On a sunny day (Mon, 09 Mar 2020 06:59:02 GMT) it happened Jan Panteltje
 wrote in :

>>By the time the IBM PC appeared most CP/M-80 machines I was seeing
>>wrote either 10 512-byte or 5 1024-byte sectors on a track on a 5.25"
>>floppy -- making 400kB of data in all. The mostly used WD controller
>>chips that allowed that. The floppy controller in the PC writes too
>>much lead data on each track to achieve that, so the best it can do is
>>9 sectors of 512 bytes -- giving 360k (DOS originally wrote only 8
>>sectors to keep the addressing simple(r), giving 320k in all).
>
>Not sure you should blame the IBM PC floppy controller,
>as I used it in the CP/M clone I wrote, am talking about the 8272 chip.
>I used the Kaypro II format with its 40 tracks x 10 sectors, 400kB.
>Description of that system here:
> http://panteltje.com/panteltje/z80/system14/system14.doc.txt
>
>Circuit diagram of the floppy controller board I designed:
> http://panteltje.com/panteltje/z80/system14/diagrams/fdc-1.jpg
>  IC1 is the 8272, circuit is dated 13-7-1984
>
>Wrote the drivers too of course.
>
>
>>IBM really screwed up there!
>
>No, maybe it was Microsoft?

PS, could have been an IBM BIOS limitation?

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