Expandable microprogram memory

Abstract From the Canadian patent

A method and an apparatus for increasing the capacity and speed of access of a large microprogram control store. The method or apparatus incorporates the use of a 4 bit control field in a microinstruction word which is set into a hardware register to extend the address beyond the normal range of the addressing capability of the microinstruction word. The 4 bits stored in the hardware registers allow 1 of a maximum of 16 read only memory control stores to be selected thereby resulting in the normal addressing circuitry to have a reduced number of components. This technique also allows the selection of another of the memory control stores in one machine cycle.