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From: jesup@cbmvax.commodore.com (Randell Jesup)
Newsgroups: comp.sys.amiga.hardware,comp.sys.amiga.programmer
Subject: Re: Decoding MFM
Message-ID: <18157@cbmvax.commodore.com>
Date: 27 Jan 91 09:14:14 GMT
References: <2786@gould.doc.ic.ac.uk>
Reply-To: jesup@cbmvax.commodore.com (Randell Jesup)
Organization: Commodore, West Chester, PA
Lines: 22

In article <2786@gould.doc.ic.ac.uk> la@doc.ic.ac.uk (Liaket Ali) writes:
>Hi Amiga Fans!
>
>Could anyone offer some help (info/source) on how to convert raw MFM data into
>intelligible information. In the RKM Libs&Devs, mention is made of the use of
>the blitter in this task but I could not find any further info on it. Also is
>it possible to use the CPU to do the decoding.

	Decode is easy.  take the two halves of the longword, and each of
them with $55555555, shift the even bits 1 left, and or them together.  Note
that in the headers, they are adjacent - in the data block, it goes <all the
odd bits> <all the even bits> (or vice versa, I forget at the moment).
There was stuff about this in the RKMs.

	Encoding isn't so simple.

-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.commodore.com  BIX: rjesup  
The compiler runs
Like a swift-flowing river
I wait in silence.  (From "The Zen of Programming")  ;-)