Addressing Mechanisms for Large Virtual Memories

Traditionally there has been a clear distinction between computational (short-term) memory and filestore (long-term memory). This distinction has been maintained mainly due to limitations of technology. Recently there has been considerable interest in programming languages and systems which support orthogonal persistence. In such systems arbitrary data structures may persist beyond the life of the program which created them and this distinction is blurred. Systems supporting orthogonal persistence require a persistent store in which to maintain the persistent objects. Such a persistent store can be implemented via an extended virtual memory with addresses large enough to address all objects. Superimposing structure and a protection scheme on these addresses may well result in them being sparsely distributed. An additional incentive for supporting large virtual addresses is an interest in exploiting the potential of very large main memories to achieve supercomputer speed. This paper presents hardware and software mechanisms to implement a paged virtual memory which can be efficiently accessed by large addresses. An implementation of these techniques for a capability-based computer, MONADS-PC, is described.