Energy-efficient physically tagged caches for embedded processors with virtual memory

Abstract

In this paper we present a low-power tag organization for physically tagged caches in embedded processors with virtual memory support. An exceedingly small subset of tag bits is identified for each application hot-spot so that only these tag bits are used for cache access with no performance sacrifice as they provide complete address resolution. The minimal subset of physical tag bits, i.e. the compressed tag, is dynamically updated following the changes in the physical address space of the application. Special support from the operating system (OS) is introduced in order to maintain the compressed tag during program execution. The compressed tag is updated by the OS to match the current set of physical memory pages allocated to the application. We have proposed efficient algorithms that are incorporated within the memory allocator and the dynamic linker in order to achieve dynamic update of the compressed tags in the cases where the mapping between virtual and physical addresses is modified; such cases include memory allocation/deallocation and swapping physical pages on the secondary memory storage. The only hardware support needed within the l/D-caches is the support for disabling bitlines of the tag arrays. An extensive set of experimental results demonstrates the efficacy of the proposed approach.