Although bus connected multiprocessors have been widely used as high-end workstations or servers, the number of connected processors is strictly limited by the maximum bandwidth of the shared bus. Instead of them, a switch connected multiprocessor which uses a crossbar or Multistage Interconnection Networks (MINs) for connecting processors and memory modules is a hopeful candidate. However, in such a system, a snoop cache technique in bus connected multiprocessors cannot be used, and consistency problems must be solved for providing the cache memory between a processor and the switch. To address this problem, hardware approaches by making the best use of advanced VLSI technology have been proposed. However, traditional methods require a large memory outside the switching element and it causes not only a large additional hardware but also the extra latency by accessing the outside memory. Moreover, the complicated MIN with cache or directory must also treat data packet which should be transferred quickly. In order to solve these problems, we proposed the MINC (MIN with Cache control mechanism). In the MINC, the MIN which only transfers a part of the address and cache coherent messages is separated from the data transfer network, and pushed into an LSI chip called the MINC chip. The coherent control is done based on the directory using the reduced Hierarchical Bit-map Directory scheme (RHBD). In order to reduce unnecessary packets, the pruning cache which is a small cache enough to implement inside the chip is introduced in the MINC chip.
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