A class of compatible cache consistency protocols and their support by the IEEE futurebus

Abstract

Standardization of a high performance blackplane bus, so that it can accommodate boards developed by different vendors, implies the need for a standardized cache consistency protocol. In this paper we define a class of compatible consistency protocols supported by the current IEEE Futurebus design. We refer to this class as the MOESI class of protocols; the term “MOESI” is derived from the names of the states. This class of protocols has the property that any system component can select (dynamically) any action permitted by any protocol in the class, and be assured that consistency is maintained throughout the system. Included in this class are actions suitable for copyback caches, write through caches and non-caching processors. We show that the Berkeley protocol and the Dragon protocol fall within this class, and can be extended to be compatible with other members of the class. The Illinois, Firefly and Write-Once protocols can be adapted to be compatible with this class; the facilities of he Futurebus currently do not support those protocols without adaptation. We discuss very briefly performance choices among protocols, and also other issues relating to a standard bus consistency protocol.