Amiga double density disk

The Amiga double density disk format' (3 1/2", double sided, double density) was the standard format for disks on all Amiga models. It had 80 tracks per side, 11 sectors per track, and 512 bytes per sector. Data was stored with MFM encoding.

Unlike the PC-DOS 720k format, the Amiga did not use the index hole and careful timing to determine where sectors were located. Instead, a synchronization words were used: 4489 4489 (the MFM-encoded form of bytes A1 A1). Amiga disks were typically read or written as entire tracks at once, rather than reading individual sectors. Sectors could be in any order on a track.

Because of the Amiga's more flexible approach to the floppy disk, the Amiga could read most other physical disk formats, including the PC disks, Apple disks and, with an suitable external disk drive, even C64 and Apple 5 1/4" disks.

No Amiga model ever came with a high-density drive. However, the Amiga did support them, and high-density drives were available from third party vendors. The Amiga high density disk format is the same as the double disk format, but with 22 sectors per track instead of 11.

Disks were generally formatted with with one of the Amiga's standard filesystems: OFS (Old File System) natively supported by Workbench 1.x, or FFS (Fast File System) supported by Workbench 2.x and higher. However, other filesystems could be used, including PFS (Professional File System, which later became AFS: Ami File Safe) and SFS (Smart File System). The Amiga could read PC disks formatted with the FAT16 filesystem using third party software called CrossDOS. Commodore licensed CrossDOS from its authors and bundled it with Workbench 2.1 and higher.

The Amiga operating system provided raw access to floppy disks via a driver called trackdisk.device. It was possible to squeeze even more onto double-density disks; Klaus Deppisch's diskspare.device allowed 12 sectors instead of 11, and, for drives that supported it and disks that had flux material that far in, 82 tracks instead of the standard 80, in total allowing 984KiB instead of the standard 880KiB.