Design and Implementation of a Simple 8-Bit CPU

Copyright © 1997, 2000 by Rex N. Fisher

Block diagrams of simple CPUs have been used in beginning computer courses for decades, because it is important to visualize how a CPU functions. To meet this need, many textbook authors have devised simple CPUs at the block diagram level to illustrate how instructions are executed and data are manipulated. Omitting many of the circuit details allows an overall understanding that is usually sufficient for students with little or no experience with digital circuits.

However, students in TAC/ABET accredited Electronics Engineering Technology programs, such as the one at BYU-Idaho, have a more thorough background in digital circuit design. They are able to understand how instructions are decoded, what control signals are required for datapath operation, and how those control signals are generated. By examining this extra level of detail, students can better tie the new material to principles they have already learned.

This report describes the development and operation of the P8 CPU. It is a working 8-bit CPU designed and built with standard memory devices and TTL logic components. It extends a student's understanding of computer operation to a deeper level than can be accomplished with only a block diagram CPU model.

Four elements of this project will be explained: (1) the selection of a suitable CPU model; (2) the details of the instruction set, with its various operations and addressing modes; (3) the design of the actual datapath and control logic hardware; and (4) the operation of the CPU, along with some examples of how the hardware implements the instruction set.

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