System for transferring selected data words between main memory and

Abstract

A data cache capable of operation in a write-back (copyback) mode. The data cache design provides a mechanism for making the data cache coherent with memory, without writing the entire cache entry to memory, thereby reducing bus utilization. Each data cache entry is comprised of three items: data, a tag address, and a mixed size status field. The mixed size status fields provide one bit to indicate the validity of the data cache entry and multiple bits to indicate if the entry contains data that has not been written to memory (dirtiness). Multiple dirty bits provide a data cache controller with sufficient information to minimize the number of memory accesses used to unload a dirty entry. The data cache controller uses the multiple dirty bits to determine the quantity and type of accesses required to write the dirty data to memory. The portions of the entry being replaced that are clean (unmodified) are not written to memory.