Fixed-block architecture
| Industry | Computer hardware |
|---|---|
| Founded | June 16, 1911 in Endicott, New York, United States |
| Products | Hard disk drives, mainframe storage devices |
Fixed-block architecture (FBA) is an IBM designation for a hard disk drive (HDD) configuration where every addressable block or sector maintains uniform size, utilizing 4-byte block identifiers alongside a unique set of command instructions. IBM launched FBA with its 3310 and 3370 HDD models starting in 1979 to set it apart from the variable record length format used in IBM mainframe HDDs since the introduction of the System/360 in 1964.
Overview
During the period from the RAMAC system until the early 1960s, HDD data addressing typically relied on a three-part scheme: cylinder, head, and sector (CHS). The release of IBM's 1301 in 1961 brought variable-length records into use, shifting industry preference away from strictly sector-based disks.
The System/360, introduced in 1964, featured a self-formatting variable-length record format applicable to disks and other random-access storage devices. IBM's mainframe storage units, known as direct access storage devices (DASD), employed a six-byte seek address along with a five-byte CCHHR format for command operations. Operating systems such as OS/360 utilized an 8-byte address scheme (MBBCCHHR) that could accommodate up to 255 variable-sized records per track, with optional key fields. Access to records was possible either by indexing their number or by searching keys within the CKD structure.
The term fixed-block architecture was introduced by IBM in 1979 to clearly differentiate this method from variable-length record formats. Under FBA, tracks are segmented into fixed-size blocks composed of an ID field plus a data field. Instead of referencing cylinder, head, and record numbers, applications access these blocks via relative block numbers. Both the 3310 and 3370 models implemented data fields sized at 512 bytes. FBA found adoption across multiple IBM mainframe HDDs during the 1970s, and compatibility with these drives persists in current DASD systems. Although MVS mandates CKD DASD usage, all IBM HDDs produced since the 1990s internally employ fixed-sector layouts.
Historically, IBM’s sectored disks featured block sizes including 100, 200, 270, 366, 512, 1024, 2048, or 4096 bytes. On each track, blocks are separated by inter-record gaps, which together with block size determine how many blocks can fit on a track.
Later formats
A significant innovation in disk addressing was the development of logical block addressing (LBA), which replaced the cylinder-head-sector trio with a single linear block number. This linear identifier is converted within the disk drive into physical cylinder, head, and sector values, allowing manufacturers to vary block counts per track without affecting software compatibility.
Subsequent magnetic hard drives advanced LBA further by enabling addressable disk sector sizes that differ from physical block dimensions. For example, Advanced Format (AF) 512e drives use physical sectors of 4096 bytes but simulate virtual sectors of 512 bytes through firmware; "512e" stands for "512-byte emulation."
See also
- Block (data storage)
- Data set (IBM mainframe)
- Count key data (CKD)
- Record (computer science)
- Track (disk drive)
- Volume Table of Contents (VTOC)