 | This article relies largely or entirely on a single source .(October 2015) |
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. [1]
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.
An OFS datablock stores block size BSIZE-24 bytes (i.e. normally 488 bytes at most frequently used BSIZE of 512 bytes).
The rootblock is located at the physical middle of the media: block number 880 for DD disks, block 1760 for HDs. This helps minimize seek times.
The exact calculation for where it is stored is as follows:
numCyls = highCyl - lowCyl + 1 highKey = numCyls * numSurfaces * numBlocksPerTrack - 1 rootKey = INT (numReserved + highKey) / 2
The rootblock contains information about the disk: its name, its formatting date, etc. It also contains information on accessing the files/directories/links located at the uppermost (root) directory.
The characters '/' and ':' are forbidden in file and volume names, but *!@#$%|^+&_()=\-[]{}';",<>.? and letters with diacritical marks like âè are allowed.
The date fields in the root block (and other blocks) are structured in the form of DAYS, MINS and TICKS. The DAYS field contains the number of days since January 1. 1978. MINS is the number of minutes that have passed since midnight and TICKS are expressed in 1/50s of a second. A day value of zero is considered illegal by most programs. Since the DAYS value is stored as a 32-bit number, the Amiga filesystem does not have an inherent Year 2000 problem or Year 2038 problem.
To reach a file, directory or link, AmigaDOS uses a hash function to calculate which 32-bit word in the disk block to use as a pointer to a hash bucket list, which in turn contains the file, directory, or link record. A bucket list is used to support filesystem objects with names that hash to the same offset. For example: file_1a, file_24 and file_5u have the same hash value.
Filename characters can be lowercase and uppercase, but are not case sensitive when accessed. That is to say, "MyFile" and "myfile" in the same directory refer to the same file.
Files are composed of a file header block, which contains information about the file (size, last access time, data block pointers, etc.), and the data blocks, which contain the actual data. The file header block contains up to BSIZE/4-56 data block pointers (which amounts to 72 entries with the usual 512 byte blocks). If a file is larger than that, file extension blocks will be allocated to hold the data block pointers. File extension blocks are organised in a linked list, which starts in the file header block ('extension' field).
The Commodore 1581 is a 3½-inch double-sided double-density floppy disk drive that was released by Commodore Business Machines (CBM) in 1987, primarily for its C64 and C128 home/personal computers. The drive stores 800 kilobytes using an MFM encoding but formats different from the MS-DOS, Amiga, and Mac Plus formats. With special software it's possible to read C1581 disks on an x86 PC system, and likewise, read MS-DOS and other formats of disks in the C1581, provided that the PC or other floppy handles the "720 kB" size format. This capability was most frequently used to read MS-DOS disks. The drive was released in the summer of 1987 and quickly became popular with bulletin board system (BBS) operators and other users.
File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by current operating systems for personal computers and many mobile devices and embedded systems, allowing interchange of data between disparate systems. The increase in disk drives capacity required three major variants: FAT12, FAT16 and FAT32. The FAT standard has also been expanded in other ways while generally preserving backward compatibility with existing software.
Hierarchical File System (HFS) is a proprietary file system developed by Apple Inc. for use in computer systems running Mac OS. Originally designed for use on floppy and hard disks, it can also be found on read-only media such as CD-ROMs. HFS is also referred to as Mac OS Standard, while its successor, HFS Plus, is also called Mac OS Extended.
AmigaDOS is the disk operating system of the AmigaOS, which includes file systems, file and directory manipulation, the command-line interface, and file redirection.
ProDOS is the name of two similar operating systems for the Apple II series of personal computers. The original ProDOS, renamed ProDOS 8 in version 1.2, is the last official operating system usable by all 8-bit Apple II series computers, and was distributed from 1983 to 1993. The other, ProDOS 16, was a stop-gap solution for the 16-bit Apple IIGS that was replaced by GS/OS within two years.
Disk formatting is the process of preparing a data storage device such as a hard disk drive, solid-state drive, floppy disk or USB flash drive for initial use. In some cases, the formatting operation may also create one or more new file systems. The first part of the formatting process that performs basic medium preparation is often referred to as "low-level formatting". Partitioning is the common term for the second part of the process, dividing the device into several sub-devices and, in some cases, writing information to the device allowing an operating system to be booted from it. The third part of the process, usually termed "high-level formatting" most often refers to the process of generating a new file system. In some operating systems all or parts of these three processes can be combined or repeated at different levels and the term "format" is understood to mean an operation in which a new disk medium is fully prepared to store files. Some formatting utilities allow distinguishing between a quick format, which does not erase all existing data and a long option that does erase all existing data.
The Amiga Fast File System is a file system used on the Amiga personal computer. The previous Amiga filesystem was never given a specific name and known originally simply as "DOS" or AmigaDOS. Upon the release of FFS, the original filesystem became known as Amiga Old File System (OFS). OFS, which was primarily designed for use with floppy disks, had been proving slow to keep up with hard drives of the era. FFS was designed as a full replacement for the original Amiga filesystem. FFS differs from its predecessor mainly in the removal of redundant information. Data blocks contain nothing but data, allowing the filesystem to manage the transfer of large chunks of data directly from the host adapter to the final destination.
The Smart File System (SFS) is a journaling filesystem used on Amiga computers and AmigaOS-derived operating systems. It is designed for performance, scalability and integrity, offering improvements over standard Amiga filesystems as well as some special or unique features.
In computing, the BIOS parameter block, often shortened to BPB, is a data structure in the volume boot record (VBR) describing the physical layout of a data storage volume. On partitioned devices, such as hard disks, the BPB describes the volume partition, whereas, on unpartitioned devices, such as floppy disks, it describes the entire medium. A basic BPB can appear and be used on any partition, including floppy disks where its presence is often necessary; however, certain filesystems also make use of it in describing basic filesystem structures. Filesystems making use of a BIOS parameter block include FAT12, FAT16, FAT32, HPFS, and NTFS. Due to different types of fields and the amount of data they contain, the length of the BPB is different for FAT16, FAT32, and NTFS boot sectors. Combined with the 11-byte data structure at the very start of volume boot records immediately preceding the BPB or EBPB, this is also called FDC descriptor or extended FDC descriptor in ECMA-107 or ISO/IEC 9293.
Floppy disk format and density refer to the logical and physical layout of data stored on a floppy disk. Since their introduction, there have been many popular and rare floppy disk types, densities, and formats used in computing, leading to much confusion over their differences. In the early 2000s, most floppy disk types and formats became obsolete, leaving the 3+1⁄2-inch disk, using an IBM PC compatible format of 1440 KB, as the only remaining popular format.
Amiga Disk File (ADF) is a file format used by Amiga computers and emulators to store images of floppy disks. It has been around almost as long as the Amiga itself, although it was not initially called by any particular name. Before it was known as ADF, it was used in commercial game production, backup and disk virtualization. ADF is a track-by-track dump of the disk data as read by the Amiga operating system, and so the "format" is really fixed-width AmigaDOS data tracks appended one after another and held in a file. This file would, typically, be formatted, like the disk, in Amiga Old File System (OFS).
In computer file systems, a cluster is a unit of disk space allocation for files and directories. To reduce the overhead of managing on-disk data structures, the filesystem does not allocate individual disk sectors by default, but contiguous groups of sectors, called clusters.
IMG, in computing, refers to binary files with the .img filename extension that store raw disk images of floppy disks, hard drives, and optical discs or a bitmap image – .img.
The following tables compare general and technical information for a number of file systems.
Amiga support and maintenance software performs service functions such as formatting media for a specific filesystem, diagnosing failures that occur on formatted media, data recovery after media failure, and installation of new software for the Amiga family of personal computers—as opposed to application software, which performs business, education, and recreation functions.
SpartaDOS X is a disk operating system for the Atari 8-bit family of computers that closely resembles MS-DOS. It was developed and sold by ICD, Inc. in 1987-1993, and many years later picked up by the third-party community SpartaDOS X Upgrade Project, which still maintains the software.
AmigaOS is a family of proprietary native operating systems of the Amiga and AmigaOne personal computers. It was developed first by Commodore International and introduced with the launch of the first Amiga, the Amiga 1000, in 1985. Early versions of AmigaOS required the Motorola 68000 series of 16-bit and 32-bit microprocessors. Later versions were developed by Haage & Partner and then Hyperion Entertainment. A PowerPC microprocessor is required for the most recent release, AmigaOS 4.
The floppy disk is a data storage and transfer medium that was ubiquitous from the mid-1970s well into the 2000s. Besides the 3½-inch and 5¼-inch formats used in IBM PC compatible systems, or the 8-inch format that preceded them, many proprietary floppy disk formats were developed, either using a different disk design or special layout and encoding methods for the data held on the disk.
A FAT file system is a specific type of computer file system architecture and a family of industry-standard file systems utilizing it.
