In a distributed shared memory system, clusters of symmetric multi-processors are connected to each other by a network. Each symmetric multi-processor includes a plurality of processors, a memory having addresses, and an input/output interface to interconnect the processors. A software implemented method enables data sharing between the clusters of symmetric multi-processors using variable sized quantities of data called blocks. A set of the addresses of the memories are designated as virtual shared addresses to store shared data, and a portion of the virtual shared addresses are allocated to store a shared data structure as one or more blocks. The size of a particular allocated block can vary for different shared data structures. Each block includes an integer number of lines, and each line includes a predetermined number of bytes of shared data. Directory information of a particular block is stored in the memory of a processor designed as the home of the block. The directory information includes the size of the particular block, the identity of the processor that last modified the data in the particular block and the identity of all processors having a copy of the block.