Cache intervention from a cache line exclusively holding an unmodified

Abstract

A method of improving memory latency associated with a read-type operation in a multiprocessor computer system is disclosed. After a value (data or instruction) is loaded from system memory into a cache, the cache is marked as containing an exclusively held, unmodified copy of the value and, when a requesting processing unit issues a message indicating that it desires to read the value, and the cache transmits a response indicating that the cache can source the value. The response is transmitted in response to the cache snooping the message from an interconnect which is connected to the requesting processing unit. The response is detected by system logic and forwarded from the system logic to the requesting processing unit. The cache then sources the value to an interconnect which is connected to the requesting processing unit. The system memory detects the message and would normally source the value, but the response informs the memory device that the value is to be sourced by the cache instead. Since the cache latency can be much less than the memory latency, the read performance can be improved substantially with this new protocol.