The cache coherence protocol of the data diffusion machine

The Data Diffusion Machine (DDM) is a scalable shared memory multiprocessor in which the location of a datum in the machine is completely decoupled from its address. A data access "snooping" protocol provides an automatic duplication and migration of the data to wherever needed. The protocol also handles data coherence and replacement. The hardware organization consists of a hierarchy of buses and data controllers linking an arbitrary number of processors each having a large set-associative memory. Each data controller has a set-associative directory containing status bits for data under its control. The rest of the system appears to one processor like shared memory system, which makes the DDM a general architecture. The DDM is scalable in that there may be any number of levels in the hierarchy. The logical topmost bus (or any other bus) can be implemented by an unlimited number of physical buses removing an anticipated bottleneck.A different version to appear in PARLE-89 (Conference on Parallel Architectures and Languages Europe), Eindhoven, The Netherlands, June 12-16, 1989. Published by Springer-Verlag. Original report number R89004.