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From: v089pfrb@ubvmsd.cc.buffalo.edu (Jeffrey C Murphy)
Newsgroups: comp.sys.amiga.hardware,comp.sys.amiga.programmer
Subject: Re: Decoding MFM
Message-ID: <56321@eerie.acsu.Buffalo.EDU>
Date: 26 Jan 91 23:27:12 GMT
References: <2786@gould.doc.ic.ac.uk>
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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
>[del...snip....del...cut..]

 With MFM every data bit is followed by a clock bit (the clock bit is toggled
according to the status of the adjacent  clock bits: if an adjacent data bit
is set then the clock bit is reset and versa vice-a)

Data:    1 0 1 0 0 0 0 1
Clock:  0 0 0 0 1 1 1 0
MFM:    0100010010101001

To decode you can use this simple routine (assuming you already have the
data in memory):

 ... 
 MOVE.L (A0)+,D0     ; get first longword
 MOVE.L (A0)+,D1     ; get second
 ANDI.L #$55555555,D0 ;remove clock bits
 ANDI.L #$55555555,D1 ;for d1 too
 LSL.L #1,D1         ; Shift bits in D1 over one position
 OR.L D1,D0          ; Overlay D1 on D0 and put in D0
 ...

As far as the blitter goes, it can do this much faster than the cpu can
(unless you have a fast cpu) but requires you to set up structures for the
blitter (which I have not really learned about  yet....)

I got the decode routine from Amiga Disk Drives Inside and Out by Abacus,
which is about the worst proging book I have ever read (they repeatedly
refer to absolute memory addresses! and routines that are not always at the
addresses that they say.) This is because they get the book from Data Becker,
a german company which thinks the amiga is really a C64 with everything 
permanently in rom. sigh... Anywaze... a pretty good book (although not the
best) is the amiga programmer's handbook by sybex. I don't know if it is
still available.

Hope this helps.

Jeff M. SUNYAB
v089pfrb@ubvmsd.bitnet