On internal organization in compressed random-access memories

The design of a compressed random-access memory (C-RAM) is considered. Using a C-RAM at the lowest level of a system's main-memory hierarchy, cache lines are stored in a compressed format and dynamically decompressed to handle cache misses at the next higher level of memory. The requirement that compression/decompression, address translation, and memory management be performed by hardware has implications for the directory structure and storage allocation designs used within the C-RAM. Various new approaches, summarized here, are necessary in these areas in order to have methods that are amenable to hardware implementation. Furthermore, there are numerous design issues for the directory and storage management architectures. We consider a number of these issues, and present the results of evaluations of various approaches using analytic methods and simulations. This research was done as part of a project to explore the feasibility of compressed-memory systems; it forms the basis for the memory organization of IBM Memory Expansion Technology (MXT).