Count key data

Count key data (CKD) is a direct-access storage device (DASD)^[a] data recording format introduced in 1964, by IBM with its IBM System/360 and still being emulated on IBM mainframes. It is a self-defining format with each data record represented by a Count Area that identifies the record and provides the number of bytes in an optional Key Area and an optional Data Area. This is in contrast to devices using fixed sector size or a separate format track.

Count key data (CKD) also refers to the set of channel commands (collectively Channel Command Words, CCWs) that are generated by an IBM mainframe for execution by a DASD subsystem employing the CKD recording format.^[1] The initial set of CKD CCWs, introduced in 1964, was substantially enhanced and improved into the 1990s.

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