, a way to read and
write Mac disks may be viable on the Amiga, so if there's not other way by
then, it would be possible to go through an Amiga.

> Christopher D. Johnson

-- 
Dave Haynie     Commodore-Amiga    Usenet: {ihnp4|caip|rutgers}!cbmvax!daveh
"The A2000 Guy"                    PLINK : D-DAVE H             BIX   : hazy
     "Catch a wave and you're sittin' on top of the world" -Beach Boys