Computer Hardware What is a CPU

Objectives Identify and describe computer data bus, address bus, control bus and connectivity. Demonstrate an understanding of the CPU (Components of the CPU: control unit, arithmetic/logic unit, internal memory, registers, buses. Explain the role of the CPU in fetching and executing instructions stored in memory

Bus Terminology

Buses What is a bus? An electronic traffic lane through which electrical signals are carried from one chip to another chip. Also used to carry data and instructions to and from the CPU and memory. The width of a bus tells you how many lanes it has, more lanes increases the amount data it can transfer.

A Potential Bird’s Nest Imagine connecting four chips to each other Now try to imagine what would happen with twenty chips!

The Bus Solution the bus A bus comprises the actual physical components (wires, connectors, chips) and a set of protocols defining how it is used (rules)

History Lesson – The System Bus A system bus is a single computer bus that connects the major components of a computer system. The technique was developed to reduce costs and connect different components together. Although popular in the 1970s and 1980s, modern computers now use separate buses for more specific needs and to increase the speed of the system.

Transferring Data Data Bus Used for transferring data to and from the memory, or from the central processing unit (CPU) to other components. Each data bus is designed to handle many bits of data at a time. The amount of data a bus can handle is called bandwidth. A typical data bus is 32-bits wide. Newer computers have data buses that can handle 64-bit and even 96-bit data paths.

Transferring Data Address Bus An address bus is a computer bus that is used to specify a physical address. When a processor needs to read or write to a memory location, it specifies that memory location on the address bus The width of the address bus determines the amount of memory a system can address at on point in time.

Transferring Data Control Bus  This carries command signals from the CPU and returns status signals from the devices. For example, one line of the bus is used to indicate whether the CPU is currently reading from or writing to main memory.

Transferring Data Computer buses

CPU Often described as 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 A standard process that describes the steps needed for processing to take place. Sometimes simply called the Fetch-Execute Cycle. Fetch The first step the CPU carries out is to fetch some data and instructions from main memory, then store them in its own internal temporary memory areas. These memory areas are called 'registers'. This is where the Address and the Data buses are used!

The fetch-decode-execute cycle The next step is for the CPU to make sense of the instruction it has just fetched. The CPU is designed to understand a specific set of commands. These are called the 'instruction set' of the CPU. Each make of CPU has a different instruction set. The CPU decodes the instruction and prepares various areas within the chip in readiness of the next step.

The fetch-decode-execute cycle This is the part of the cycle when data processing actually takes place. The instruction is carried out upon the data (executed). The result of this processing is stored in yet another register or in the main memory (RAM). Once the execute stage is complete, the CPU sets itself up to begin another cycle.

The Fetch / Execute Cycle control unit arithmetic / logic unit decode execute fetch (store) RAM

Parts of the CPU There are 3 main parts of the CPU these are: The Control Unit The Arithmetic and Logical Unit (ALU) The CPU also contains registers In addition modern processors also have a type of temporary memory called Cache.

Control Unit This has 3 main jobs: 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. For example, if you send something to print, the control unit will keep a check that the instructions are sent to the printer correctly. It controls the input and output of data so that the signals go to the right place at the right time. It controls the flow of data within the CPU - which is the Fetch-Execute cycle.

Arithmetic and Logical Unit (ALU) This is where the computer processes data either by manipulating it or acting upon it. It consists of two parts: Arithmetic part - which does exactly what you think it should - it performs the calculations on the data e.g. 3 + 2 = 5 Logic part - this deals with logic and comparisons. For example, it works out if one value is greater, less than or equal to another.

Registers Registers are special, high-speed storage areas within the CPU. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register. Registers can contain the address of a memory location where data is stored rather than the actual data itself.

Registers

Cache 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 Cache memory. The CPU needs to do this because Backing Storage (e.g. the hard disk) is much too slow to be able to run applications directly.

Cache In modern computers Cache memory is located on the CPU this is known as level 1, level 2 and level 3 cache. Level 3 cache used to be located on the motherboard but with increases in technology it is now integrated with the CPU. The larger the number the larger in size the cache is Also the level 1 and 2 cache is much faster and more expensive.

The Cache

The Cache

Clock Speed Computers can only do one thing at a time! A CPU processes data by taking each piece of data one-at-a-time and doing something with it. The speed at which it processes each piece of data is controlled by a vibrating quartz chip inside the CPU known as the system clock. With every tick of the clock, the CPU is able to process one piece of data or execute one instruction. Each fetch execute cycle takes one tick.

Clock Speed The CPU clock speed is measured in cycles per second. 1 cycle per second is also known as 1 Hertz The clock on a modern desktop computer runs extremely quickly, typically three thousand million times a second (3 GHz). The speed of the clock determines the basic performance of the CPU - the faster it goes, the more powerful the computer. Over clocking This means adjusting the clock to run faster than the CPU was really designed, used for extreme performance. However, this makes the CPU run hotter and so extra cooling fans also have to be purchased.

Cores To further improve speed using multiple CPU ‘s at the same time would increase performance! In an ideal world, running two CPUs would give you twice the performance. In real life this is not quite the case because many applications have not been written to take advantage of extra processors. A modern processing device may contain two or four or even eight CPUs. Some chip-making companies call these CPUs 'cores'. So a dual-core device means it contains two CPUs and a quad-core contains four.

Your Task Find a quad core processor and find out what is speed is, how much cache memory does it have and is it 32 bit or 64 bit? Can you find a CPU with more than 4 cores? Now find out what is speed is, how much cache memory does it have and is it 32 bit or 64 bit? Why do CPU have multiple cores?

32 bit verses 64 bit For homework find out the difference between a 32 bit CPU and a 64 bit CPU