Disk File Index Detection
Original Publication Date: 1989-Mar-01
Included in the Prior Art Database: 2005-Jan-27
The index signal is used to mark a fixed physical position on the rotating disk of a direct access storage device (DASD) file. This is accomplished by writing a unique pattern on the file's servo surface once per revolution that is later detected by the file's electronic circuits, creating a pulse on the index signal line. Defects on the servo surface may alter the servo pattern such that a false index pattern is created thus producing an "extra" index, or the index pattern is destroyed thus producing a "missing" index. Even in the absence of defects, the servo information is unreliable during a file seek operation and may cause extra or missing index patterns as well. The presence of extra or missing index patterns has the potential of creating both soft and hard file errors during read and write operations.