Editing Amiga Original Chip Set (section)

===Floppy disk controller===
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 at 500 ([[double density]]) or 250&nbsp;kbit/s ([[single density]] or GCR). [[Modified Frequency Modulation|MFM]] or [[Group Coded Recording|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&nbsp;inch DD disks was increased from the typical 720&nbsp;KB to 880&nbsp;KB, although the less-than-ideal [[Amiga Old File System|file system]] of the earlier Amiga models reduced this again to approximately 830&nbsp;KB of actual payload data.

In addition to the native 880&nbsp;KB 3.5-inch disk format, the controller can handle many foreign formats, such as:

* [[IBM PC]]
* [[Apple II]]
* [[Mac (computer)|Mac]] 800&nbsp;KB (requires a Mac drive)
* [[AMAX Mac emulator]] (a special floppy of only 200&nbsp;KB to exchange data between Amiga and Macintosh could be formatted by the Amiga, and it could be read and written by floppy drives of both systems)
* [[Commodore 1541]] (requires 5¼-inch drive slowed to 280&nbsp;rpm)
* [[Commodore 1581]] formatted {{frac|3|1|2}}-floppy for C64 and C128

The [[Amiga 3000]] introduced a special, dual-speed floppy drive that also allowed use of high density disks with double capacity without any change to Paula's floppy controller.