Most often, the key is omitted and the record is located sequentially or by direct cylinder-head-record addressing. If it is present, the key is typically a copy of the first bytes of the data record (for "unblocked" records, or a copy of the highest key in the block, for "blocked" records), but can be any data which will be used to find the record, usually using the Search Key Equal or Search Key High or Equal CCW. The key (and hence the record) is locatable via hardware commands. Since the introduction of IBM's System/360 in 1964, nearly all IBM large and intermediate system DASDs have used the count key data record format.