Fixed-block architecture (FBA) is an IBM term for the
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
(HDD) layout in which each addressable
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
(more commonly,
sector
Sector may refer to:
Places
* Sector, West Virginia, U.S.
Geometry
* Circular sector, the portion of a disc enclosed by two radii and a circular arc
* Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc
* Spherical sector, a po ...
) on the disk has the same size, utilizing 4 byte block numbers and a new set of command codes. FBA as a term was created and used by
IBM for its
3310 and
3370 HDDs beginning in 1979 to distinguish such drives as IBM transitioned away from their variable record size format used on
IBM's mainframe hard disk drives
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk platter, platters ...
beginning in 1964 with its
System/360
The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...
.
Overview
From
RAMAC until the early 1960s most hard disk drive data were addressed in the form of a three number block addressing scheme
Cylinder, Head & Sector (CHS); the cylinder number, which positioned the head access mechanism; the head number, which selected the read-write head; and the sector number, which specified the rotational position of a fixed size block. On June 2, 1961, IBM introduced the
1301
Year 1301 ( MCCCI) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar.
Events
By place
Europe
* January 14 – With the death of King Andrew III (the Venetian) (probably poisoned), ...
, which had variable length records, and the market for sector-oriented disks was eclipsed for decades.
IBM's 1964
System/360
The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applica ...
introduced their new self-formatting
variable-length record format for disk and other random-access drives, wherein each record had an optional variable length key field and a variable length data field. IBM mainframe disk, drum and mass storage devices, called
direct access storage device
A direct-access storage device (DASD) (pronounced ) is a secondary storage device in which "each physical record has a discrete location and a unique address". The term was coined by IBM to describe devices that allowed random access to data, ...
s (DASD) are addressed using a six byte seek address for seek commands and a five byte CCHHR for search count commands.
OS/360
OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB ...
and other S/360 operating systems used an 8-byte address structured as MBBCCHHR (Extent (M)-Bin (BB)-Cylinder (CC)-Head (HH)-Record (R), which was capable of storing records of varying size, up to 255 such records per track, with the zeroth record (R0) being reserved for certain error correction information, such as skip defects). In addition to data, records could also contain a key. The length of the key, like the length of the data, was specified by the application writing the record. In addition to addressing records by number, it was possible to search disks by key, using the underlying
count key data
Count key data (CKD) is a direct-access storage device (DASD) data recording format introduced in 1964, by IBM with its IBM System/360 and still being emulated on IBM mainframes. It is a self-defining format with each data record represented by a ...
(CKD) structure.
The term fixed-block architecture was created by IBM in 1979 to distinguish this format from its variable-length record format. Each track is divided into fixed-length blocks, consisting of an ID field and a data field. Application programs refer to blocks by relative block number, and cannot address them by cylinder, head and record. Although the FBA commands allowed a query to determine the data area size, the 3310
[
] and 3370
[
] have the same size data fields, 512 bytes. Fixed-block architecture was adopted for a few mainframe HDDs produced by IBM beginning in the 1970s, and contemporary DASD systems continue to support
3310 and
3370 compatibility.
MVS
Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unrelated ...
continues to require CKD DASD, although by the 1990s all new IBM HDDs used fixed sectors internally.
IBM's various sectored disks had block sizes of
100
or 200 characters,
and 270,
366,
512,
[ 1024, 2048, or 4096 bytes.
Blocks are typically separated on the ]track
Track or Tracks may refer to:
Routes or imprints
* Ancient trackway, any track or trail whose origin is lost in antiquity
* Animal track, imprints left on surfaces that an animal walks across
* Desire path, a line worn by people taking the shorte ...
by ''inter-record gaps''. Together, the block size and the size of the inter-record gap determine how many blocks can fit in each track.
Later formats
A later development in disk addressing was ''logical block addressing'' ( LBA), in which the cylinder-head-sector
Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a hard disk drive.
It is a 3D-coordinate system made out of a vertical coordinate ''head'', a horizontal (or radial) coordinate ''cylinder'', an ...
triplet was replaced by a single number, called the block number. Within the disk drive, this linear block number was translated into a cylinder number, head number and sector number. Moving the translation into the disk drive allowed drive manufacturers to place a different number of blocks on each track transparently to the accessing software.
Still later, magnetic hard disks employed an evolution of LBA where the size of the addressable disk sector
In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user-accessible data, traditionally 512 bytes for hard disk drives (HDDs) and 2048 bytes for CD-ROMs and D ...
s can differ from the physical block size. For example, Advanced Format
Advanced Format (AF) is any disk sector format used to store data on magnetic disks in hard disk drives (HDDs) that exceeds 512, 520, or 528 bytes per sector, such as the 4096, 4112, 4160, and 4224-byte (4 KB) sectors of an Advanced Format ...
(AF) 512e
Advanced Format (AF) is any disk sector format used to store data on magnetic disks in hard disk drives (HDDs) that exceeds 512, 520, or 528 bytes per sector, such as the 4096, 4112, 4160, and 4224-byte (4 KB) sectors of an Advanced Format ...
HDDs use 4096-byte physical sectors, while their firmware provides emulation for a virtual sector size of 512 bytes; thus, "512e" stands for "512-byte emulation".
See also
* Block (data storage)
In computing (specifically data transmission and data storage), a block, sometimes called a physical record, is a sequence of bytes or bits, usually containing some whole number of records, having a maximum length; a ''block size''. Data thus ...
* Data set (IBM mainframe)
In the context of IBM mainframe computers in the S/360 line, a data set (IBM preferred) or dataset is a computer file having a record organization. Use of this term began with, e.g., DOS/360, OS/360, and is still used by their successors, incl ...
* Count key data
Count key data (CKD) is a direct-access storage device (DASD) data recording format introduced in 1964, by IBM with its IBM System/360 and still being emulated on IBM mainframes. It is a self-defining format with each data record represented by a ...
(CKD)
* Record (computer science)
In computer science, a record (also called a structure, struct, or compound data) is a basic data structure. Records in a database or spreadsheet are usually called "rows".
A record is a collection of ''fields'', possibly of different data typ ...
* Track (disk drive) {{Unreferenced, date=November 2009
A disk drive track is a circular path on the surface of a disk or diskette on which information is magnetically recorded and from which recorded information is read.
A track is a physical division of data in a d ...
* Volume Table of Contents In the IBM System/360Including the successors S/370 through z/Architecture storage architecture, the Volume Table of Contents (VTOC), is a data structure that provides a way of locating the data sets that reside on a particular DASD volume. With th ...
(VTOC)
Notes
References
{{Reflist
IBM storage devices