Known in the prior art are semiconductor devices which provide volatile storage of information in a dynamic random access memory (RAM) portion and non-volatile storage in a read only memory (ROM) portion. For example, U.S. patent application Ser. No. 654,332, (now U.S. Pat. No. 4,611,309) filed Sept. 24, 1984, on behalf of Patrick Chuang, Robert Yau and Ron Maltiel, entitled "An Improved Non-Volatile Dynamic RAM Cell", assigned to the assignee of the instant application discloses one such device.
Oftentimes it is necessary to non-destructively access both the volatile and non-volatile data for such purposes as comparison in image processing and pattern recognition. It is desirable that both of these accesses take place over a short period of time, ideally within one memory clock cycle, because of the large number of data points used in such applications as image processing and pattern recognition. If each such comparison requires several clock cycles, an undesirable time penalty would be imposed during pattern recognition.
However, the memory cells of the prior art are incapable of concurrently accessing both the volatile and non-volatile data without over-writing the volatile data. In addition, in these so-called shadow-memories, the non-volatile portion merely duplicated the data stored in the volatile portion. This duplication was necessary to provide back-up for data which would be lost from the volatile portion during a power failure. Therefore, only one-half of the shadow-memory is being used, the other one-half serving as back-up. On approach overcoming these difficulties is to temporarily store the data from, for example, the volatile portion of the memory during one memory access cycle and then access the data from the non-volatile portion of the memory during the following memory cycle for subsequent comparison. Such approach is described in a co-pending application Ser. No. 717,348, filed Mar. 29, 1985, on behalf of Ron Maltiel and Robert Yau, entitled "Method and Apparatus for Non-Destructive Access of Volatile and Non-Volatile Data in a Shadow Memory Array", and assigned to the assignee of the instant application.