An experimental 1-Mbit cache DRAM with ECC

A cache DRAM which consists of a dynamic RAM (DRAM) as main memory and a static RAM (SRAM) as cache memory is proposed. An error checking and correcting (ECC) scheme utilizing the wide internal data bus is also proposed. It is constructed to be suitable for a four-way set associated cache scheme with more than a 90% hit rate estimated to be obtained. An experimental cache DRAM with 1-Mb DRAM and 8-kb SRAM has been fabricated using a 1.2-μm, triple-polysilicon, single-metal CMOS process. A SRAM access time of 12 ns and a DRAM access time of 80 ns, including an ECC time of 12 ns, have been obtained. Accordingly, an average access time of 20 ns is expected under the condition that the hit rate is 90%. The cache DRAM has a high-speed data mapping capability and high reliability suitable for low-end workstations and personal computers