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In the late 1970s to early 80s incompatible system software (i.e. MicroSoft Basic and CP/M 1.4) for S100 computers could be released in 8 inch, hard and soft sector, floppy disk formats. In that era what computer systems allowed you simply swap backwards and forwards between these incompatible hard (MicroSoft BASIC) and soft (CP/M 1.4) sector disc formats on the one 8 inch floppy disc drive by a reboot?

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  • You might largely overestimate the concepts of compatibility and data exchange requirements for early computing. These came up only relatively late in the CP/M history and with 5 1/4" drives. Most people were absolutely happy when they could read their own disks :) Data exchange between computing islands wasn't really a thing.
    – tofro
    Mar 16 at 9:06
  • There was no data exchange or transfer of information between the IMS hard and soft sector 8 inch floppy disks. System booted in one mode or the other but did not have a dual mode. Hard sector mode was only to access historical documents
    – PDP11
    Mar 20 at 18:03

2 Answers 2

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On a "standard" soft-sectored 8" disk, the drive generates one (index) pulse per rotation. On a hard-sectored floppy, the same drive will generate 33 (or 17) pulses (32 sector pulses plus one index pulse which is out-of-sync with the others - The sector and index holes were (on most drives, but not on all) located on the same circumference and picked up by the same sensor).

The Shugart SA 801 8" floppy drives provide a separate sector and index signal (many other drives could do this as well, the SA 801 is only an example. They filter out the one out-of sync pulse and provide it as an index pulse). This makes it possible to build a controller that can read and write both soft and hard sectored disks. (For soft-sectored disks, simply ignore the sector pulses and work on index only). In practice, you could connect the SA 801 drive to either a controller for hard- or soft-sectored (simply don't connect the sector signal) floppies, it would work on both. By doing that, you could even format a hard-sectored floppy as a soft-sectored one (That could, obviously, not be read by a soft-sector-only drive, so this was relatively useless) .

Drive electronics providing the necessary signals, however, is only part of the picture. To be able to handle both hard-sectored and soft-sectored floppies in the same drive, you'd need a controller that can do both (Note a disk controller for a hard-sectored drive can be built a lot simpler than for soft-sectored, so from a pure cost perspective, you'd probably prefer hard-sectored).

The TMS-9909 is one example of a floppy disk controller that could handle both hard and soft-sectored disks with specific commands (I have yet to find a controller board that used it, though, although we can be sure there was one and I didn't do an exhaustive search. The TMS9909 apparently only had a very limited lifespan). The rest is "only software".... Because the hardware preconditions could all be fulfilled, there were specific "format conversion boxes" that could basically read and write anything and convert between various formats. Modern µ-Controller-based floppy hardware (Fluxcopy/Fluxteen, for example) support both hard and soft-sectored disks.

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  • I worked for a reseller of Industrial Micro Systems, S100 systems with the Shugart 8in drives and lawyers had purchased it with software for document editing. It was configured with a hard sector controller and MicroSoft software. CP/M 1.4 was a step up but incompatible and it was delivered on soft-sectored disks. As it was S100 system a soft sectored controller could also be installed. Add the TTL logic to switch the drive between hard and soft sectored and you could easily boot into either system. I was unaware that the TMS-9909 existed
    – PDP11
    Mar 18 at 7:28
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I'm not aware of any systems that had configurable hard/soft sector floppy drives. Typically the floppy drive internal electronics were different, so switching between hard and soft sectoring would be difficult.

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    “I’ve never heard of any, because it was hard to do” is a comment than an answer.
    – RonJohn
    Mar 16 at 19:25

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