Dynamic spare column replacement memory system

Abstract

An MxN dynamic spare column replacement memory system for storing M N-bit data words includes a random access memory (RAM) formed by a rectangular array of M rows and N+S columns of single-bit memory cells. Each row has a unique address and stores an N-bit word using a selected set of N of its N+S cells. An N-line parallel data bus provides data access to the DRAM. Responding to a switching instruction from a switch controller at the start of each memory access cycle, a crossbar switch selectively connects each of the N lines of the data bus to a separate one of the N+S columns. Thus during a memory read or write access cycle the N data lines access N cells of an addressed row columns. The remaining S cells of the row are unused. A host computer occasionally checks the DRAM for defective memory cells, and upon finding a defective cell or cells in any row, the host stores the row address and a switching instruction in the switch controller. At the beginning of each memory access cycle, the switch controller compares the DRAM address to its stored list of addresses of rows having a defective cell. If the current DRAM address matches a stored address, the switch controller switches data bus lines from columns containing the defective cell to spare columns in accordance with a switching instruction stored with the address. Thus spare cells are assigned for replacement of defective cells on an address-by-address basis.