Page "Original Chip Set" Paragraph 45

The floppy controller is unusually flexible.

It can read and write raw bit sequences directly from and to the disk via DMA or programmed I / O.

MFM or GCR were the two most commonly used formats though in theory any run-length limited code could be used.

It also provides a number of convenient features, such as sync-on-word ( in MFM coding, $ 4489 is usually used as the sync word ).

MFM encoding / decoding is usually done with the blitter — one pass for decode, three passes for encode.

Normally the entire track is read or written in one shot, rather than sector-by-sector ; this made it possible to get rid of most of the inter-sector gaps that most floppy disk formats need to safely prevent the " bleeding " of a written sector into the previously-existing header of the next sector due to speed variations of the drive.

If all sectors and their headers are always written in one go, such bleeding is only an issue at the end of the track ( which still must not bleed back into its beginning ), so that only one gap per track is needed.

This way, for the native Amiga disk format, the raw storage capacity of 3. 5 inch DD disks was increased from the typical 720 kB to 880 kB, although the less-than-ideal file system of the earlier Amiga models reduced this again to ca.

830 kB of actual payload data.