Re: "Fat tracks" are not really fat tracks

Re: "Fat tracks" are not really fat tracks

From: Pete Rittwage <>
Date: Sun, 22 Mar 2009 20:53:30 -0500
Message-ID: <>
Pete Rittwage wrote:
> 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.

Another finding.  XEMAG 2.0 does work, it's just that the Activision 
loader crashes on the 1571.  Even in VICE you can test this- change the 
drive to 1571 and try to load Fireworks Celebration Kit or Gamemaker- it 
will crash.

Write out a disk with IHS in the 1571 and use it in a 1541 and it boots 
fine.  :)

Pete Rittwage

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Received on 2009-03-23 02:59:54

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