Explains how a CPU works by implementing the LC-3 instruction set. Includes an in-depth look at the instruction cycle phases. The inquiry “How do computers do math introduced the components needed to build a microprocessor. This article series continues by introducing the microprocessor. It uses a top-down approach.

We use a top down approach to help break up the complex microprocessor into simple, more manageable parts. Starting from the architecture, it dives down to an instruction set. In the second half we build a microprocessor using a field programmable gate array.

We assume a familiarity with assembly code.


This text leans on chapter 4 from the excellent book “Introduction to Computer Systems” by Patt and Partel. The implementation borrows from Davis’ Project#1 description at NC State University.


World War II bought widespread destruction, but also spurred a flurry of computer innovations. The first electric computer was built using electro-mechanical relays in Nazi Germany (1941). Two years later the US army built ENIAC using 18,000 vacuum tubes for calculating artillery firing tables and simulating the H bomb.

Glenn A. Beck and Betty Snyder program ENIAC
Source: US Army photo, PD

This computer could perform complex sequences of operations, including loops, branches and subroutines. The program in this computer was hardwired using switches and dials. It was a thousand times faster as the electro-mechanical machine, but it took great effort to change the program.

The programming was hard-wired into their design, meaning that “reprogramming” a computer simply wasn’t possible: Instead, computers would have to be physically disassembled and redesigned. To explain how a CPU works, we take a look at the leading architecture.

von Neuman Architecture

Src: Encyclopædia
In 1945, the mathematician John von Neumann formalized processor methods developed at the University of Pennsylvania. His computer architecture design consists of a Control Unit, Arithmetic and Logic Unit (ALU), Memory Unit, Registers and Inputs/Outputs. These methods became known as the von Neumann architecture and still forms the foundation for today’s computers.

Using the von Neumann architecture, computers were able to be modified and programmed via the input of instructions in computer code. This way, the functionality could be simply rewritten using a programming language.

The von Neuman Architecture is based on the principle of:

  1. Fetch an instruction from memory
  2. Decode the instruction
  3. Execute the Instruction
This process is repeated indefinitely, and is known as the fetch-decode-execute cycle.

The Central Processing Unit (CPU) is the electronic circuit responsible for executing the instructions of a computer program. It is also referred to as the microprocessor. The CPU contains the Control Unit, ALU and Registers. The Control Unit interprets program instructions and orchestrates the execution of these instructions. Registers store data before it can be processed. The ALU carries out arithmetic and logical operations.

von Neumann architecture


Instructions are a fundamental unit of work that are executed completely, or not at all. In memory, the instructions look just like data — a collection of bits. They are just interpreted differently.

An instructions includes:

  • an opcode, the operation to be performed;
  • operands, the data (locations) to be used in the operation.

There are three instruction types:

  • arithmetic and logical instructions, such as addition and subtraction, or logical operations such as AND, OR and NOT;
  • memory access instructions, such as load and store;
  • control instructions, that may change the address in the program counter, permitting loops or conditional branches.

This article “How a CPU works” continues with Instruction Set on the next page.

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