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
http://rittwage.com


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