GB1459540A - Digital data processing apparatus - Google Patents

Abstract

1459540 Data processing system INTERNATIONAL BUSINESS MACHINES CORP 29 Nov 1974 [28 Dec 1973] 51738/74 Heading G4A In a system in which data segments may be transferred between a primary working store and a secondary, e.g. disc, back-up store, a header is maintained for each stored segment indicating the time of transfer and counts of the total number of transfers which had taken place previously whereby examination of the header associated with a particular segment enables detection of a possibly erroneous state of the segment caused by an earlier primary store failure (any modified segment in primary store at the time of a failure may have influenced segments which were returned to secondary store before the failure occurred). The header checking and maintenance system is controlled by a special microprogram and includes registers in which system time, iota count and omega count are maintained, the iota count being incremented after each transfer of a modifiable segment from the secondary store to the main store, and the omega count being updated to the lowest value of the iota counts in segment headings in the primary store prior to removal of a segment from primary store. On entry of a segment into primary storage, comparison of the iota count and time recorded in its header with recorded values of omega count and time relating to previous system crashes enables a possibly "premature" segment to be identified. Whenever a segment is to be moved out of primary storage, the headings of segments in primary storage are scanned to determine the lowest value of iota count therein, this value then becoming the omega count. Modification of the heading checking and maintenance procedures when the data segments are paged is also discussed.