Fixed-block architecture

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Further information: Logical block addressing

Fixed-block architecture (FBA) is an IBM term for the hard disk drive (HDD) layout in which each addressable block (more commonly, sector) on the disk has the same size, utilizing the well-established three-number block addressing scheme.[1] FBA as a term was created and used by IBM for its 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 beginning in 1964 with its System/360; later, FBA evolved into the logical block addressing (LBA), which is a linear block addressing scheme for the fixed length blocks.


From RAMAC until the early 1990s 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.

IBM's 1964 System/360 introduced their new variable-length record format for their mainframes wherein each record had an optional variable length key block and a variable length data block. IBM mainframe disk storage, called direct access storage device (DASD) are addressed using an 8-byte address structured as MBBCCHHR (Extent (M)[a]-Bin (BB)[b]-Cylinder (CC)-Head (HH)-Record (R),[2] 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. In addition to addressing records by number, it was possible to search disks by key, using the underlying count-key-data (CKD) structure.

The term "fixed-block architecture" was created by IBM in 1979[3] to distinguish this format from its variable-length record format. Fixed-block architecture was adopted for a few mainframe HDDs produced by IBM beginning in the 1970s, and by the 1990s all new IBM HDDs used FBA. IBM's various FBA disks had block sizes of 100[4] or 200 characters,[5] and 270,[6][7] 366,[8]:11 512,[9]:2-1 1024, 2048, or 4096 bytes.

Blocks are typically separated on the track 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. To read or write a block on an FBA disk, the computer would address the block using the traditional CHS three number address.

Later formats[edit]

A later development in disk addressing was logical block addressing (LBA), in which the cylinder-head-sector 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 sectors can differ from the physical block size. For example, Advanced Format (AF) 512e HDDs use 4096-byte physical sectors, while their firmwares provide emulation for a virtual sector size of 512 bytes; thus, "512e" stands for "512-byte emulation".

See also[edit]


  1. ^ The "extent" can be 0x00 through 0xff, with 16 extents being reserved for each device in a "concatination" (association) of up to 16 physically separate devices, even of different device types, although each physical device may have only 16 such "extents".
  2. ^ Applies only to "Data Cell" devices, and is 0x00 otherwise.