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.
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 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.
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 continues to require CKD DASD, although by the 1990s all new IBM HDDs used fixed sectors internally.