An Asynchronous Victim Cache

Memory bandwidth is a limiting factor with many modern microprocessors and it is usual to include a cache to reduce the amount of memory traffic.Of the two commonly used cache write-policies, the copy-back approach is better than the write-through approach in this respect. The performance of both approaches can be further aided by the inclusion of a small buffer in the path of outgoing writes to the main memory, especially if this buffer is capable of forwarding its contents back into the main cache if they are needed again before they are emptied from the buffer. This is what is known as a victim cache. For an asynchronous microprocessor it is logical that the cache system should be asynchronous as well; since a large degree of the flexibility of an asynchronous microprocessor would be lost if it were to use a standard synchronous memory interface. However, implementing a forwarding mechanism in an asynchronous system is more difficult because the data to be forwarded is flowing in a manner unsynchronised to the process which requires it. This paper presents an architecture for a victim cache to resolve forwarding in a totally asynchronous environment. The resultant structure forms a key part of an asynchronous copy-back cache system for the Amulet3, a third generation asynchronous implementation of the ARM processor.