A digital computer with the capability of incorporating multiple central processing units (CPU's), utilizes an address and data bus between each central processing unit and from one to fifteen intelligent composite memory and input/output modules (MIO). Data is transferred to and from each MIO and the CPU synchronously by a bus during one phase of a three phase clocking cycle. During a second phase of the clocking cycle data on one or more low speed serial data channels within each MIO is transferred to and from the MIO and external devices. During the third phase of the clocking cycle data on a high speed direct memory access channel (DMA) is transferred to and from the MIO and one or more external devices. Additional CPU's can be interconnected with the first CPU by means of an inter-processor buffer module (IPB) which interconnects to the bus at one end and the additional CPU, by means of a bus, at its other end. The IPB may be a software modifiable MIO and can store data addressable by the two interconnected CPU's. In turn, the additional CPU and its associated bus interconnects by the second bus with from one to fifteen additional MIO's or IPB's, allowing cascading of CPU's and associated MIO's and IPB's. Since all data transfers to and from the MIO's and external devices occur at time phases separate from the first time phase in which the CPU communicates with the MIO's and IPB's, the computational speed of any CPU is independent of the quantity of data transferred between the MIO's and IPB's and associated external devices or additional CPU's.