Enhance the 8-Bit CPU

1. Purpose

We have learned an 8-bit CPU in the last lab. The ALU of the 8-bit CPU has operations such as AND, NOT, and XOR. Can you enhance the ALU by adding more operations?

2. The 8-bit CPU:

Obtain the following 8-bit CPU working circuit file:


The circuit just obtained is a 8 bit version of the 1 bit CPU we built in the previous lab with a few additions:

3. More Explanations:

  1. The circuit is simply an extension of what you have already done.
    Notice the similarities e.g. temp, accumulator, IR, B and C registers, and the external input.
  2. Notice the differences. For example, the memory has a memory address register;
    there is a decoder associated with the ALU;
    and there is a PC and an adder to produce a PC+1 value.
  3. The difference between absolute and immediate addressing:


Many devices may be connected to a common bus by using tri-state buffers. 
4-bit tri-state buffers are used in the 8-bit CPU circuit to control the
8-bit data flow.

5. Lab Assignments

Copyright: Department of Computer Science, University of Regina.