Count key data

Count key data (CKD) is a direct access storage device (DASD) 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. The initial set of CKD CCW introduced in 1964 was substantially enhanced and improved into the 1990s.

CKD Track Format

The principle of CKD records is that since data record lengths can vary, they all have an associated count field which identifies the record and indicates the size of the key (if used) and the size of the data area, if used. The count field has the identification of the record in cylinder-head-record format, the length of the key, and the length of the data. The key may be omitted or consist of a string of characters.