Building an 8-bit computer in Logisim (Part 3 — Basic ALU)
This is the third article in a series. If you haven’t read part 1 and 2, you can find them here:
In this part, we will be looking at how to build the Arithmetic Logic Unit (ALU).
What is an ALU?
The arithmetic logic unit is the part of the CPU that deals with doing arithmetic calculations. It is a combination of many arithmetic circuits that all work together to perform these calculations.
Typically, an ALU accepts two input numbers and lets you select the operation you want to perform. It then does the operation and outputs the result.
What should our ALU be Able to Do?
This sort of depends on the computer you want to build. Maybe you are making a basic computer, and all you need is addition and subtraction. Or perhaps you need something more elaborate that can handle a wide range of operations.
In this section, we will be making an ALU that can add, subtract, and make logical comparisons (AND, OR, NOT, etc.). In a future section, we will be extending our ALU to also include multiplication, division, comparisons and bit shifting.
Building a 1-Bit ALU
To keep things simple, let’s start with a 1-bit ALU. This ALU will take in two bits, (A and B) and perform some…