2. Computer Hardware A computer is made of internal components Central Processor Unit Internal External and external components

3. External components are also known as peripherals: 4 Input devices –keyboard –mouse CPU Keyboard Disk Drive VDU Output devices VDU printer Storage devices disk drive

4. Internal components are contained in the Central Processor Unit CPU CPU Main Memory Processor I/O controllers Buses

5. Central Processor Unit Processor Main Memory Keyboard Input Controller VDU Output Controller (Disk) I/O Controller Keyboard Secondary Storage VDU Control Bus Address Bus Data Bus System Buses

6. Memory 4 Memory holds program instructions and data (Von Neumann machine). 4 Memory provides high speed storage and is volatile. So contents are lost when the computer is switched off (unlike secondary storage) 4 Memory is organised into same size memory locations, sometimes called a ‘word of memory’. –Bit: Binary digit, each having value 0 or 1 –Byte: a set of 8 bits. –Word: a set of bytes (typically giving 16, 32, 64 or 128 bits)

7. Processor 4 The processor executes programs stored in memory. It contains three main components: 4 control unit –co-ordinates and controls computer by generating control signals and performing the fetch/execution cycle 4 arithmetic and logic unit –performs arithmetic and logical operations on data 4 registers –are high speed memory cells (eg program counter)

8. Fetch/execute cycle 4 In order to execute program instructions stored in main memory the processor repeats the cycle: 4 fetch the next instruction from main memory –copy the contents of the program counter to the address bus –request memory to be read –increment program counter to point to next instruction 4 decode the instruction –decode instruction into commands 4 execute the instruction –execute decoded instruction commands

9. Input/Output Controllers 4 Because peripherals work in different ways and are continually being developed, the processor communicates indirectly with peripherals through I/O controllers (also called device controllers). 4 Device controllers contain: –interface connection to system bus –a set of data, command and status registers –interface connection to peripheral cable

10. System buses 4 A bus is made of parallel wires connecting components of a computer. The three buses connecting the processor to other components, called system buses, are: –control bus –address bus –data bus 4 Each bus is made of lines, with each line capable of carrying a 0 or 1 value bit at a time

11. Control Bus 4 The control bus carries bi-directional signals that control the operation of the system components. 4 status signals enable sharing of the address and data buses –bus request = a component needs control of bus –bus grant = a component is granted control of bus 4 command signals specify operations to perform –memory write = data on data bus written to location given by address bus –memory read =data in location given by address bus read intodata bus 4 timing signals synchronise components. –Clock provides regular pulses (giving speed of processor)

12. Address bus 4 The address bus carries address signals in one direction only: from the processor to a component. 4 The address bus carries the address of the location of an instruction, or data, to be written to or to be read to memory or to an I/O controller. 4 The width of the address bus determines the maximum memory size, –for example if an address bus had a width of 3 lines, the number of locations that can be addressed would be = 8 locations. (at least 20 lines would be more realistic)

13. Data bus 4 The data bus is bi-directional and carries program instructions or data. 4 The number of lines determines the amount of data that can be transferred at a time (in a single fetch) Word size 4 The word size of a computer is the number of bits the processor can process simultaneously. 4 This may be the same or a multiple of the data bus size

14. Central Processor Unit Processor Main Memory Keyboard Input Controller VDU Output Controller (Disk) I/O Controller Keyboard Secondary Storage VDU Control Bus Address Bus Data Bus System Buses