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the cpu (central processing unit) is the part of a computer system that is commonly refereed to as the brains of a computer. the cpu is also know as the processor or microprocessor

The cpu is responsible for executing a sequence of stored instructions called a program. this program will take inputs from an input device, process the input in some way and output the results to an output device.

cpus arent only found in desktop or laptop computers, many electronic devices now rely on them for their operation. mobile phones,DVD players and washing machines are examples that have a cpu.

a computer uses two types of storage. a main store consisting of ROm and Ram, and backing stores which can be internal, eg hard disk, or external, eg a cd or usb flash drive.

RAM and ROM

main store(or computer memory) is divided into read only memory (ROM) and random access memory(RAM).

ROM

ROM is memory that cannot be changed by a program or user. ROM retains its memory even after the computer is turned off. for example,ROm stores the instructions for the computer to start up when it is turned on again.

RAM

RAM is a fast temporary type of memory in which programs, applications and data are stored. here are some examples of whats stored in RAM:

the operating system

applications

the graphics user interface(GUI)

when a computer loses its power

Bridges

bridges manage how data and instructions are transferred between the cpun memory and other devices.

many pc motherboards use a northbridge and a southbridge to connect devices;

the northbridge connects the cpu, RAM and gpu through the fsb.

the southbridge connects to the northbridge.all other hardware, such as the hard drive, connects to the cpu through the southbridge.

ALU

the AlU stands for arithmetic and logic unit

the ALU is where the CPU performs the arithmetic and logic operations. every task that your computer carries out is completed here. even typing into a word processor involves adding binary digits to the file, and then calculating which pixels on the screen should change so that you can see the characters. the ALUs operations fall into two parts:

the arithmetic part,which deals with calculations eh 1+2 = 3

the logic part, which deals with any logical comparisons eg 2>1

Cores

a CPu is traditionally made up of a processor with a single core. most modern Cpus have two, four or even more cores.

A CPu with two cores, called a dual core processor, is like having two processors in one. a dual core processor can fetch and execute two instructions in the same time it takes a single core processor to fetch and execute just one instruction. a quad cor processor has four cores and can carry out even more instructions in the same period of time.

the main downside of using quad core processors is that they are more expensive to design and make, and they also use more power than single or dual core processors. another disadvantage is that the instructions have to be split up to decide which core will execute them and the results have to be merged together again at the end, which slows the processor down a little.

Cache

a cache is a tiny block of memory built right onto the processoir. the most commonly used instructions and data are stored in the cache so that they are close at hand. the bigger the cahce is, the quickly the commonly used instructions and data can be brought into the processor and used.

the fetch- execute cycle

the basic operation of a computer is called the fetch-execute cycle. the computer fetches the instruction from itsa memory and then executes it . this is done repeatedly from when the computer is booted up to when it is shut down.

fetching the instuction

the first step the cycle carries out is fetching the instruction. the cpu fetches this from the main memory(the hard drive) and stores it in the cpu temporary memory,the immediate access store.

the hardware that defines a computer is the cpu and memory. the cpu executes programs and memory stores program operations and data while a program is being executed. storage stores programs and files long term, even when they're not in use.

level-1 cache memory is the fastest kind of memory for the cpu to access. it is made up of faster memory circuitry and is the closest to the circuits of the cpu.

the RISC mobile processor usually works at a slower speed than a CISC processor to conserve battery life. however,RISC processors are usually fast enough for the device to run games and playback video smoothly.

when swapping data in and out of virtual memory, data from RAM is often transferred to and from a temporary file on a hard disk. data that is needed from the cache gets transferred back to RAM, not to the hard disk.