Directory-based coherency system using two bits to maintain coherency on

Abstract

An improved directory-based cache coherency memory system for a multiprocessor computer system. The memory system includes a dual ported system memory shared by the multiple processors within the computer system; a plurality of data cache memories, at least one data cache memory associated with each processor; and first and second memory busses, the first memory bus connecting a first subset of processors and associated data cache memories to a first port (PORT A) of the system memory, and the second memory bus connecting a second subset of processors and associated data cache memories to a second port (PORT B) of the system memory. Cache coherency is maintained through the use of memory line state information saved with each line of memory within the system memory and data cache memories. The system memory contains a system memory line state for each line of memory saved within the system memory, the system memory line state being any one of the group: SHARED PORT A, SHARED BOTH, OWNED PORT A and OWNED PORT B. Each one of these states is represented by a different two bit code saved with each line of memory in system memory. Additionally, each data cache memory contains a data cache memory line state for each line of memory saved within the data cache memory, the data cache memory line state being any one of the group: MODIFIED, EXCLUSIVE, SHARED, or INVALID.