Storage protection keys in two level cache system

The cache system comprises a level one (L1) data cache, a level one (L1) key cache for storing a plurality of access keys for respective pages or blocks of data referenced by the central processor. A level three (L3) storage stores the data requested by the central processor and an access key array including the plurality of access keys. A level two (L2) data cache is coupled between the L3 storage and the L1 data cache and stores a copy of data fetched from the L3 storage for the L1 data cache pursuant to a read request and data written by the central processor. The level two (L2) key cache is coupled between the L3 storage access key array and the L1 key cache and stores the plurality of access keys for respective pages or blocks of data in the L2 data cache. The general strategy of the cache system is to copy the access key corresponding to each central processor request into the L1 key cache the first time that any page of data is inpaged pursuant to a read or write request, so that the access key is readily available to the central processor for the current and subsequent requests. If the central processor request is to store data to the L2 data cache, the corresponding access key but not the data is copied into the L1 level, and the data is updated only in the L2 data cache, assuming that the data did not reside in the L1 data cache due to a previous read request.