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This is a low-level tech question, as I am developing a driver for an FDC at its command-set level.

Having reviewed a number of sources, I cannot find the algorithm to format a 360K diskette using a newer 1.2M drive. Lots talk about the differences in track size, magnetic material, how to degauss the media. But none lays out the algorithm in where heads must be positioned and in what order.

The goal is having a diskette formatted using a 1.2M drive to be portable to a 360K drive as much as possible. I do not have a 360K drive, thus I cannot perform trial-and-error to empirically find out the best way of dealing with formatting.

I had formatted two 360K diskettes using a Windows XP machine recently, and noticed a peculiar behavior: heads start at track 0, to somewhere at the ½ of the surface towards the center in double-track step, then heads go back about 20 tracks, and perform formatting in single track step up to the position where they retracted, and then continue as usual in double track step. This happened on two diskettes, thus it is not caused by the media defect.

There must be some explanation for this behavior, and formatting a 360K disk in a 1.2M drive may not be that straightforward as going from outer to inner in double-track step and writing (formatting) the tracks. What can, and must, I do with the floppy disk controller to format a 360K diskette?

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    If your disks have previously been written in a genuine 360K drive, this is unlikely to be successful, because the width of the track written by a 1.2M drive head is about half the width of a 360K head track, so it will never completely erase the original data. The reliable solution is to bulk erase the floppy disks, but I guess you don't have a bulk eraser available. A 360K drive will (usually) read the narrow tracks successfully, so long as there is no "magnetic noise" either side of the narrower strip of data.
    – alephzero
    Apr 25 '20 at 21:34
  • "and noticed the distinct behavior: ..." I've never observed this, and I've been using 360k diskettes on MS-DOS. But then, I've never formatted a 360k disk on XP. I don't see any reason why 360k couldn't be formatted in the usual way (track by track).
    – dirkt
    Apr 25 '20 at 22:25
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    @Anonymous Sorry, but the only "practical" advice I can offer is this won't work. Unless "as successful as possible" means "100% successful every time," the result is worthless in practice.
    – alephzero
    Apr 26 '20 at 0:38
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    Congrats, you found a 30 year old bug in the 360kB on 1.2MB drive formatting method.
    – Janka
    Apr 26 '20 at 1:14
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    I would do a double check with an older Linux distribution first.
    – Janka
    Apr 26 '20 at 1:16
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Apple's Disk II could control drive head motion in increments of 1/4 track, which made it possible for it to ensure that there could be no residual data between tracks, and I think some other drives like the Commodore allowed for half-track increments. PC drives and others that control the stepper motor using up/down pulses rather than motor phases, however, are limited to moving in full-step increments. Further, while the Apple drive (and I think the Commodore drive) could enable the write signal without writing out any data, the PC cannot.

It would thus be possible to write a program for the Apple II which would erase all flux transitions from the surface of a disk that the Apple drives can reach. The one problem is that PC drives expect to move the head over a range of 40 low-density tracks, but the Apple drives can't move the head in quite that far, and would thus have no way of erasing the areas between the last few low-density tracks.

I think the most reliable way to format and write a disk using a high-density drive that would be readable with a low-density 5.25" drive would be to use something like a television set degausser to neutralize any existing magnetic fields on the disk, and then write the disk once with a 5.25" drive. Rewriting the disk with another drive whose alignment doesn't precisely match the first could leave residual data that might confuse a low-density 5.25" drive, but if the disk is only written once that shouldn't be a problem.

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