Amiga double density disk

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File Format
Name 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.

The Amiga uses an MFM sync word to mark the start of sectors (0x4489, a special MFM-encoding of the byte 0xA1, so it can be distinguished from normal 0xA1 bytes which would be MFM-encoded as 0x44A9). This contrasts with the PC-DOS 720K format which used the floppy disk index hole and careful timing to determine where sectors were located. 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; each sector had a header with its track and sector number.

Because of the Amiga's more flexible control over the floppy disk, the Amiga could read most other physical disk formats, including the PC disks, Apple disks and even Commodore 64 and Apple 5¼" disks with a suitable external disk drive.

Only the Amiga 4000 ever came with a built-in high-density drive. High-density drives were also 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 AmigaOS 1.0-1.2, or FFS (Fast File System) supported by AmigaOS 1.3 and higher. However, other filesystems could be used, including PFS (Professional File System), AFS (Ami File Safe) and SFS (Smart File System). The Amiga could read PC disks formatted with the FAT12 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.

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