A survey of read-only memories

Consider the problem of the design of a combinational circuit with A inputs and B outputs, where each of the output variables is given as a Boolean function of the input variables. Such a circuit might be part of the control unit of a digital computer, where the A inputs are the operation code of an instruction, and the B outputs are the signals which directly control the opening and closing of gates throughout the machine to effect an execution of that instruction. The circuit might be a code converter, where the A inputs are an input code (for example, the machine-code of an alphanumeric character); and the B outputs are an output code (for example, the pattern of signals required for a display of that character). The circuit might be a table look-up device, where, for example, the input variables are a code for the numeric value of a given argument, and the output variables are a code for the value of some function of that argument. Finally, the circuit might be considered as a memory, with fixed information stored, where the A input bits are an address, and the B output bits are the word stored at that address. It is called a "read-only" or "fixed" memory if the information stored is not alterable at electronic speeds.