Computing hardware CPU

Candidates should be State the purpose of the CPU Describe the function of the CPU as fetching and executing instructions stored in memory Explain how common characteristics of CPUs such as clock speed, cache size and number of cores affect their performance

CPU The CPU is sometimes referred to as the brain of the computer. However, unlike the human brain it cannot think for itself. It requires humans to supply programs and data before it can process. It is where all the searching, sorting, calculations and decision making takes place. A typical CPU consists of three main elements: - Control unit: responsible for co-ordinating the input and output devices and all related activities. - Immediate Access Store (IAS) - Arithmetic and logic unit (ALU): calculations are done here and logical devices are made.

Control Unit It controls and monitors the hardware attached to the system to ensure the commands given to it by the application software are used It acts under the direction of the clock, and ensures that all the data goes to and from the correct places in memory It instructs the ALU as to what arithmetic calculation to perform E.g. if you send something to print, the control unit will keep a check that the instructions are sent to the printer correctly.

Control Unit E.g. if you send something to print, the control unit will keep a check that the instructions are sent to the printer correctly.

The Immediate Access Store This holds the data and programs needed at that instant by the Control Unit The CPU reads data and programs kept on the backing storage and stores them temporarily in the IAS’s memory CPU needs to do this because Backing Storage (e.g. hard disk) is much too slow to be able to run applications directly So data and programs are first loaded into main memory then the CPU fetches just the part it immediately needs and loads it into its own internal registers as part of the Fetch-Execute Cycle E.g. If modern CPU was slowed down to carry out one instruction per second. If the hard disk slowed down to an equivalent rate, it would take three months to supply the data needed to the CPU.

The Immediate Access Store This holds the data and programs needed at that instant by the Control Unit The CPU reads data and programs kept on the backing storage and stores them temporarily in the IAS’s memory CPU needs to do this because Backing Storage (e.g. hard disk) is much too slow to be able to run applications directly So data and programs are first loaded into main memory then the CPU fetches just the part it immediately needs and loads it into its own internal registers as part of the Fetch-Execute Cycle E.g. If modern CPU was slowed down to carry out one instruction per second. If the hard disk slowed down to an equivalent rate, it would take three months to supply the data needed to the CPU.

The Arithmetic and Logic Unit This is where all the calculations take place within a computer system The computer processed data either by manipulating it or acting upon it There are basically two operations which can be performed Arithmetic part - Basic mathematical operations (+, -, *, /) Logic part – Comparisons, works out if one value is greater, less than or equal to another( < or >)

The fetch-decode-execute cycle This describes the steps needed for processing to take place. Sometimes called Fetch-Execute Cycle Operating system loads the data and the program into main memory (RAM)

Fetch CPU fetches some data and instructions (program) from the main memory and stores them in its internal temporary memory areas called ‘registers’ For this to happen, the CPU makes use of a vital hardware path called the ‘address bus’ CPU places the address of the next item to be fetched on to the address bus Data from this address moves from main memory into the CPU travelling along another hardware path called ‘data bus’

Decode The CPU must make sense of the instructions it has just fetched. This process is called ‘decode’ CPU is designed to understand a specific set of commands called the ‘instruction set’ of the CPU. Each make of CPU has a different instruction set CPU decodes the instructions and prepares various areas within the chip in readiness of the next step

Execute This is the part of the cycle where data processing actually takes place The instruction is carried out upon the data (executed). The result of this process is stored in yet another register Once the execute stage is complete, the CPU sets itself up to begin another cycle once more

Clock speed Computers can only process one instruction at a time. It happens so fast that it appears as though many things are happening simultaneously The amount of time it has to process one piece of data is controlled by a quartz clock inside the CPU CPU clock speed is measured in cycles per second. 1 cycle per second is also known as 1 Hertz A computer running at 1GHz can carry out a thousand million instructions per second Clock on a modern desktop computer runs extremely quickly, typically 3 thousand million times a second (3 GHz)

Clock speed cont. Clock speed higher Clock speed lower Faster performance but runs hotter and consumes more power Clock speed lower Lower performance, less costly, needs less power so good for battery life in laptops Overclocking - Adjusting the clock to run faster than the CPU was designed for. This makes the CPU run hotter and so extra cooling fans also have to be purchased

Number of cores If the clock cannot be sped up any more, how would you improve performance? Using two CPUs at the same time means the job in hand is shared between the two CPUs But many applications have not been written to take advantage of extra processors A modern processing device may contain two or even four CPUs. These CPUs are often called ‘cores’. So a dual-core device means it contains two CPUs and a quad-core contains four

The Cache Most data is stored on the hard disk, when we use the data it is loaded into RAM because it is much faster to access from RAM than from the hard disk Data can be stored as close as possible to the CPU to avoid delays. This nearby storage area is called the ‘cache’ It can be accessed much faster than RAM. CPU looks in the cache for the data it needs. If it is there it will retrieve it and process it. If not, the CPU accesses the system memory and then puts a copy of the new data in the cache before processing it

The Cache

Cache cont. Cache memory is volatile i.e. when computer is shut down, the data stored there is lost To improve computer performance, it is important to keep the local cache full at all times so that the CPU doesn’t have to wait around with nothing to do The larger the cache size, the better the performance as you are able to store more data locally

Summary The purpose of a CPU is to: Control movement of data & instructions Fetch data and instructions from memory Decode and execute instructions Perform arithmetic & logical calculations The performance of the CPU depends upon the: Processor speed Bus speed Amount of cache available Number of processor cores