Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Do all 3.5 inch drives use the same encoding
Message-ID: <5231@cbmvax.UUCP>
Date: 14 Nov 88 18:40:25 GMT
References: <8811101847.AA22940@godzilla.eecg.toronto.edu>
Organization: Commodore Technology, West Chester, PA
Lines: 48

in article <8811101847.AA22940@godzilla.eecg.toronto.edu>, leblanc@eecg.toronto.edu (Marcel LeBlanc) says:
> Summary: 3.5 inch sector encoding question
> 
> []
> 	Can somebody tell me if Amigas, IBMs, 1581s (CBM 8-bit), and Ataris
> use a compatible encoding method for 3.5" disk sectors?  

Yes and no.  They all use a form of MFM encoding (good to leave out
Macs, since they use a GCR format).  IBMs and Ataris use pretty much the
same 9 sector/track format (720K), nothing really tricky there.  1581s
store 10 sectors/track (800K), and Amigas store 11 sectors/track (880K). 
The IBM/ST format lets the controller write arbitrary sectors; there's
enough space left between them to account for head positioning slop. 
The 1581 always writes full tracks, so sectors are always positioned
accurately, and there's no need for slop areas between them.  Amigas
always read write full track, and in fact leave no space between sectors
at all. 

> I expect differences in sector length and file/directory structure, but 
> what about the encoding itself?

The Amiga actually does the MFM encoding and decoding with the blitter in
normal operation; it reads raw MFM in from disk.  This allows the Amiga to
read or write any MFM format that uses the 1-meg unformatted density encoding,
this includes both IBM/ST and 1581, though the arrangement of these formats
make lend itself better to CPU driven decoding than blitter driven decoding.
While the 1581 uses hardware encoding and decoding, it's capable of reading,
but not writing, full raw tracks.  So a C128 program could, at least in
theory, read an Amiga disk, but it couldn't write one.  With IBM type
formats, it probably depends on the floppy controller.  These all do their
encoding and decoding in hardware, and are only guaranteed to read and 
write individual sectors.  Any IBM based system with a floppy controller
chip than can read and write full raw tracks could read and write Amiga
disks with the appropriate program, but I suspect most controllers can't
do this.  An IBM controller could probably read a 1581 disk with the right
program, but again, without that full track write capability, it can't
write the 1581 format accurately.

> 	Can someone illuminate me on this question?

There's a candle.  If need a flashlight, let me know.

> Marcel A. LeBlanc	  | University of Toronto -- Toronto, Canada

-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
              Amiga -- It's not just a job, it's an obsession