1. In a digital computer having at least one addressable memory bank for storing data signals therein, each said addressable memory bank having multiple circuit connectors for address and control signals to store and retrieve said data signals including one of said multiple circuit connectors from which said addressable memory bank has been removed, the improvement comprising: (a) a memory expansion circuit including a plurality of expansion memory banks, said expansion memory banks electrically connected in parallel to one another and one of said expansion memory banks connected to said one of said multiple connectors from which said addressable memory bank has been removed, each said expansion memory bank corresponding to said addressable memory banks in said digital computer; (b) logic control means connected in parallel to said plurality of expansion memory banks and having first and second operating modes; (c) a plurality of input/output means interconnecting said plurality of expansion memory banks and said one multiple circuit connector for operationally connecting said address and control signals to said expansion memory banks; (d) said logic control means having digital decoder means responsive to a predetermined combination of said data, address and control signals to operate in said first operating mode and to select and make operational a particular one of said plurality of expansion memory banks in which information is to be stored; and (e) said logic control means operating in said second mode in the absence of said predetermined combination of signals to enable said data signals to be stored in and retrieved from said expansion memory bank selected by said digital decoder means to be operational in said first operational mode, said digital decoder means including an N-bit latch for inserting and storing said predetermined combination of said data signals when said logic control means is in said first operating mode, and for retaining said predetermined combination of said data signals when said logic control means is in said second operating mode.