Internal and External Hardware Components of a Computer
3 Box modelCPU Main Memory I/O Control Bus System Bus Address Bus Data Bus
The Processor The most complex and expensive component of the computer. Transistor count is currently 2.6 billion (2011). The processor executes programs and supervises the rest of the system at the same time. Transistors are devices that amplify and switch electronic signals and power. A voltage or current applied to one pair of the transistors terminals changes the currents flowing through another pair of terminals.
Main MemoryMain memory is data stores that can be directlyaddressed by the CPU. It’s used to storeprogram instruction and data and uses thesystem bus to communicate with the CPU. Mainmemory is made up of RAM and ROM chips.Today Ram is measured in Gb and largeprograms can run or multiple programs can runat the same time. The larger the memory themore/bigger the program you can run. Mainmemory is data stored in different addressesand so the more memory you have the moreaddresses you have.
RAMRandom access memory or RAM is used for anymemory that is readable and writable so the processuses it to store data and load it back. However RAM isvolatile, meaning the contents of the RAM isforgotten when the computer is turned off.
ROMRead only memory or ROM remembers data whenthe computer is turned off making it non-volatile.ROM can provide random access like RAM but it can’tbe written to. This makes it useful to store fixedprograms on.
I/O Controllers and PortsI/O controllers regulate the power between the devicesand the processor. Can handle different types ofdevices now whereas before the processor wouldn’trecognise new devices and whole new CPU would haveto be installed.Generic ports are now used for all devices e.g. USBbecause all computers have them and everyone canplug in devices. This is the method of performing I/Obetween the CPU and the device.
Connection between devicesIn the 3 box model each components is connected by abus, known as the system bus. The different buses aremade up of wires that connect to each component ofthe computer system and passes signals between them.Signals either represent an address, data or controlinformation and so the system bus is made up of theaddress bus, the data bus and the control bus.
Address BusUnidirectional and has 32 wires connecting to theaddress memory which gives 110 locations. It carriessignals from the CPU to the main memory and I/Odevices. The CPU could request some data from themain memory and it does this by sending the address ofthe data to the main memory and then the mainmemory returns the data along the data bus.
Control BusBidirectional and has more than 16 wires (usually 32)and is transports control signals between the 3components of the box model. It’s used to carryimportant information such as messages to say when adevice has finished a job or when a device has beenplugged in. It controls interrupt signals as well whichallow devices to signal that they have finished arequest. The CPU temporarily suspends its currentprogram, services the device and then resumes itsprogram. Ctrl + Alt + Del is the highest priority ofinterrupt.
Data BusBidirectional and has 32 wires used to transport dataand instruction between all the components in the 3box model. The larger the data bus is the more datathat can be transported at one time. E.g. 11111111 = 8 digit’s which means the data bus is 8 bit.
PeripheralsPeripherals are any device thatisn’t connected to the CPU.I/O devices aren’t connected tothe CPU – meaning they areperipheral. The system uses themto get information in or out. E.g.a keyboard gets information inand a VDU gets information out.
Secondary StorageSecondary storage is not directly connected to theprocessor making it a peripheral device but it ispermanent storage. An example of secondary storageis a magnetic hard disk. Any secondary storage isconnected to the processor and the main memory byan I/O controller and data is transferred by using thesystem bus.