Fixed-block architecture

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 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 beginning in 1964 with its System/360.

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, which had variable length records, and the market for sector-oriented disks was eclipsed for decades.

IBM's 1964 System/360 introduced their new self-formatting variable-length record format for their S/360 mainframes 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 devices (DASD) are addressed using a six byte seek address for seek commands and a five byte CCHHR for search count commands. OS/360 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 (CKD) structure.

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