Segmented Addressing Solves the Virtual Cache Synonym Problem

Abstract

If one is interested solely in processor speed, one must use virtually indexed caches. The traditional purported weakness of virtual caches is their inability to support shared memory. Many implementations of shared memory are at odds with virtual caches—ASID aliasing and virtual-address aliasing (techniques used to provide shared memory) can cause false cache misses and/or give rise to data inconsistencies in a virtual cache, but are necessary features of many virtual memory implementations. By appropriately using a segmented architecture one can solve these problems. In this tech report we describe a virtual memory system developed for a segmented microarchitecture and present the following benefits derived from such an organization: (a) the need to flush virtual caches can be eliminated, (b) virtual cache consistency management can be eliminated, (c) page table space requirements can be cut in half by eliminating the need to replicate page table entries for shared pages, and (d) the virtual memory system can be made less complex because it does not have to deal with the virtual-cache synonym problem.