METHOD AND APPARATUS FOR IMPLEMENTING PCI DMA SPECULATIVE PREFETCHING IN A MESSAGE PASSING QUEUE ORIENTED BUS SYSTEM

摘要

Speculative prefetching during DMA reads in a message-passing, queue-oriented bus system is controlled by creating a special data struture, called a 'DMA scoreboard', for each work queue entry associated with a DMA read. The DMA scoreboard tracks the completion of DMA writes and reads by monitoring acknowledgementes received from DMA writes and data tags received from DMA read PCI addres, and size and number of prefetches to be performed. After a DMA read has completed, the PCI current address is incremented to obtain a new PCI address from the first prefetch request. A new work queue entry is then created from the information in the DMA scoreboard to perform the prefetch. If the amount of data to be fetched exceeds the maximum amount of data that can be retrieved by a single read request, when the read request has been copleted, the address and another work queue entry is created. Operaton continues in this manner until the numer of prefetches specified in the DMA scoreboard have been performed.