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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.〔(Definition of “Fixed block” from the IBM Knowledge Center )〕 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. == Overview == 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)-Bin (BB)-Cylinder (CC)-Head (HH)-Record (R),〔(IBM System/360 Operating System: System Control Blocks )〕 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〔 〕 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〔(IBM 350 disk storage unit )〕 or 200 characters, and 270,〔(IBM 2311 Field Engineering Theory of Operation ) describes the internals of the IBM 2311, which had both a count-key-data model and two models with fixed-size data blocks〕〔(IBM System/360 Model 20 Functional Characteristics )〕 366,〔(IBM System/360 model 44 Functional Characteristics )〕 512,〔(IBM 3310 Direct Access Storage Reference Manual )〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fixed-block architecture」の詳細全文を読む スポンサード リンク
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