Tom Greene wrote:
> On Jan 23, 5:47 pm, "Michael J. Mahon"  wrote:
>> Tom Greene wrote:
>>> That is followed by two bytes indicating the number of tracks in the
>>> image, then the track data. Each track has a 4-byte header containing
>>> the track number (in quarter-tracks) and length. Unused tracks are
>>> omitted.
>> How do you tell that a "track" is unused??
>>
> 
> The A2V2 imaging program that comes with Virtual II does some kind of
> analysis on the data to decide if the track is valid, and the tracks
> deemed invalid are not stored in the image. I'm not sure what that
> decision is based on.
> 
> In practice it doesn't seem to work very well for the few protected
> disks I tried. None of the resulting images worked, and for one disk
> it even discarded *all* tracks. It works OK for normally formatted
> disks, discarding most of the half-tracks.

Just for the record, the only analysis that can be guaranteed to
work is to faithfully (cycle-accurately) simulate the execution of
the disk *from boot* on an Apple II.  This is equivalent to a fully
automatic "boot trace" of the disk.

The reason for this is that the Disk ][ and Disk ][ Controller allow
for so much variability in the recording and reading of information
from a disk that there is *no other way* to correctly determine which
magnetic transitions (or transition-free regions) are necessary for
the correct execution of the program.

In fact, the original disk may not have been written using a Disk ][
drive and Controller, so that the disk may have characteristics not
writeable (but distinguishable) on a Disk ][.

-michael

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