1.2 - Memory and storage

1.2.1 Primary storage

The need for primary storage

• A computer needs primary storage because access times are faster than secondary storage

• Primary storage holds data and instructions that the CPU needs to access whilst the computer is turned on

• Consists of:

  • Random access memory (RAM)

  • Read-only memory (ROM)

  • Registers

  • Cache

• Why would RAM be faster than a hard disk drive:
  

• RAM has no moving parts

• Hard disk has a head - which is a mechanical component

• RAM is physically closer to the CPU

• Hard disk is physically further away from the CPU

Difference between RAM and ROM

Volatile = when power is switched off it no longer retains any data

Non-volatile = retains/stores data even when not powered

RAM:

• Volatile

• Can change contents

• High capacity

• Holds data and instructions

ROM:

• Non-volatile

• Cannot change contents (Read only)

• Low capacity

• Holds start-up instructions

  • Contents of ROM are fixed in the factory

  • BIOS runs as soon as the computer is switched on

    • It checks that the hardware is functioning correctly, then runs a second program known as the bootup or bootstrap program that loads the computer's operating system

Virtual memory

Virtual memory = the use of secondary storage as primary storage:

• If there isn’t enough main memory for all the programs to run, the CPU uses virtual memory which is slower to access

• The CPU must fetch data/programs from virtual memory instead of from RAM

• Memory is divided into small fixed chunks called pages

• The least recently used page is swapped out of main memory and into virtual memory when main memory is full

• If a page is swapped out then needed again - it is swapped back into main memory then another page is swapped out

• Disk thrashing:

  • Happens when a computer spends too much time swapping pages back and forth between main memory and virtual memory

    • Slowing down the computer

• Using virtual memory makes a computer run slower:

  • The processor has to wait while data is swapped between hard disk and RAM

  • As secondary storage devices have slower access times than RAM, the computer's processing performance can be decreased

  • Processing performance can be improved by increasing the size of the RAM, thereby reducing the need for virtual memory

1.2.2 Secondary storage

The need for secondary storage

Secondary storage = non-volatile, long term storage for programs and data:

• Saves files over a long period of time

• Retains its contents when the computer is switched off

• Stores both programs and data

• Modifiable

Need:

• Used to keep programs and data even when the computer is switched off and is used as a long-term storage

Common types of storage

Optical storage:

• Examples include - 

  • CDs

  • DVDs

  • Blu Ray

• Applications - 

  • CD - Music

  • DVD - Video/film

  • Blu ray - HD video/film

• Advantages -

  • Hold reasonable amount of data

    • 650-600MB for CD / 4.7GM for DVD

  • Portable

  • Fast data transfer rates

• Disadvantages - 

  • Easily scratched - data can’t be read

  • Memory capacity not great

  • Cannot overwrite data

• How it works:

  • Holds data using pits burned into a plastic surface that is read by a laser beam

  • Flat areas known as the lands

  • A sensor looks at the reflected light:

    • Reflection (from lands) = a binary “1”

    • No reflection (from pits) = a binary “0”

Magnetic storage:

• Examples include - 

  • Hard disk drive / magnetic tape

• Applications - 

  • Main storage for a typical computer

  • Used to store:

    • OS

    • Application software

    • Files such as documents

    • Music, videos, etc

• Advantages -

  • Very fast data access

  • Data can be read directly from any part of the hard disc (random access)

  • Holds a lot of data (1TB+)

• Disadvantages - 

  • Moving parts can break when dropped

    • And eventually fails

  • Not portable (takes up space and heavy to carry)

  • Vulnerable to magnetic fields 

  • A strong magnet might erase the data the device holds

• How it works:

  • Use magnetic fields to magnetise tiny individual section of a metal spinning disk

  • A magnetised section = a binary “1”

  • A demagnetized section = a binary “0”

  • As the disk is spinning a read/write head moves across the surface to magnetise/ demagnetise a section of the disk spinning under it

Solid state storage:

• Examples include - 

  • USB stick, solid-state drive, SD card

• Applications - 

  • Personal storage

  • Mobile devices

  • Portable storage

  • Same as hard disk drives

• Advantages - 

  • Less susceptible to physical shock

  • Faster access time than hard disk

  • Not affected by magnets

  • Lower power consumption

  • No noise

• Disadvantages - 

  • Low storage capacity compared to hard disk drives

  • Limited number of writes

  • Expensive

• How it works -

  • Holds data using electrons trapped in solid matter

Characteristics of secondary storage

• Capacity

  • How much data can it store?

• Speed

  • How fast can it access data/ read and write data?

• Portabilit

  • How easy it is to move it from one place to another?

• Durability

  • How robust it is?

• Reliability

  • How consistently does it perform?

• Cost

  • How much does it cost per KB, MB or GB?

1.2.3 Units

All data in a computer is stored in binary form:

• A binary digit is known as a bit

• A bit is the smallest unit of data a computer can use

How to calculate file sizes of sound, images and text files:

• Sound file size = sample rate X duration (s) X bit depth

  • Sample rate = number of samples per second measured in Hz

  • Bit depth = number of bits required to store each sample

• Image file size = colour depth X image height (px) X image width (px)

  • Colour depth = Number of bits required to store each pixel

• Text file size = bits per character X number of characters

Why data must be stored in binary format:

Store data in binary because they use electronic switches with two states:

• CPUs, memory, and storage devices use transistors

• Transistors have two stable states: on or off

• These correspond to 1 and 0 in binary

1.2.4 Data storage

Numbers

HAVE TO DO ALL BINARY, DENARY, BINARY ADDITION, BINARY SHIFTS AND HEXADECIMAL PRACTICE QUESTIONS AND LEARN MYSELF

Characters

A character set =

• A defined list of characters recognised by the computer hardware and software, with each character being represented by a single number

• Important that every computing device uses the same binary number to represent each character

• Each character is logically ordered

  • E.g - character code for “B” will be one more than the character code for “A”

• A character can be:

  • 26 Uppercase (A-Z)

  • 26 Lowercase (a-z)

  • Numbers (0-9)

  • Punctuation codes @;/?~!$* etc…

  • Whitespace tabs, line breaks etc…

• Two standard character sets in common use are:

  • ASCII (American Standard Code for Information Interchange)

    • ASCII uses 7 bits per character - giving a character set of 128 characters (2^7 = 128)

    • Extended ASCII uses 8 bits per character - giving a character set of 256 characters (2^7 = 256)

      • Enough to only fit the english language

    • BUT for the exam, ASCII will use 8 bits!!!

  • Unicode

    • Unicode uses 16 bits per character - giving a character set of 65,536 characters (2^16 = 65,536)

    • Enough for every language in the world

Images

Types of digital images:

• Vector graphics - 

  • Stored as a set of mathematical instructions - how to draw each shape

    • Uses simple geometric objects such as points, lines, curves, and shapes and polygons…

    • Uses mathematical expressions

  • Usually has smaller file sizes than bitmap images

  • Can be scaled infinitely without any loss in quality - 

    • Every line and shape has a value that changes when the image expands

    • When a vector image is enlarges - entire image is redrawn

      • no pixelation occurs… smooth rescaling

• Bitmap images - 

  • Rely on a series of square blocks called pixels, arranged on a grid

  • Quality of images depends of the amount of pixels per square inch

  

  • Pixels =

    • A picture element

    • Smallest unit of data that can be represented in an image

    • A single, solid block of colour

    • More pixels = more detailed images

    • Pixelated = a term that is used when the pixels in an image are visible

    • Resolution = how many pixels are displayed in a specific area of an image 

      • Expressed as dots per inch (dpt) or pixels per inch (ppi)

      • More pixels = a more detailed image/ better image quality

      • BUT more resolution = a bigger file size

        • more pixels means more data to store

    • Colour depth = number of bits per pixel:

      • Formula for number of colours in a pixel = 2ⁿ

        • 1 bit: 2¹ = 2 colours

        • 2 bits: 2² = 4 colours

        • 3 bits: 2³ = 8 colours

      • Greater colour depth -

        • more realistic colours and a better quality image

        • more bits need to be stored and the file size gets larger

  • When bitmap image is enlarged - pixelation starts to occur

Sound

Sound waves are analogue (continuously changing) but digital signals have a distinct value separated by time - 

• Computers only understand binary

• So the continuously variable sound must be converted into discrete binary values

• So you measure the amplitude (height) of the wave at regular intervals (sampling)

• Then you store each sample as a binary number

What affects the quality of digitally converted sound waves?

• Sample rate - how often the amplitude of a sound is recorded

  • The more often a sample is recorded - a better quality of playback

  • BUT more often means more data so a bigger file

  • Number of samples taken per second

  • Measured in hertz (Hz)

• Bit depth - number of bits used to record each sample

  • The greater the bit depth - a better quality of playback

  • BUT more bits to store = a bigger a file

1.2.5 Compression

Compression = the process of encoding data so that it needs fewer bits to represent it and reduces the size of files:

• Why do we use compression?

  • Takes up less space on storage devices

  • Quicker to transfer or stream over the internet

• Compressed data must be decompressed to be used

Lossless compression:

• Methods of lossless compression:

  • Dictionary coding - for text based documents

    • The algorithm scans the original data

    • It identifies repeated sequences

    • It stores these sequences in a dictionary

    • Each time a sequence appears, instead of writing the full data, it writes a short code that points to the dictionary entry

  • Run-length encoding - for images images

    • Looks at the data in a file for consecutive runs of the same data

    • These runs are stored as one item of data

    • 00000011111111000000 (20 characters) becomes:

      608160 (6 characters) -

      • (number of repetitions)(value repeated)

  • Both lossless compression methods allow us to recreate the file in its original quality

Lossy compression:

• Lossy compression compresses data files but does lose come of the information

• Used is some loss of accuracy is acceptable (not used on text files as it could remove or change characters)

• Produces smaller data files compared to lossless compression

• Image file can be compressed by reducing the colour depth

• Sound file can be compressed reducing the bit depth