Cache/disk subsystem with dual aging of cache entries

摘要

When a processor issues a read or write command to read one or more words from a disk, a cache store is checked to see if a copy of the segment(s) containing the word(s) are present therein. If a copy of the segment is not present in the cache store then it is moved from disk to the cache store and sent to the processor. A segment descriptor table is maintained and the entries in the table are linked by forward and backward age links. When a segment is brought into the cache store from a disk because it contains the word or words specified by a command, its segment descriptor is linked in the age chain as the most recently used. Provision is made for reading into the cache store one or more segments in addition to the segment(s) containing the word(s) specified by a command, on speculation that the additional segment(s) contain words most likely to be accessed soon by the processor. The segment descriptor for these additional segments are linked into the age chain at an age which is intermediate the most recently used and the least recently used.