A digital electronic hardware design and construction system is described. Basic to the system is a microprogrammed control processor which is used unmodified in all devices constructed using this technique. This processor has no data manipulation capabilities; all arithmetic, logic, and I/O capabilities are supplied by logically complete hardware modules called functional units. The processor supplies control signals on 256 unique instruction lines. Each of these instruction lines can be wired as a control signal to one or more functional units. A control signal may cause data transfer between functional units, test the status of a functional unit, or cause a functional unit to perform a specific internal function. A single data bus connects each of the functional units and the control processor such that the flow and manipulation of data are controlled by a microprogram. A logic machine refers to the ensemble of control processor, single data bus, several functional units, and a microprogram all arranged so that a specific digital system is constructed.