CIDF—Control Interval Definition Field

The CIDF is a 4-byte field that contains two 2-byte binary numbers.

Offset	Length	Description
0(0)	2	The displacement from the beginning of the control interval to the beginning of the unused space, or, if there is no unused space, to the beginning of the control information. The displacement is equal to the length of the data (records, record slots, or record segment). In a control interval without data, the number is 0.
2(2)	2	The length of the unused space. This number is equal to the length of the control interval, minus the length of the control information, minus the 2-byte value at CIDF+0. In a control interval without data (records, record slots, or record segment), the number is the length of the control interval, minus 4 (the length of the CIDF; there are no RDFs). In a control interval without unused space, the number is 0.
2(2)	1... ....	Busy flag; set when the control interval is being split; reset when the split is complete.

In an entry-sequenced data set, when there are unused control intervals beyond the last one that contains data, the first of the unused control intervals contains a CIDF filled with 0s. In a key-sequenced data set or an RRDS, the first control interval in the first unused control area (if any) contains a CIDF filled with 0s. A CIDF filled with 0s represents the software end-of-file.