Cpu & Memory

Front 1

The CPU is also known as the 'brain' of the computer.
The main purpose of the CPU is to process data.
searching, sorting, calculating and decision making are all done within the CPU.

Back 1

There are 3 Components of the CPU:
1. The Control Unit (CU)
2. Arithmetic and Logic Unit (ALU)
3. Immediate Access Store (IAS)

Front 2

The Control Unit (CU) performs the Fetch - Decode Cycle
The Arithmetic & Logic Unit (ALU) performs calculations, comparisons of values and Boolean operations
Immediate Access Store (IAS) holds data and programs currently in use

Back 2

The Fetch - Decode - Execute Cycle

All programs, from word processors to games, are all made up of a series of simple instructions called "Machine Code" - this must be followed in order for the program to work. Humans are only able to write simple instructions in this form.
Programmers then write the "source code" of programs in languages such as C++, Java, Dephi or visual basic.
This is then translated into machine code, the format that the CPU can understand, often referred to as .exe files.

Front 3

1. Fetch
The first step the CPU carries out is to fetch main memory then store them in their own internal temporary memory areas. These memory are called "registers".
For this to happen the CPU makes use of vital hardware called the "Address Bus".
The CPU places the address of the next item on the address bus.
Data from this address moves from main memory to CPU by traveling along the "Data bus"

Back 3

2. Decode
The next step is for the CPU to make sense of the instruction it has just fetched.
This is called DECODE.
The CPU is designed to understand a specific set of commands which are called the "instruction set" of the CPU. Each make of CPU has a different instruction set.
The CPU decodes the instructions and prepares various areas within the chip in readiness of the next step.

Front 4

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

Back 4

The Control Unit (CU)

Has 3 main jobs:
1. Controls and monitors the hardware attached to the system to make sure the commands given to it by the application software are used.
2. Controls the input and output of data so that signals go to the right place at the right time.
3. Controls the flow within the CPU - which is the fetch execute cycle.

Front 5

Immediate Access Store (IAS)

Often referred to as RAM as it is the technology chiefly associated with it.
Data can be read anywhere in IAS without going through all the memory addresses.
This holds the data and instructions needed that instant by the CU.
The CPU reads data and programs held on backing storage and stores them temporarily in the IAS memory. CPU needs to do this because backing storage is much to slow to be able to run the applications directly.
Data and programs are first of all loaded into the main memory, then the CPU fetches just the part it immediately needs and loads it into it's own internal registers.

Back 5

The Arithmetic and Logic Unit (ALU)

Where the computer processes data either by manipulating it or acting upon it. It consists of 3 parts:
1. Arithmetic part - perfroms calculations on the data e.g. 3+2=5
2. Logic Comparisons - deals with the logic and comparisons. Compares data items such as numbers, letters, or special characters to see if they meet a particular condition
3. Logical Functions - Based on Boolean algebra. Used to compare numbers which can only be 1 or 0 which usually mean "true" or "false"

There are 3 basic types:
- The OR function
- The AND function
- The NOT function

Front 6

Clock Speed

Computers can only do one thing at a time, it may appear the many things are happening simultaneously, but in reality a computer can only process one instruction at a time, just because the computer is so fast, everything seems to be happening at once.
The CPU processes digital data by taking data one-at-a-time and doing something with it. The amount of time it has to process each piece of data is controlled by a quartz clock in the CPU.
With every Click of the clock, the CPU is able to process one type of data or execute one instruction.

Back 6

The CPU clock speed is measured in cycles per second. 1 cycle per second is known as 1 Hertz.
A computer running at 1GHZ can carry out 1 thousand million instructions per second.
The Clock on a modern desktop runs extremely quickly, typically 3 thousand million times a second (36Hz).

Front 7

Its hard to imagine how fast this is. But it does mean that the CPU running at this clock speed can handle a huge amount of data in a very short time.
The speed of the clock determines the basic performance of the CPU - the faster it goes the more powerful the computer.

Back 7

Clock Speed Higher - faster performance but runs hotter and consumes more power
Clock Speed Lower - lower performance, less costly, needs less power - good for battery life in laptops
Over Clocking - some people try to get the most extreme performance out of their CPU. They do this by over clocking, this means that they adjust the clock to run faster than the CPU was designed for, this makes the CPU hotter and so extra cooling are needed

Front 8

Cache Memory

Although RAM is faster than the hard disk, it still isnt fast enough to cope with the speed the CPU needs to dead with.
The way around this is to store as much data as possible, as close to the CPU to avoid delays. This nearby storage is "Cache".

Back 8

This special type of memory can be accessed faster than RAM. The CPU looks in the cache for the data it needs.
If the data is there, it will retrieve and process it. If it isnt there, the CPU accesses the system memory and then puts a copy of the new data in the cache before processing it.

Cache memory is Volatile - data is lost when computer switched off.

The larger the cache size, the better the performance because you are able to store data locally.

Front 9

RAM - Random Access Memory

RAM is temporary memory.
Work is safe there while the computer is working but as soon as the computer is switched off, everything disappears - Volatile Memory.

RAM stores the data you are working and also stores the modules that are needed to make your applications work.

RAM is needed so you can have multiple windows open and so you can switch between them.

Back 9

ROM - Read Only Memory

Special type of memory which stores the instructions which the computer uses when it "boots - up" - the BIOS (Basic Input Output System)
Because the data is "Read Only", it can be read but not changed by the user.
ROM is Non Volatile - Data is not erased when the computer is shut down. The data is stored permanently.