Processor Architecture

By: 2Lt Avery

Processor Architecture

This is a guide to understanding basic processor architecture based off the Intel 8088 CPU.

This information will be presented in a conceptual form as to make it easier to understand.

CPU stands for ‘Central Processing Unit’

You can think of the CPU as nothing more than an advanced and really fast calculator. It is able to add, subtract, multiply, and divide at great speed. This study of the CPU will focus on the Intel 8088 CPU. I chose this processor due to the fact that many ideas, and some basic parts in current chip designs are still based off of the 8088’s design. The Intel 8088 was released in 1979 and forever changed computing.

To understand how a CPU works we need to think of it as a college math professor. We can ask him any mathematical question and he can perform virtually any mathematical function. There’s a catch though. This math professor stays locked up in his room and refuses to speak to anyone but still wants to solve math problems. We need to find a way to communicate with this math professor.

Now imagine we are able to install 16 light bulbs , 8 inside his office and 8 outside his office.

Each of the 8 bulbs outside connect to the ones inside to form a pair. Each pair is either always on or always off. You can control the pairs via a set of 8 switches from outside his office and he can also control the pairs via a set of 8 switches inside his office. This light bulb analogy is used to explain how the External Data Bus communicates with the CPU.

By creating on/off patterns with the lights we can represent data or commands in a form the CPU (math professor) can understand. Thus we can send information to him and he can send information back to us.

This information is in the form of binary 1’s and 0’s.

Now that we can communicate with the math professor we can start sending him work. But to do this work he need worktables. These work tables are called Registers. In fact the processor needs at least 4 worktables. Think of the worktables as a desk with 16 light bulbs that are lined up in a straight line. (also binary) These lights can only be turned on or off by the math professor. (CPU)

By creating on/off patterns like the ones in the External Data Bus the CPU can use the 4 registers to work out math problems. All CPU’s these days contain a bunch of registers but for now we will look at the 4 most common or General Purpose Registers . These registers were named by Intel . AX, BX, CX, DX.

Because this is more of a basic overview we will not discuss low-order bits, or high-order bits.

With what is cover so far we know that if we turn on a certain combination of lights (binary code) on the External Data Bus we can instruct the CPU to do a specific task. Each time this is done it is called a line of code. The complete set of binary codes/combinations is known as the CPU’s Instruction Set .

Ok great, so now we have given our math professor a setup to receive information, 4 tables to work out the problem and a way to send information back out to us. But how does the math professor know when its time to work? Well imagine we have also installed a bell inside the math professor’s office. We have a little button on the outside that will ring the bell whenever we press it. Each time you press the button to ring the bell the math professor will read his set of lights. The bell in a CPU is called the Clock Wire. When there is a charge on the clock wire it tells the CPU that there is another piece of information waiting to be processed. For the CPU to process a command placed on the external data bus, a certain minimum voltage must be applied to the Clock Wire. This is called a Clock Cycle. The maximum number of Clock Cycles a CPU can handle in a given period of time is called its Clock Speed. The Intel 8088 had a Clock Speed of 4.77 MHz or 4,770,000 Cycles per second.

Ok so we’ve seen how the CPU works out problems but left out once crucial piece. How does the code get on the external data bus? The program is stored on the hard drive but the hard drive is too slow to get information to the processor. CPU’s needs something that will copy the program one line at time and fast. Machine code is just a pattern of eight one’s and zero’s so anything that can store that information is known as memory.

Memory will be covered in the next section.

Sources:

http://en.wikipedia.org/wiki/Intel_8088

http://www.cpu-world.com/CPUs/8088/index.html

http://www.pcguide.com/ref/cpu/fam/g1I8088-c.html

http://www.cpu-world.com/Arch/8088.html

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