Formatted Disk Image

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FDI is a file image of a disc created with the Disk2FDI PC software. It is often used to read Amiga or C64 discs and other discs formatted in a way that normally can't be read with a PC. ^[1]

The software uses two methods:

Connect two floppy disk drives to a PC and fill drive-0 with an ordinary PC formatted floppy disc. Drive-1 receives a foreign format disc. Then the software reads as normal from drive-0, but then selects drive-1 instead. It thus bypasses obstacles that would make the floppy controller otherwise refuse the format of the foreign disk. In particular it gets the PLL to sync.

Connect one disc drive to the PC via a cable that lets the parallel port interrupt pin "/strobe" trigger on the read signal. The PC only needs to make the drive spin and use correct track and head. The reading is done by software exploiting interrupt timings.