count-key-data

A self-defining, track recording structure created by IBM for its System/360 disk drives in the 1960s. Each track includes fields that define its beginning and space used, followed by a repeating three-field record pattern, with each field separated by a blank gap: (1) "C"ount field holds length of record, (2) optional "K"ey field holds account number, etc., and (3) the "D"ata field.

Count-key-data (CKD) was built into IBM hardware through the 3330 and then implemented in microcode through the 3390 and 9340 series. In today's disk subsystems, which use generic drives with fixed-length sectors, CKD records are emulated.