Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!killer!tness7!tness1!flatline!erict
From: erict@flatline.UUCP (j eric townsend)
Newsgroups: comp.sys.cbm
Subject: Re: is it possible to read/write Commodore disks on an IBM PC?
Summary: Head stepping
Message-ID: <935@flatline.UUCP>
Date: 22 Jun 88 05:38:54 GMT
References: <22a54e8a@ralf> <19371@watmath.waterloo.edu> <565@mannix.iros1.UUCP>
Organization: a flat near the Montrose, Houston, Tx.
Lines: 30

In article <565@mannix.iros1.UUCP>, desmarai@iros13.iro.umontreal.ca (Stephane Desmarais) writes:

> (Why didn't they find a way do avoid doing more than one or two bump on
> the head stop?)


It has to do with the way the head resets to track 0.  Instead of making
a "where is the head now" sensor, they used GCR encoding.  The drive
reads the GCR encoding data off of the disk, and can figure out
where it is.   So how does it decide a reference point?  Bring the
head in towards track zero for X amount of time, where X > amount of
time for the head to go all the way across from track 40 to track 0.

Voila.  Tell the head to go to track 5 when the drive doesn't know
where the head is:  Reset head towards 0 (clatter clatter clatter).
"Ok, I've run the head all the way in, and found some sync marks,
this must be track zero.  Now I go out to track 5." (Step Step Step
Step Step).  "Here's some sync marks, this must be track 5."

For some real fun, get a simple disk diagnosis program and fiddle
with the disk head while the program is running.  I convinced my drive
that the directory was on track 17 -- when the readout said track 17
I was reading the directory info.  The problem was, it looked for it
on track 18 (which was really track 19 on the disk) and never found
it.  So much for that method of copy protection... :-)
-- 
"It was men made her that way,             Skate UNIX or go home, boogie boy...
it was us made her that way." -- from "Airhead" by Thomas Dolby
J. Eric Townsend ->uunet!nuchat!flatline!erict smail:511Parker#2,Hstn,Tx,77007
             ..!bellcore!tness1!/