In order to realize high performance database systems, it is very much important to develop a method for high-speed backup as well as method to realize high-speed database operations. Although semiconductor memory is getting less expensive, disks are still important to realize reliable large database systems due to their characteristics; low cost, large capacity and nonvolatileness. If system down or power failure occurs, main memory data may be lost. To avoid losing data, battery backup memories and battery backup disks are usually used. This method, however, still has problems, since each data item is not duplicated. Some system fault may change data in main memory, which cannot be recovered.

If all the contents of a main memory at a certain time are stored in disks, high-speed recovery can be achieve using sequential access. Continuously backed-up RAMs are designed for such a purpose. Dual-port-RAMs developed for video display systems are good candidates to realize such systems, since sequential backup can be realized simultaneously with normal read/write operations. As we can show that currently available dual-port-RAMs are not sufficient for our purpose, we have designed continuously backed-up RAMs using conventional RAMs. Two-plane backup RAMs and double buffer backup RAMs are designed and effectiveness of these approaches is examined by actually developing hardwired circuits. Since to realize the whole main memory by continuously backed-up RAMs is not realistic, we propose a memory system consisting of continuous backedup RAMs and conventional RAMs. High-speed recovery can be achieved by storing hotspot data (data which are modified frequently) in continuously backed-up RAMs. discussed.