"Fat tracks" are not really fat tracks

"Fat tracks" are not really fat tracks

From: Pete Rittwage <peter_at_rittwage.com>
Date: Sun, 22 Mar 2009 11:01:26 -0500
Message-ID: <49C660D6.20103@rittwage.com>
Hi guys,

With the help of "TeaRex" on my forum, I added a simple way to 
read/write disk images using the index hole sensor built into the 1571. 
  Through this addition, a revelation occurred.

We had always assumed (since the 80's) that the Electronic Arts' "fat 
tracks" protection had 2 tracks (34 and 35) perfectly aligned.  I added 
the index hole code and wrote out the disk perfectly aligned to track 0 
and thought it was beaten.  :)

Well, it turns out this assumption is *not* true.  When reading against 
the index hole, track 35 is actually skewed back 1/4 track or so on all 
the original disks.  If I write it back out skewed in this way, it 
boots.  These were never "FAT" tracks at all, just a specific track skew 
between two identical copies of a track.

XEMAG 2.0 (the Activision variety of this protection) is skewed about 
1/2 track between 35 and 36 (and some amount between 34 and 35 also that 
doesn't appear to be checked).

I noticed years back that this protection boots if the drive motor is 
slowed down to 298.5 or lower (within reason) no matter what the skew. 
I guess something about how their timing check is setup allows this to pass.

Pete Rittwage
C64 Preservation Project

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Received on 2009-03-23 02:59:40

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