University of California, Santa Barbara: Behrooz Parhami's "Addition"
Study slides 1-29 to learn how addition is implemented and carried out at the gate level. In the state of the practice, i.e., in the current profession, computers are architected using larger components. For example, to perform addition and subtraction, computer architects utilize ALU's, arithmetic logic units. You can design a computer without knowing the details of an ALU or of an adder, similar to using a calculator to find the square root of a number without knowing how to manually compute the square root (or in computer science terminology, without knowing the algorithm that the calculator performs to find the square root). However, we want you to have the strongest foundation in your study of computer architecture. Hence, study the assigned slides for Chapter 5. If you feel very ambitious, optionally you can study Chapters 6-8, which expand on basic addition.
Knowing the underlying algorithm for larger components, you will be able to better use them in constructing larger components, for example, using half adders to construct a full adder. A half adder takes 2 bits as input and outputs a sum and a carry bit; a full adder takes 2 operand bits and a carry bit as input, and outputs a sum bit and a carry bit. Note that to add two floating point numbers, one or both need to be put in a form such that they have the same exponent. Then, the mantissas are added. Lastly, the result is normalized. We will cover floating point addition in subunit 4.4. Subtraction is not discussed explicitly, because it is done via addition by reversing the sign of the number to be subtracted (subtrahend) and adding the result to the number subtracted from (minuend). You have to be careful when subtracting floating point numbers, because of the possible large round off error in the result.