Count Key Data:
This is a self-defining record format used on DASD devices. The count field contains the length of the key (optional) and the length of the data, the key field is optional and if present contains a string used to uniquely identify the data record (ex. account number), the data field contains the actual record. This format was introduced in the IBM 360 as a way to off-load low level I/O functions from the main processor. A key search is performed by the channel and device saving the processor from becomming involved.