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Path: utzoo!mnetor!seismo!lll-lcc!ames!ucla-cs!sdcrdcf!darrelj
From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: comp.sys.apple
Subject: Re: IBM & Apple disks.....
Message-ID: <3682@sdcrdcf.UUCP>
Date: Tue, 6-Jan-87 16:06:03 EST
Article-I.D.: sdcrdcf.3682
Posted: Tue Jan  6 16:06:03 1987
Date-Received: Wed, 7-Jan-87 06:38:25 EST
References: <8701051528.aa00868@SPARK.BRL.ARPA>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Organization: System Development Corporation R&D, Santa Monica
Lines: 30

In article <8701051528.aa00868@SPARK.BRL.ARPA> ELORANTA@BRANDEIS.BITNET writes:
>
> IBM drives are using double denstity and apple is using single density.
                                                           ^^^^^^^^^^^^
> So IBM could read apple disks (with special program) and apple can't
> read IBM (normal standard) disks.
>                                 Jussi Eloranta.

No, no no!!!  Apple Disks use their own, unique 1.5 density recording scheme
which no "standard" disk controllers can do anything with. With some real 
grungy assembly language programming, an Apple controller could do single
density because it has the ability to selectively write both clock and
data pulses on the disk.  Double density is impossible because some pulses
have to be written halfway between normal pulse positions, and some have to
be shifted about 12% either way to compensate for bit shift on the media.
(There may be complications in Apple reads of single density; I don't know if
missing clock timing schemes for standard FM are sufficiently compatible with
the Apple controller hardware state machine which sets initial syncronization
with the incoming data.  The remainder of the hardware is two 8-bit shift
registers so that software can (must) do just about anything at the bit
level on the disk.
-- 
Darrel J. Van Buer, PhD
System Development Corp.
2525 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
                                                            !sdcrdcf!darrelj
darrel@sdc-camarillo.ARPA or VANBUER@USC-ECL.ARPA