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.