GCSE OCR Computer Science - Memory & Storage

RAM (Random Access Memory)

Volatile memory which can be read to and written from. It stores all data, files and programs currently in use.

Virtual Memory

An area of secondary storage used as extra RAM

Why does using virtual memory make a computer slow to respond?

• The CPU has to read data in virtual memory and move it back into RAM

• Secondary storage is much slower than RAM

ROM (Read Only Memory)

Non-volatile memory which can only be read from.

What does ROM contain?

The BIOS (Basic Input Output System) which is the instructions required for the computer to boot up.

Primary Storage

Memory areas that can be accessed very quickly by the CPU and are typically volatile.

Secondary Storage

Non-volatile storage that stores all data not in use and is much slower than primary storage.

How does a HDD (Hard Disk Drive) work?

• Data is stored magnetically on small areas of a spinning magnetised metal disk platter’s circular tracks

• A moving arm can access this data and read and write

Advantages of HDDs in comparison to SSDs (3)

• Higher storage capacity

• Cheaper

• Longer read/write life

Advantages of SSDs in comparison to HDDs (4)

• Faster access times

• No moving parts so more durable and portable

• Don’t make noise

• Don’t need defragmenting

How does an optical disk work?

• Data is stored on a spinning disk as ‘pits and lands’

• A laser beam shines onto the surface

• A sensor detects how it shines back, determining if it was a pit or a land

How does flash memory work?

Data is stored in tiny ‘floating-gate’ transistors.

Advantages of Optical Disk

• Very cheap

• Portable

• Take up less physical space

Disadvantages of Optical Disk

• Slow read/write speed

• Low capacity

• Easily damaged (scratched)

Why do computers store data in binary?

Computers consist of transistors which only have two values (on or off) so data must be processed in this way.

Units of data

Bit, nibble, Byte, Kilobyte, Megabyte, Gigabyte, Terabyte, Petabyte

Nibble

4 bits

Byte

8 bits

Binary

(base-2) Uses 2 different digits (1 and 0)

Hexadecimal

(base-16) Uses 16 different digits (0-9 & A-F)

How to convert Hex to Denary?

• Multiply the first digit by 16

• Add the second digit

How to convert Denary to Hex?

• Divide by 16

• Add on the remainder

How to convert Binary to Hex?

• Divide the binary number into nibbles

• Convert each nibble to it’s corresponding value

How to convert Hex to Binary?

• Find the denary value of each nibble

• Find the binary value of each denary number and put them together

Character Set

A collection of characters the computer recognises through binary representation.

How many bits does ASCII use?

8 bits (1 byte)

How many characters are available with ASCII?

256

How many bits does Unicode use?

16 bits (2 bytes)

How many characters are available with Unicode?

65536

Colour depth

The number of different colours which can be used in an image file.

Resolution

The number of pixels in an image (width x height)

Metadata

Data about the file itself

Examples of metadata

Format, heigh, width, colour depth, resolution

File size (Image) =

colour depth x resolution

How can analogue signals be converted to digital?

The amplitude of the analogue sound wave is sampled at regular intervals

Sample rate

(Hz) Samples taken a second

Bit depth

The number of bits available for each sample

File Size (sound) =

Sample Rate (Hz) x bit depth x duration (s)

Compression

Decreasing the file size whilst trying to keep the file true to the original.

Lossy Compression

Compression that permanently removes data from the file, reducing the overall quality but greatly reducing file size.

Lossless Compression

Compression that temporarily removes data from the file with no reduction in quality, but only a slight reduction in file size.

Why can’t lossy compression be used on text or program files?

They need to retain all the data from their original files.