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Unlike many drives, the Apple Disk II has inputs that control the individual head stepper coils. These are commonly described as advancing by half-tracks, since advancing a full track requires activating two coils in sequence. Four-pole stepper motors, however, can not only be moved by driving coils individually, but also by driving pairs of coils. Since advancing by one pole moves a half track, energizing a pair of cells will access a quarter-track location between two half-track locations.

Experimentally, at least on the Apple //c, when writing data sequentially, it's possible to write data at 3/4-track intervals and then read back previously written tracks. For test purposes, I used a program that packs twelve double-high-res graphics pictures into the space of 27 normal tracks (2.25 tracks each) and allows random access via keyboard. I likely could have used 15 if I'd had that many DHGR pictures handy. This would likely not be reliable if tracks were being written by different drives at different times, and I'm not sure how reliable it would be for data written using one drive and read using another (I just have one drive on my Apple //c) but the fact that this works at all would suggest that it could be expected to be reliable if information were written on quality media using a drive specialized for the purpose--something that could have been really great for copy-protection purposes if a game would make use of data in a way that effectively required 200K to fit on a single disk ("normal" Apply formatting is 35 tracks of 16 sectors of 256 bytes, but the game Prince of Persia used a format that packed 4.5K bytes per track, which would allow 35 tracks to hold 157.5K); 200K would have exceeded that by a substantial margin.

Were there any other floppy drive systems that offered quarter-track control, or high-density systems that offered half-track control, or was such ability unique to the Disk-II style systems?

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  • You mean beside other Apple drives? --- Also, nice hack, like it. Has been done back in the days. Yields about10-15 extra tracks (upper limit when going beyond track 35 - most drives can do 40 to 44). It's worth to keep the first tree tracks separated by 4 steps. to ease unusual loaders. Hacking RWTS for 3 steps is possible
    – Raffzahn
    Dec 9 '20 at 17:40
  • @Raffzahn: Is it reliable for uses cases other than writing tracks sequentially using only one drive? I would think that writing a track would slightly disturb the contents of tracks that are 0.75 away; such disturbance seems tolerable when it only occurs on one side, but writing the tracks both 0.75 above and 0.75 below a track would increase the likelihood of corruption. Further, if the track below were written from a drive that wrote it 0.05 tracks above the expected position, while the track above were written by one that wrote it 0.05 tracks below, that would encroach even more.
    – supercat
    Dec 9 '20 at 17:43
  • @Raffzahn: If this were at all common back in the day, I would think emulators would support it. In any case, my question is about drives which aren't designed for media compatibility with the Disk II.
    – supercat
    Dec 9 '20 at 17:45
  • Didn't say all that common, just it has been used. After all, it's only used in a closed setup with a modified RWTS, so anyone who wanted to exchange disks outside had to use standard format, as Apple DOS cold not be adjusted dynamic. So essential only for private datadisks or alike. Track stepping width of SA400 is ~500 µm (48 TPI), while trackwidth (defined by the head) is 300 µm (330 with some security). So this allows outting tracks closer and still be (fairly) transportable.
    – Raffzahn
    Dec 9 '20 at 18:27
  • 1
    It has definitely been used. Some Apple II games had the data written is a single spiral track as copyright protection.
    – RETRAC
    Dec 10 '20 at 19:55
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The Disk-II mechanism is essentially a floppy drive with the low-level controller ripped out. Few, if any, other manufacturers were willing to do that, as it means a more complex controller and connection to the computer.

Drive mechanisms from the Shugart SA-400 onwards used just two lines — Direction and Step — to manage the heads. To manage the four bipolar stepper motor lines required far more complex timing and switching, and while the Disk-II controller is a marvel of electronics, it was also implementing a solved problem. It seemed to be more important to Apple at the time to produce a low parts cost design that maximised profit than to get a working drive to market quickly.

By the time other competitors to Apple were implementing disk drives, rapid time to market was more important than gaining a few more bytes on a disk by designing your own low-level controller. It was also likely that the disk drive market was larger, and no one buyer would have enough clout to demand their own weird controller-less variant.

I know of no other computer company that went Apple's way. That's not to say that someone else didn't, but the reasons for Apple making the choices they did were very specific to their market entry time and their technical team being very close to management.

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  • 2
    Ironically, the Apple Disk Controller II is among the simplest anywhere. I think Woz pushed his penny pinching a bit further than would be ideal, since it took many years before disk routines appeared that could read and decode a full track worth of 6:2 data in a single revolution.
    – supercat
    Jan 4 at 3:10
  • sure, it's simple: but it takes up all of the CPU's cycles to manage the process, and sending stepper drive pulses the full length of a cable probably seemed a great idea at the time.
    – scruss
    Jan 4 at 17:27
  • The current for the stepper motor is switched within the drive itself, so the cable is simply carrying four logic-level signals for the heads. Were there any mass-market drives that could use GCR encoding without using the CPU to read or write individual bytes?
    – supercat
    Jan 4 at 17:58
  • Can't say for certain. Apple's IWM/SWIM looks like it can return bytes with only an address and a clock (??). The Amiga's controller was a track-at-once DMA thing for 3½" drives could do GCR, in theory. After the IBM PC, GCR became pretty niche and likely not worth the effort unless you had compatibility to maintain
    – scruss
    Jan 7 at 18:10
  • FM requires two encoded bits for each bit of data, and can generate one flux transition per bit. GCR on the Apple requires 8 for 6, and on the Commodore requires 5 for 4, again with up to one flux transition per bit. MFM uses two encoded bits per bit of data, but never writes two consecutive flux transitions, thus allowing use of a raw data rate twice as high as FM.
    – supercat
    Jan 7 at 19:18
0

There used to be games that used half tracks, and I believe, quarter tracks for copyright protection.

Skipping half tracks in the middle was fairly common, there were special copy programs that could copy those disks but they where unreliable and took forever. If a normal floppy was written as 1 3 5 7 9 half track such a disk would be written as 1 3 6 8 10 and such pattens. It would reduce the capacity of the disk a bit but make it harder to copy.

As for quarter tracks I recall one program that wrote the floppy disk as a sort of quantum vinyl record with a spiral jumping a quarter track to the inside every quarter rotation.

You could read by reading all the quarter quarter tracks to find the legitimate block of data with a marker. I suspect you would need a special modified disk drive to write such things, or very precise timing.

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