Basic CPU Architecture - The Arithmetic Logic Unit
By Stephen Bucaro
Understanding computers requires an understanding of the basics. In this article I
explain basic CPU architecture in simple basic terms. Most people agree that the CPU
(Central Processing Unit) is the "brains" of the computer. The CPU is a large integrated
circuit mounted on the motherboard inside the computer's case. A CPU is a very complex
circuit, so in this article, I focus on the Arithmetic Logic Unit part of the CPU.
Will you need to know any of this to pass the A+ certification exam? The A+ certification
exam is designed for a technician that removes and replaces modules. In other words, the
exam expects you to be able to determine when the CPU is bad, and to be able to replace it,
but not necessarily to understand its inner working in detail. So, no you probably will
not need to know any of this to pass the A+ certification exam.
Will knowing any of this help you to troubleshoot a computer? Any understanding you have
about something will always aid in figuring out what's wrong with it, so it very well could
help you to troubleshoot a computer.
Will it help you understand articles and specifications about new CPU's coming out and be
able to converse with your colleagues in an intelligent manner? Yes, it definitely will put
you in the computer geek category.
In order to explain basic CPU architecture in an understandable way, I'm going to
describe a simple fictitious CPU that doesn't really exist. The closest thing to the CPU
I describe here is the 8088, a simple 8-bit processor developed by Intel in 1979. Even so,
I'm going to simplify it even further by leaving out many complex details.
The Intel 8088 has the height
of advanced computer technology
when it released in 1979.
Computers today are tasked with everything from controlling your car to controlling
satellites, but in the early days computers weren't thought of as controllers, they where
designed primarily to perform mathematical calculations. Therefore, they were designed
around a circuit called the Arithmetic Logic Unit.
Highly simplified 8088 diagram
Let's study the diagram above from right-to left (computer professionals commonly think from
right-to-left). In my explanation of this diagram, I'm going to leave out a whole lot of detail,
which I'll explain later. On the right side of the diagram, you see the Memory or RAM.
Basically the Bus Interface Unit puts an address on the Address bus along with
a read signal on the Control signals causing the Memory to place the data
at that address on the Data bus.
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