Dennis Doms replied: >Andy McFadden wrote: > >> Bryan Parkoff wrote: >>> I understand that it is SpiraDisc. How does SpiraDisc block >>> Apple II >>> users from reading data using nibble editor. Is it true that we have >>> to read 6502 code (assembler) from the track 0 sector 0 before we can >>> understand what 6502 code (assembler) manipulate with the disk? >> >> It uses quarter-tracking. Apple II 5.25" disk drive controllers give >> software direct control over the stepper motor, so you can do funky >> things >> like freeze the head partway between tracks. This didn't work on some >> 3rd-party 5.25" drives, causing certain people to lend certain other >> people copies of Frogger, which subsequently got cracked, written up, >> and submitted to COMPUTIST. >> >> (Try that today and the DMCA police would have you in jail.) >> >> Anyway. You need to get the data from the tracks *and* have it >> properly synchronized, so that when they move from track 0 to track >> 0.25 >> the next set of data is right under the read head. If the data >> doesn't show up quickly enough, they know it's a copy and the boot >> fails. > >I don't know that I ever saw a disk that formerly "quarter-tracked". >What I saw were disks that wrote a segment of a track, stepped a half >track, wrote another segment of a track, etc., so that the segments >never overlapped on adjacent tracks. The controlling software could >time out whem to "step", but a copy program might was usually unable to >duplicate this since most of them worked solely on half-track steps and >with the assumption that no tracks were laid closer than one track >apart (that is, you might find a track at 3.0 and 4.5 and 5.5, but not >3.0 and 4.0 and 4.5). Quarter-tracking with Apple drives is a bit less "positive" than half- tracking, but it works. Apple drives have a track-to-track distance corresponding to two "full steps" of the head positioning stepper motor. Half-tracking simply does one step to get to the midpoint between two standard tracks. The motor is moved by energizing four "phases" which correspond to two sets of interleaved stator poles and two magnetic polarities. The rotor is a multi-pole permanent magnet having the same number of poles as each set of stator poles. The normal way of advancing the motor smoothly from one position (say, with phase 0 on, to lock the rotor) to another is to turn on phase 1, then turn off phase 0, then turn on phase 2, then turn off phase 1. This will advance the rotor two full steps. If you stop after turning off phase 0, you have advanced one step, or a half track. If you stop with both phase 0 and 1 on, you have advanced one-half step, or a quarter track. >> Copying is tricky because there's a lot of "leakage" from adjacent >> half-tracks. If you tried to write the disc as a series of tracks, >> you'd be stepping on the half-tracks and quarter-tracks to either >> side. Which might work out okay if you could get the synchronization >> just right. > >I was told (by people I presumed knew more than me :) that the stepper >motor design of Apple 5.25 drives only positioned reliably to >half-track positions. (Bit) Copy programs _did_ implement an effort to >position the head at quarter-track increments in order to attempt to >copy spiralled disks; the idea was that if you copied the spiralled >tracks at the quarter-track position between the half-track spiral that >you would get 3/4 of each segment. That hopefully would provide enough >signal for the disk head to "bite" on. The head was positioned by >starting a half-track positioning move and then "aborting" in such a >way that the head only moved part of the distance, but the people I >talked to said the positioning was not precise on typical drives. >Therefore you might need to retry on any tracks where the boot >"stalled" to see if you lucked into better positioning. The trick in any fractional-track scheme is that only partial tracks are written, so that no data written actually overlaps any other. For example, three sectors of data are written on track 0, then the head is stepped a quarter track (in one sector's time) and three more sectors are written on track 0.25, then the head is stepped another quarter track (in the time/angle required by one sector) and three more sectors are written on track 0.5, then, etc., etc. You can see why this is called "SpiraDisc". Note that by the time that the disk has completed one whole revolution, it has quarter-stepped four times, so the next three sectors can be written synchronized (in angular position) with the first sectors written, and they will be spaced a full track apart. This scheme does sacrifice one fourth of the data capacity, since the head movement must settle before reading or writing, and that takes a little time. Clever choices of format can reduce (but not eliminate) the capacity loss. >You could check the success of tracks on the original Disk II drives by >removing the cover and marking the head positioning cam in track >increments. You booted the attempted replicant disk and watched for the >head movement to "hang"; that was usually an indication the read at >that position had failed (some peograms would reboot on failure to make >it more difficult to see). > >> Spiradisc was a thing of beauty. I wish I could find my write-up of >> the Maze Craze disk, which had a disk-present check that Frogger >> didn't. I also truncated the Frogger write-up severely because I just >> referred to the Maze Craze discussion (i.e. there's not a lot of >> details about >> the protection itself). Sadly, the Maze Craze "softkey" was rejected >> for publication, so if I can't find it then it probably doesn't exist. > >The first one I saw was Lifesaver (a very useful disk recovery utility; >can't recall the software company at the moment). The boot process was >almost an artistic experience, and _fast_; a single-load disk where the >head appeared to make two continuous swipes (actually half-track steps, >but so rapid as to seem smooth) across the disk and the program was up. >Just the distinctive sound of the head sweep was sometimes a giveaway. Exactly. -michael Check out amazing quality 8-bit Apple sound on my Home page: http://members.aol.com/MJMahon/