1. THE CPU
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2. LEARNING OBJECTIVES By the end of this lesson, you will:
be able to describe the purpose of the CPU
be able to state the function of the CPU (fetch and execute instructions stored in memory)
be able to describe how common characteristics of CPUs affect their performance:

3. Remember the computer system?
We’ve looked at what we mean by computer system, we’ve looked at the data input and output devices that might be used, today we need to look at data processing – the microprocessor, typically the Central Processing Unit (CPU)
CPU
(Processing)
Input
Output
Backing Storage
Main memory

4. What is the cpu? The CPU has 2 main components:
The control unit – uses electrical signals to direct the system to execute the instructions in stored programs
The Arithmetic and Logical Unit (ALU) carries out all of the arithmetic and logical operations including addition, subtraction, comparisons such as less than, more than.
The CPU needs access to 2 types of memory:
The main memory or RAM where the programs are stored.
The cache memory which is used to store data waiting to be processed. This is fast memory stored close to the CPU with dedicated connections so it can quickly access frequently used data.

5. What is the purpose and function of the CPU?
The purpose of the Central Processing Unit (CPU) is to carry out program instructions.
The function of the CPU can be broken down into a series of steps that are carried out in a continuous cycle.

6. CPUs fetch data and instructions from main memory, process/execute and store the result in main memory in a continuous cycle
Fetch
Decode
Execute

7. The fetch step:
This involves retrieving an instruction from a memory address. The address of the instruction is stored in a register called the program counter (PC).
After an instruction is fetched, the PC is updated so the CPU knows the address of the next instruction it has to fetch.

8. The decode step:
This involves the CPU identifying the operation code (op-code) part of the instruction which tells it which operation to perform.
If the op-code requires the CPU to act on some data then the second part of the instruction will contain either the data or the memory address where the data is stored.

9. The execute step:
In this step the parts of the CPU are connected that are needed to execute the instruction that was decoded.
For example, if the instruction involved integer arithmetic or logical operations then the arithmetic logic unit (ALU) would be connected to the relevant memory locations so that:
The data for the calculation can be passed along a data bus to the ALU as input.
The ALU can execute the required operation
The result of the operation can then be passed from the ALU along a data bus as output.

10. So how fast can a processor process??
The performance of the CPU depends upon the:
Processor speed: this is determined by an electronic clock chip. It uses a vibrating crystal to maintain a constant rate. It is measured in hertz (Hz) or cycles per second. Everything a computer does is in time with this chip. A processor that is 3Ghz means that the processor can compile 3 billion cycles very second. So, a single CPU doing nothing else, running at 3 GHz, could complete around 300 million simple calculations every second.
Amount of cache available: cache memory is very fast but relatively expensive. It is usually measured in Megabytes rather than Gigabytes.
Bus speed: even with a high processor speed and a large cache, performance may be limited by how fast data and signals can move around the system on buses. This is simply a circuit that connects one part of the mother board to another, a bit like a road. The speed of the bus is usually measured in megahertz (MHz). This tells you how much data can move across the bus at the same time.

11. HOW TO IMPROVE PERFORMANCE
The clock speeds in use today are close to the limit for our existing technology so a different way of improving processor performance was developed – the multi-core processor.
A dual core processor has 2 CPUs working together. They may have their own cache or may share a cache, but because you’ve got 2 cores working together on the fetch-decode- execute cycle at the same time, they can process more instructions as a whole.

12. CPU SUMMARY The purpose of the CPU is to:
Control the movement of data and instructions
Fetch data and instructions from memory
Decode and execute instructions
Perform arithmetic operations such as add and subtract, and logical operations such as AND, OR, NOT
The performance of the CPU depends upon the:
Processor speed
Bus speed
Amount of cache available
Number of processor cores

13. WHAT TO DO NOW Complete the questions on the CPU on the worksheet.
In PowerPoint/Word, using autoshapes, draw the Fetch-decode-execute cycle (could use Smart Art) and explain briefly in a text box at the side what happens at each stage.
On the next slide(s), explain what is meant by the following and how these affect the processor’s performance: (1)clock speed (2) cache size (3) number of cores (4) bus speed
BBC bitesize will help you with this

14. plenary

15. RAM core execute Processing cache clock bus RAM Hertz motherboard
Central Processing Unit (CPU)
The core of every computer is the Central Unit. This directs the system to computer programs. In order to do this the computer needs access to:
random access memory or , where programs are stored
(b) memory, which is used to store data waiting to be processed.
As soon as the computer starts up, the CPU runs an endless fetch-execute cycle, constantly fetching instructions and decoding them.
The speed of the CPU is set by a chip which determines the CPUs speed, which is measured in Data is moved around the system along a or pathway. This simply connects one part of the motherboard to another. To improve performance of a computer we can use dual processors, which is simply two CPUs working together.
RAM core execute Processing cache
clock bus RAM Hertz motherboard