Amiga Old File System
On the Amiga, the Old File System was the filesystem for AmigaOS before the Amiga Fast File System. Even though it used 512-byte blocks, it reserved the first small portion of each block for metadata, leaving an actual data block capacity of 488 bytes per block. It wasn't very suitable for anything except floppy disks, and it was soon replaced.
Originally known as the Amiga File System, it came from the filesystem of TRIPOS, which formed the basis of the first versions of AmigaDOS. It received the nickname of "Old" or "Original" File System when Fast File System was released with AmigaOS 1.3.
OFS is very good for repairing the filesystem in the event of a problem, although the so-called DiskDoctor provided by Commodore quickly earned the name DiskDestroyer, because it could not repair No-DOS type autostart disks provided by third-party software manufacturers as bootable disks for games. The idea to create non-standard autobootable disks was born in a primitive attempt to prevent copy of such disks and to avoid the loading and launch of Amiga DOS, in order to directly access the Amiga graphic, audio and memory chipsets. DiskDoctor in fact changed autostart disks bootblocks into standard AmigaDOS-based ones, renaming a disk with "Lazarus" namedisk, and made the autostart disk unusable.
Amiga uses MFM encoding/decoding by default when handling floppy disks. There are 80 cylinders on an Amiga floppy disk. Each cylinder has 2 MFM tracks, one on each side of the disk. Double density (DD) disks have 11 sectors per MFM track, high density (HD) disks have 22 sectors.
The geometry of an Amiga floppy disk is as follows:
The DD disk has 11 * 2 * 80 = 1760 (0 to 1759) blocks, while the HD disk has 22 * 2 * 80 = 3520 blocks.
Amiga stores 880 KiB on a DD disk and 1760 KiB on an HD floppy disk.
Prior to AmigaOS 3.5, AmigaDOS file handles maintained a 32-bit wide offset parameter (unsigned), telling where to start the next read or write operation. The biggest size for any single Amiga file under these operating systems therefore comes to 232 = 4 GiB. After Amiga OS 3.5, file handles may reference 264 = 16 EiB files. However, OFS-formatted disks continue to retain the 32-bit limitations, for that is an intrinsic limitation of the format as recorded on the media.
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