Tom Greene wrote:
On Jan 23, 5:47 pm, "Michael J. Mahon" <mjma...@aol.com> 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 ******** Note new website URL ******** NadaNet and AppleCrate II for Apple II parallel computing! Home page: http://home.comcast.net/~mjmahon/ "The wastebasket is our most important design tool--and it's seriously underused."