RISC OS is the original OS for the ARM chip, and the Raspberry Pi can run it natively. It apparently doesn't support floppy disc drives (the Pi lacks the interface, and USB drives appear like USB memory sticks to the OS). If it did, the ADFS (Advanced Disc Filing System) would format discs to 800 or 1600KB.

https://en.wikipedia.org/wiki/Advanced_Disc_Filing_System

(PS, it's 90mm. Odd that even outside the USA we said 3½ inches.)