CENTRAL PROCESSING UNIT2.1.2 – a,b,c & d

2.1.2. a - The Purpose of a CPU

The CPU is the brain of the computer.

The Purpose of the CPU is to process data. It is where all the searching, sorting, calculating and decision making takes place in the computer.

The fetch-decode-execute Cycle

The job of the processor is to FETCH the first instruction in its memory that makes up a program (e.g. Word, IE. These are all ‘programs’ made up of ‘instructions’)

Once if has fetched an instruction, it then has to decode what to do with it (called ‘DECODING’ an instruction).

Finally, it carries out the instruction (known as ‘EXCUTING’ an instruction).

Once the first instruction has been fetched, decoded and excuted, it then does the same to the next instruction, and the next, and the next, untilthe program is finished. Hence the CYCLE!

2.1.2 b –The Function of the CPU

You now understand the fetch-decode-execute cycle, but where does it fetch instructions from?

The role of the RAM in FDE cycle

The RAM is Random Access Memory – this is the short term memory of the computer.

When you load a program, most instructions are stored in the Hard disk.

When you double-click on a icon, a copy of the program instructions are moved to the computers main memory, known as the RAM.

The CPU fetches instructions one at a time from the main memory.

Primary Memory (IAS) V Secondary Storage and the FDE cycleThe CPU can not fetch instructions for programs stored on the hard drive. Therefore, the main memory, called the RAM is also known as the primary storage as well as immediate access storage.

The hard disk is the also known as secondary storage.

Note: You will see storage or memory.

Parts of the CPU

There are three parts to a CPU:• Control Unit• Immediate Access Store• Arithmetic and Logic Unit

The Control Unit

The control Unit has three main jobs:

1. It controls and monitors the hardware attached to the system to make sure that the commands given to it by the application software are used. i.e. printing, the CU will keep a check that the instructions are sent to the print correctly.

2. It controls the input and output of data so that the signals go to the right place at the right time

3. It controls the flow of data within the CPU – which is the Fetch-Execute Cycle

Immediate Access Store

This holds the data and the programs needed at that instant by the control unit. The RAM.

The arithmetic and Logic Unit (ALU)• This is where the computer processes data either by

manipulating it or acting upon it. It consists of two parts:1. Arithmetic part – which does exactly that, performs the

calculation on the data e.g. 2+2=4

2. Logic part – this deals with logic and comparisons. For example, it works out if one value is greater, less than or equal to another.

2.1.2 c – CPU Characteristics and their impact on performanceAll CPU’s are not the same. A quick Amazon search shows 2,910 results of different processors. These are the main differences:• Materials,• Clock Speed,• Cache Size• Cores,• Word Size

December 2012

Materials

CPU are made of materials, no different to a car. You have your basic VW fox to your Aston Martin's.

Some are made with better quality materials than others. The materials used will effect;- Reliability- Speed- Performance

A slow component will slow down the overall performance of the CPU.

Clock Speed

CPU’s have a clock inside them, ticking away.

Instructions area carried out each time a tick of the clock happens. The faster the clock, the more instructions can be carried out in any second.

So 3.40GHz Processor can complete 3.40 billion ticks a second.

OverclockingSome people like to play with the BIOS settings that directly talks to the processor.

They increase the clock speed to a much higher speed compared to their designed speed.

Only one problem though! The fast the speed, the hotter it gets, therefore extra cooling is needed.Click Here for video example

Cache

The CPU fetch’s instructions from the RAM, but there is another place, the CACHE.

Cache is like RAM, but it is local to the CPU and therefore much faster to read from and write to incomparison.

The computer puts the information hat it will need again and again. The more cache the CPU has, the better the CPU will perform.

CoresToday’s basic CPU’s have one core, but most dual or even quad cores!

Therefore if it is dual core, it has two processors, both with their own caches.

Word SizeProcessors work on instructions and data, which it has fetched from RAM.

These are in binary, if a CPU can work on this number

0111 0011 1110 1111

Then it is said to be 16 bit processor, but most processors are either 32 or 64 bit processor.

This means that they can work on more data and instructions per tick on the CPU’s clock.

Therefore, bigger the word size, better the performance.