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Access control method for a shared main memory in a multiprocessor based upon a directory held at a storage location of data in the memory after reading data to a processor

  • US 5,606,686 A
  • Filed: 10/24/1994
  • Issued: 02/25/1997
  • Est. Priority Date: 10/22/1993
  • Status: Expired due to Fees
First Claim
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1. A method for controlling main memory accesses in a computer system which includes a plurality of processing units and a main memory shared by said plurality of processing units, wherein each processing unit includes a processor and a cache memory which holds a part of data held in said main memory, the method comprising the steps of:

  • (a) transferring a data line requested by a read request provided by one of said processing units to a cache memory therewithin from a storage location for said data line within said main memory, when there is no processing unit which has already cached said data line;

    (b) storing directory information into said storage location in said main memory in place of said data line, in response to said transferring step (a), said directory information indicating that said one processing unit is a processing unit which has cached said data line;

    (c) transferring said data line from said cache memory within said one processing unit as designated by said directory information to another of said plurality of processing units, in response to a new read request provided by said another processing unit for said data line after said data line has been cached by said one processing unit;

    (d) renewing the directory information stored in said storage location so that said renewed directory information indicates that said another processing unit is also a processing unit which has cached said data line; and

    (e) responsive to replacement of said data line by one of said plurality of processing units which has cached said data line, controlling writing back of said replaced data line, depending upon said directory information, so that said replaced data line is written back into said storage location of said main memory in place of said directory information stored therein, when there is no processing unit in which said data line is still cached, and so that said replaced data line is not written back, when said data line is still cached in one of said plurality of processing units.

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