GCSE A* REVISION NOTES


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Hardware

CPU

The system bus connected to the CPU is made of three buses: address, data and control

The address bus carries memory addresses between the CPU and primary memory.

The data bus carries data which could be an instruction e.g. add or it could carry a piece of data e.g. a number.

The control bus carries control signals from the control unit. These coordinate CPU activity.

Typical bus speeds are between 66MHz and over 800MHz.

The CPU carries out the fetch, decode and execute cycle over and over again

ALU (Arithmetic Logic Unit) – Performs all the arithmetic and logical operations within the CPU

Control Unit – The control unit works with the CPU to control the flow of data within the system

Registers - very fast, small areas of memory in the CPU were specific items are stored while executing in the CPU such as program counter

Program counter – Holds the address in RAM of the next instruction to be executed

CPU Clock - produces a regular pulse that the control unit uses to generate control signals to coordinate the running of programs

State the purpose of the CPU

Describe the function of the CPU as fetching and executing instructions stored in memory

Explain how common characteristics of CPUs such as clock speed, cache size and number of cores affect their performance

Clock speed – indicates how many clock cycles/operations are executed each second

Overclocking can be used to increase the clock speed of the CPU.

Overclocking can cause instability and will generate more heat reducing the life of the CPU

Cores - Independent processing units inside the CPU that work simultaneously

Cache – Special high speed memory used by a computer. The more cache the less often the CPU needs to retrieve data from RAM which is further away and slower.

Binary Logic

Explain why data is represented in computer systems in binary form

So that computers can be based on logic circuits.

Each part of the circuit can be in one of two states – on/off or true/false

As there are only two states computers use the binary system which has two digits – 0 and 1.

It is faster to determine if it is on or off than the analogue property

Understand and produce logic diagrams using the operations NOT, AND and OR

Produce a truth table from a given logic diagram.

 AND

INPUT

OUTPUT

A

B

A AND B

0

0

0

0

1

0

1

0

0

1

1

1

OR

INPUT

OUTPUT

A

B

A OR B

0

0

0

0

1

1

1

0

1

1

1

1

NOT

INPUT

OUTPUT

A

NOT A

0

1

1

0

Memory

Describe the difference between RAM and ROM

RAM (Random Access Memory)

ROM (Read Only Memory)

Read and Write

Read Only

Volatile

Non-Volatile

Stores in use programs and data

Stores the boot up sequence

Explain the need for ROM in a computer system

ROM permanently stores the programs and data needed to boot the system. An example being the BIOS in a PC.

Describe the purpose of RAM in a computer system

It is where the program instructions are stroed after they have been loaded from the disk so that they can be accessed more quickly by the CPU. Instructions and data are stored in binary format at specific locations called memory addresses which can be accessed in any order. The main memory is made of dynamic RAM which is slower than static RAM as it constantly needs to be refreshed.

Explain how the amount of RAM in a personal computer affects the performance of the computer

The more RAM a computer has the less virtual memory is needed and so the speed will be slowed down less by the Hard Drive Disk. So adding more RAM will speed up the computer.

The computer will be able to multitask more programs as there is more space to use without using virtual memory

Explain the need for virtual memory

Virtual Memory – A section of the hard drive used to store items in RAM which are not currently being used

When the RAM is full, the OS will swap out pages from the RAM on the hard disk drive called virtual memory. Virtual memory creates a swap file/paging file in unused disk storage locations and swaps the least used pages of a fixed size. The OS keeps track of what has been swapped using a page table and any errors that occur are called page faults. If memory is very low pages will have to be continually swapped in and out called disk thrashing. Having to use virtual memory slows down the computer as swapping pages takes a long time so the more memory a computer has the less virtual memory it will use and the faster it will run.

Describe cache memory

There are three levels of cache memory L1, L2 and L3 referring to the distance they are from the CPU with L1 the closest and smallest and L3 the largest and furthest. Cache uses static ram that does not lose its charge and can store data while they have power without being refreshed. This makes accessing the data much quicker, however it is also very expensive to buy.

Describe flash memory

EEPROM (Electrically Erasable Programmable Read-Only Memory)

It is non-volatile storage that contains no moving parts and so is very reliable. It is very compact and fast. It wears out of time after a million writes. More expensive and much lower capacity than normal hard drives.

Discuss how changes in memory technologies are leading to innovative computer designs

Devices can be made smaller, lighter and more compact when their storage is based on flash memory. The storage within it needs to be small, light and have low power consumption. It must also be able to withstand being carried around and potentially moved quite suddenly. As the price of solid state memory falls, its capacity is increased and it becomes physically smaller, meaning new designs of computer based products will be possible.

Input and Output Devices

Understand the need for input and output devices

Input Device - Enter data into a computer system

Output Device - Present data to the user

Describe suitable input devices for a wide range of computer controlled situations

Input Devices

Keyboard

The keyboard, along with the mouse, is one of the most used input devices.  Once you get the hang of where the keys are, it allows people to enter text- based data quickly into the computer.  A numerical keypad (found on most keyboards on the right) provides a fast way for accountants and people who work in finance to enter numbers.

Mouse

The computer mouse is another common input device. It allows the user to interact with the software on screen.  The mouse is quite versatile, with at least two buttons and the ability to move in any direction.  However, it can be difficult to be precise – e.g. drawing using the mouse is quite hard!

Scanner

Scanners convert physical documents and photos into digital format.  They can be used in conjunction with special software to automatically read handwriting and convert it into computer text.  This is called Optical Character Recognition. Some tick-box forms are scanned in, like the Census form or your multiple choice Science exams; this is called Optical Mark Recognition. This is a form of automated input which is much quicker and more reliable than human input.

Touchscreen

Touchscreens have been around for years in one form or another.  The latest versions, like the Apple iPad interface use multi-touch technology to allow greater control.

Microphone

Microphones convert analogue sounds into a digital format that the computer can understand and playback.  Sounds are usually recorded as .wav or .mp3 files (the latter uses compression to make the file size smaller and is more likely to be used to store music tracks).

Digital Camera

Digital cameras convert live images into digital format.  Photos are usually stored as JPEG files.  The quality and size of the picture is determined by the number of megapixels the camera is capable of detecting, e.g. photos taken with an 8 megapixel camera are made up of 8 million pixels.

Sensors

Sensors measure the physical world and translate that into a digital format that a computer can understand.  There are various sensors that detect all manner of things: movement, light, heat, moisture, gases (e.g. carbon monoxide levels), location (GPS), etc.  For example, the iPhone has an accelerometer so it knows which way round it is to ensure the screen is rotated the correct way. These can be used for automated input or store values to be analysed (data logging).

Describe suitable output devices for a wide range of computer controlled situations

Printer

Printers produce physical documents and photos from digital files.  Most photo printing is done by inkjet printers which can be expensive to run.  Laser printers use toner cartridges which are more expensive initially but can print more pages before being replaced.

Monitor

Monitors/screens provide the visual output from the computer system.  Most computer monitors and mobile phones use colour LCD (liquid crystal display).

Speakers

Working the opposite way to the microphone input, the speaker converts digital sound into analogue waves.

Motor

A motor can be driven with precision by a computer system.  Often used in manufacturing, e.g. building cars, motors are used to control robotic arms.

Discuss input and output devices for users with specific needs.

Puff-suck Switch

Suck or puff down the tube to activate a switch that can be interpreted as input

Foot mouse

Can be used by someone with limited use of their upper arms.

Eye Typer

A camera mounted on the computer determines were the user is looking and movements made by the eye. They can click with slow eye blinks.

Braille Keyboard

Allows user to type and enter text in Braille

Braille Display

Reads screen text and presents it via a refreshable Braille display

Braille printer/embosser

Presses pins into one side of paper in order to create raised dots on the other side of the paper printing in Braille.

Speakers

Can be useful for visually impaired. Text can be converted into speech and can be output via the speaker in a sound format using screen readers. The computer generated voice can be difficult to understand. Speakers can also signal when something has been successful or not.

Microphone

Speech can be converted from sound to text using specialist programs

Magnifiers

The zoom tool in software can be useful for people with poor eyesight

Screen magnifiers (hardware) can also be used to magnify what is displayed on screen

Predictive Text

Allows words to be entered quicker by predicting the most likely word. This reduces the number of keystrokes that you make helping people with typing difficulties

Sticky Keys

Sticky keys allows the user to press a 'modifier' key such as Shift, Ctrl, Alt and have it remain active until another key is pressed.

Secondary Storage

Explain the need for secondary storage

To store data permanently.

Describe common storage technologies such as optical, magnetic and solid state

Solid State

Uses banks of EEPROM chips that can be read and written to many times. The chips do have finite lifetime but this type of storage has not been around long enough to know how long. It is small and light weight making it ideal for portable devices. It also has faster read and write times due to the lack of moving parts. However, it is much more expensive than other types of secondary storage

Magnetic

A magnetised rigid plate with heads to read the data as the platters spin around. Slow access times as have to wait for moving parts. If jogged while spinning it can damage the disk and it is affected by heat and magnetic fields. Provides large storage space at low cost.

Optical

Uses light from lasers to detect reflections from the surface of the disk medium. They are made rewritable by using a laser to change the colour of a dye layer in the disk. Very cheap and robust but not permanent as the dye layer deteriorates over time. Very easy to scratch.

Select suitable storage devices and storage media for a given application and justify their    choice using characteristics such as capacity, speed, portability, durability and reliability.

Capacity: How much space there is

Speed: How quickly data can be read from it

Portability: How easily it is carry around

Durability: How easily it is damaged

Reliability: How long it will last


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