Compaq Computer Corporation, Inc.

Mike T Jackson

Jeffrey C Stevens

Roger E Tipley

Sompong Paul Olarig

Jens K Ramsey

Philip C Kelly

A method for arbitrating between processor and host bus snoop accesses to a cache subsystem in a multiprocessor system where the processor does not allow for processor cycle aborts. When a processor access and a snoop access both occur and no tag access or tag modify cycle is currently being performed, the snoop access is given priority over the processor access. After an initial arbitration, if any, the processor and snoop accesses alternate tag access if both processor and snoop accesses are active. This balances any wait states incurred between the processor and the host bus and ensures that neither bus is locked out by continual accesses by the other. In addition, tag modify cycles are generally run immediately after the tag access cycles that initiate them.

Multiprocessor cache abitration

A method and apparatus for enabling a dual ported cache system in a multiprocessor system to guarantee snoop access to all host bus cycles which require snooping. The cache controller includes a set of latches coupled to the host bus which it uses to latch the state of the host bus during a snoop cycle if the cache controller is unable to immediately snoop that cycle. The cache controller latches that state of the host bus in the beginning of a cycle and preserves this state throughout the cycle due to the effects of pipelining on the host bus. In addition, the cache controller is able to delay host bus cycles to guarantee snoop access to host bus cycles which require snooping. The cache controller generally only delays a host bus cycle when it is already performing other tasks, such as servicing its local processor, and cannot snoop the host bus cycle immediately. When the cache controller latches the state of the bus during a write cycle, it only begins to delay the host bus after a subsequent cycle begins. In this manner, one write cycle can complete on the host bus before the cache controller delays any cycles, thereby reducing the impact of snooping on host bus bandwidth. Read cycles are always delayed until the cache controller can complete the snooping operation because the cache may be the owner of the data and a write back cycle may be necessary.

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Mike T Jackson