Floppy Disk Variants - Commodore Amiga

Commodore Amiga

The Commodore Amiga computers used an 880 kByte format (11×512-byte sectors per track, times 80 tracks, times two sides) on a 3½-inch floppy. Because the entire track is written at once, inter-sector gaps could be eliminated, saving space. The Amiga floppy controller was basic but much more flexible than the one on the PC: it was free of arbitrary format restrictions, encoding such as MFM and GCR could be done in software, and developers were able to create their own proprietary disc formats. Because of this, foreign formats such as the IBM PC-compatible could be handled with ease (by use of CrossDOS, which was included with later versions of AmigaOS). With the correct filesystem driver, an Amiga could theoretically read any arbitrary format on the 3½-inch floppy, including those recorded at a slightly different rotation rate. On the PC, however, there is no way to read an Amiga disk without special hardware, such as a CatWeasel, or a second floppy drive.

Commodore never upgraded the Amiga chip set to support high-density floppies, but sold a custom drive (made by Chinon) that spun at half speed (150 RPM) when a high-density floppy was inserted, enabling the existing floppy controller to be used. This drive was introduced with the launch of the Amiga 3000, although the later Amiga 1200 was only fitted with the standard DD drive. The Amiga HD disks could handle 1760 kByte, but using special software programs it could hold even more data. A company named Kolff Computer Supplies also made an external HD floppy drive (KCS Dual HD Drive) available which could handle HD format diskettes on all Amiga computer systems.

Because of storage reasons, the use of emulators and preserving data, many disks were packed into disk-images. Currently popular formats are .ADF (Amiga Disk File), .DMS (DiskMasher) and .IPF (Interchangeable Preservation Format) files. The DiskMasher format is copyright-protected and has problems storing particular sequences of bits due to bugs in the compression algorithm, but was widely used in the pirate and demo scenes. ADF has been around for almost as long as the Amiga itself though it was not initially called by that name. Only with the advent of the Internet and Amiga emulators has it become a popular way of distributing disk images. The proprietary IPF files were created to allow preservation of commercial games which have copy protection, which is something that ADF and DMS unfortunately cannot do.