The ALU for our CPU needs to perform several types of operation. The first of these is the ability to add or subtract two 32-bit numbers. To implement this I have built a stack of four boards that combine together to give the output.

Adding two numbers is simple, we just use 4 x 74283 and chain the carry output of one to the carry input of the next in sequence. This has been implemented as the 'AddSub Middle' board below.

Implementing the subtraction operation requires a bit of extra work as we need to apply 2's compliment to the second of the two inputs. 2's compliment is the method used to invert the sign of a value. Converting a positive number like 5 into the negative version -5.

This approach avoids the need to implement a separate subtraction circuit and instead allows reuse of the existing addition circuit. For example, 10 - 5 can be implemented as 10 + (-5).

2's compliment is very simple, you just invert all the bits and then add 1. The purpose of the top two boards is to invert all 32-bits of the second input when the control line indicates a subtraction is needed. Adding the extra 1 can be done in the middle layer by providing a initial carry in value to the first adder.

Finally, the bottom board is used to buffer the output, as only sometimes will the ALU want to perform an add/sub operation. All outputs from the different parts of the ALU are connected to the same output bus and so only the appropriate ALU section should put its output on the bus.