DMA exclusive cache state providing a fully pipelined input/output DMA write mechanism

摘要

A data processing system that provides a DMA Exclusive state that enables pipelining of Input/Output (I/O) DMA Write transactions. The data processing system includes a system processor, a system bus, a memory, a plurality of I/O components and an I/O processor. The data processing system further comprises operational protocol providing a pair of instructions/commands that are utilized to complete a DMA Write operation. The pair of instructions is DMA_Write_No_Data and DMA_Write With_Data. DMA_Write_No_Data is an address-only operation on the system bus that is utilized to acquire “DMA ownership” of a cache line that is to be written. The initial ownership of the cache line is marked by a weak DMA state (D1), which indicates that the cache line is being held for writing to the memory, but that the cache line cannot yet force a retry of snooped operations. When each preceding DMA Write operation has completed or each corresponding DMA_Write_No_Data operation has been placed in a DMA Exclusive (D2) state, then the next cache line in a weak DMA state transitions to a DMA Exclusive (D2) state, which forces a retry of snooped operations until the DMA_Write_With_Data transaction to memory is completed. Accordingly, DMA_Write_No_Data operations that are provided sequentially may be completed in a parallel manner on the system bus although the corresponding DMA_Write_With_Data is held until a DMA Exclusive state attaches to the cache line. Also, the DMA_Write_With_Data may be completed out of order and once the write of the cache line is completed, the coherency state transitions from the DMA Exclusive state to the MESI Exclusive (E) state or the MESI Invalidate (I) state depending on processor operating characteristics.