Ensuring orderly forward progress in granting snoop castout requests

Abstract

A method and system for ensuring orderly forward progress in granting snoop castout requests. Masters may include a tag ('request tag') in their transfer requests to a bus macro. The request tag indicates the order of the request issued by the master. If the bus macro determines that the transfer request is snoopable, then the bus macro broadcasts a snoop request that includes the request tag. If a snoop controller determines that the address in the snoop request is a hit to a modified coherency granule in an associated cache, then the master associated with that snoop controller transmits a castout request to the bus macro that includes the request tag associated with the snoop request. The bus macro uses the request tag to determine whether the castout request is a response to the oldest in a series of pipelined snoop requests to be serviced.