Code: The Hidden Language of Computer Hardware and Software, 2nd Edition

Chapter Twenty-Three

CPU Control Signals

An old adage commonly heard in engineering circles contends that the last 10% of a project requires 90% of the work. Regardless how distressing this idea might seem, it’s something to keep in mind. We’ve already made great progress in constructing a computer, but we’re not yet finished. What’s left isn’t quite 90% of the work, but the home stretch might be a little further away than it seems.

The central processing unit (CPU) that I’ve been designing is based on the Intel 8080 microprocessor. The arithmetic logic unit (ALU) and the register array shown in the previous two chapters constitute major parts of this CPU. The ALU performs arithmetic and logic operations on bytes. The register array contains ...