Chapter 10 : Computer Architecture

This chapter is only theory of computer architecture. Actually, we need to know this part before we can start design and implementation. There are a lot of concepts we should know. In this chapter, I tried to cover the most important concepts.

Computer Architecture

The term architecture usually used for everything engineers build, and it means all engineers need to know the architecture of what they made. Computer Architecture is simply operations that a computer can do. Do you remember our Addition Machine? That computer had a simple architecture, it could add two numbers, and if one of inputs was negative (according to the operator of computer, not computer itself!) it could help us subtract. That's nice, isn't that? So All instructions and operations of a computer is its architecture. This is the only concept you actually feel when you are a user. When you are engineer you need to know more concepts.

Backward Compatibility

Backward compatibility is the most important theory in computer architecture and organization. In 70's, Intel made 8085 processor. Later, they made 8086. Let's see how they implemented Backward compatibility! Imagine a program written for 8085, 8086 must support and execute that. Every newer designs, should be compatible with older ones. For now, I have a laptop with Intel Core i5 processor, and I can execute Windows XP on it. This is Backward compatibility.

Computer Organization

You know what computer architecture means, but what about organization? computer organization, is a level before architecture. In architecture, we just analyze computers instructions, and after analyzing it, we can tell what instructions this computer can do.. But, the *structure of instructions is not revealed yet! When we start studying structure of instructions, we actually study computer organization.

So, when we say our computer can multiply, we just talked about its architecture. But, when we say our computer includes a multiplier, which is made up of adders , we spoke about its organization. So, when we are talking about what a computer can do we are speaking about architecture. But, when we start talking about how instructions are implemented , we are talking about organization.

Complex or Reduced? This is the question

A computer is measured by number of its instructions, so, when this number is less than 100, we call the design Reduced Instruction Set Computer or in short, RISC. When we have more than 100 instructions in a computer, we call that Complex Instruction Set Computer or CISC. The computer we design in this book, will be a RISC, because RISCs are easy to study, learn, implement and understand. Also, RISCs are faster than CISCs.

Decisions!

In chapter two, we decided to design a computer with the word size of 8 bits. So, we need to make decisions about its architecture and organization. Next chapter, will be about our design, and then, we start making our computer!