'Nibread on no-sync tracks'
Author:Lord Crass (guest: search)
Date: Mon, Apr 11th, 2011 @ 12:00 ( . )

It's possible to "get lucky" with the current nibread and manage a successful read of these tracks. It all depends on where it started reading the track into memory. As an example, take the following layout in memory where the track was read in:

Data1
Data1
..
Track gap/write splice
Data2(start of track is actually here)
Data2
..

Data2 will be shifted differently from Data1 because of the illegal GCR in the write splice. So even if you rotate the track in memory so that the gap is at the end, you'll wind up with this:

Data2(start of track is here)
Data2
..
Data1
Data1
Track gap/write splice

Unless you know exactly how to bit-shift Data2 to match Data1 (or vice-versa), the track is ruined. These particular V-Max titles have a lot of illegal GCR in this track, not just in the write-splice area, so it exacerbates this problem.

Adding the header search ensure that you always start reading where the data is and that the illegal GCR is preferably all at the end of the track and you'll get something like this:

Data(start of track is here)
Data
Data
Data
..
Track gap/write splice

If you look at track 20 in these non-working V-Max images, some of them do have the data all at the beginning, so it's good, but it's shifted. It would probably remaster OK, but won't work in an emulator because V-Max loader specifically looks for $5A $5A, not the shifted $4B $4B which is in the image. On a real C64/disk, if it doesn't find the $5A byte pattern, the disk will just rotate past the bad GCR causing the framing to shift and hopefully it finds it on the next pass. It might take 8 reads to get the right shift, but since the disk rotates 5 times per second, it's not very noticeable to the user. Current emulators will never shift the data, it will always read back the same, so it will never work and you'll see it reading track 20 forever.

Other than the illegal GCR reducing your chances of getting a good read, the track 20 loader for this version of V-Max is always the same. You can copy it from a working image and it will boot just fine.

BTW, Into the Eagle's nest has a sync-length check as well (I forget which track, I'll have to check my notes when I get home), so you'd need to pass that after you get get a good image of track 20.


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--* Nibread on no-sync tracks
4/11/2011 @ 12:26--Lord Crass
4/11/2011 @ 19:29--Lord Crass
4/12/2011 @ 02:40--hyper active

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