1.2 Memory and storage

https://ibaguette.com/cheatsheets/gcse/ComputerScience https://www.csnewbs.com/ocr-gcse copied all from this + a bit of smart revise terms work smarter not harder

Primary storage or 'main memory'

• Low-capacity internal storage

• Includes RAM and ROM, registers and cache

• holds data and instructions CPU needs to access while computer is running

• CPU accesses data faster here than from secondary

RAM - Random Access Memory

• Temporary store of currently executing instructions and their data = volatile

• read and write

• stores parts of OS to be accessed by the CPU

• stores all currently running data and instructions

• slower than cache, much faster than secondary storage as it doesn’t have moving parts or have to retain that info

• more RAM = more programs and instructions computer can store simultaneously

ROM - Read Only Memory

• Non-volatile - stores long-term even if powered off

• read only - Instructions and data cannot be changed

• stores the first instruction, boot program/basic input/output system for when computer is switched on. BIOS then loads up OS to take over managing the computer

• smaller than RAM

Virtual memory

• Programs must be stored in RAM to be processed by CPU

• held on the hard disk

• insufficient space in RAM = hard disk drive (HDD) is an extension of RAM (virtual memory)

• new data to be stored in RAM = unused data in RAM is moved to hard drive so new data can be transferred into RAM

• data can be moved back from VM into RAM

• VM allows users to run more programs simultaneously with less system slow down

• VM is much slower than even the fastest SSDs, due to data constantly moving between it and the RAM to keep programs running

Secondary storage

• needed as ROM is read only, RAM is volatile

• for storing programs and data when power off, semi-permanent storage of data that can change, back up and archive of data files

• Slower as not directly accessed by CPU

• larger in storage size, non-volatile

• magnetic, optical and solid state storage

• not embedded on motherboard = further from CPU so slower to access then primary

Magnetic storage

• Storage medium - surfaces coated with magnetic material, flipping magnetic polarity to store bits (N/S is 1/0)

• hard disk drive

• not used - floppy disk, replaced by solid state devices; USB sticks, faster, higher storage capacity

• tape - data is accessed in order so only companies use to back up/archive large amounts of data

Magnetic Storage Characteristics

• high storage capacity and cheaper cost per GB than solid state

• Not DURABLE and not very PORTABLE when powered on because moving it can damage the device

• moving parts which eventually fail = not reliable

• Hard disks perform better if defragmented

• quick access to data

Optical storage

• Storage medium - laser scans tracks, and when light reflects, from ‘lands’ - 1, or not reflect in ‘pits’ - 0, allowing it to read data from CD, DVD or Blu-Ray

• slowest access to data

• Disc drives are traditionally internal but external disc drives can be bought for laptops

Optical Storage Characteristics

• Low storage capacity

• Not durable, discs are fragile/break or scratch easily

• Discs are thin, light and portable.

• very cheap to buy in bulk

• slowest access speed

Solid state

• no moving parts

• trapping electrons to store bits

• SSDs (Solid State Drives) are replacing magnetic HDDs in computers and video game consoles

• USB flash drive - used to transport files easily because of its small size

• Memory cards - SD card in digital camera or Micro SD card in smartphone

Solid State Characteristics

• no moving parts = reliable, fastest access speed

• medium storage capacity

• durable but cheap USB sticks can snap or break

• small size of USB sticks/memory cards = portable in bag or pocket

• no need to defragment

• most expensive

• quiet, low power

• limited number of red/write cycles

Why do computers use binary?

• PC’s consist of billions of tiny transistors - switches with two values - on (1) or off (0)

• 2 states = electrical components are easier to manufacture, cheaper, more reliable

• so all data must be represented/ processed this way

• Binary is a base 2 number system consisting of 0s and 1s

Units of data storage

• bit = 0/1

• nibble = 4 bits

• 8 bits = byte (stores 1 character, e.g “A”)

• kilobyte = 1000 bytes

• megabyte = 1000 kilobytes

• gigabyte = 1000 megabytes

• terabyte = 1000 gigabytes

• petabyte = 1000 terabytes

• kill my grandma tomorrow please

Denary

• Denary is a base 10 number system

• This means it has 10 possible values - 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9

Denary to Binary

• 128, 64, 32, 16, 8, 4, 2, 1

• subtract denary number from the star of this list

• is it can be subtracted without being negative = write 1

• continue till you reach 0

Adding binary digits

• 0 + 0 = 0

• 0 + 1 = 1

• 1 + 1 = 0, carry 1

• 1 + 1 + 1 = 1, carry 1

Hex

• Hexadecimal is a base 16 system

• hex digit: 0 1 2 3 4 5 6 7 8 9 A B C D E F

• Each hex digit represents a 4-digit binary sequence

• shorthand for binary - fewer characters to write same value

• so less prone to errors when reading or writing it, compared to binary

Denary to hex or hex to denary

1. Convert 103 into binary = 01100111

2. Split into nibbles = 0110 0111

3. Work out hex of nibble 1 = 6

4. Work out hex of nibble 2 = 7

5. Put together = 67

   OR

6. divide denary number by 16 until you can’t (here, 6 times). Then get the remainder (here, 7). Then put it together

7. do the opposite for hex to denary = turn into binary and then denary

Size equation of sound, image, text

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

• Image file size - size of image on disk in bytes = colour depth x image height (px) x image width (px)

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

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Overflow error

• occurs when binary value is too large to be stored in bits available

• With a byte the largest number that can be held is 255

• Therefore any sum of two binary numbers greater than 255 = overflow error as it is too large to be held in 8 bits

Binary shifts

• A shift of 1 to the left means multiply by 2.

• A shift of 3 to the left means multiply by 2^3 = 8

• A shift of 1 to the right means divide by 2

• A shift of 3 to the right means divide by 2^3 = 8

• left = multiply, right = divide

• 11001100 to the left by two positions = 00110000

• 11 shifted from the start is lost, and 00 replace them at the end

• no matter what side a 1/0 is shifted off, it’s replaced by a 0 on the opposite side

What is a Character Set?

• list of character recognised by a computer/table that matches together character and binary value

• Each character is represented by a unique binary number

• Character sets are necessary as they allow computers to exchange data and humans to input characters

• logically ordered - ABC, code increases by 1 with each letter

• A in binary is 0100 0001, B in binary is 0100 0010.

ASCII (extended but just call it ASCII)

• common character set which doesn’t take up much memory space

• number of characters that can be stored is limited by bits available

• ASCII uses 1 byte (8 bits) = maximum number of characters is 2^8 = 256, the highest possible character code is 255 (0-255)

• enough for English not other languages or all punctuation symbols

Unicode

• more popular character set because it uses 2 bytes (16 bits) = 65,536 possible characters

• can be also 24 bits (16mill charac) or up to 32 bits

• many different languages represented, thousands of symbols and emojis

• requires more memory to store each character than ASCII as it uses extra byte

Bitmap images

• made of pixels, single-colour squares arranged on a grid

• Each is assigned a binary value which represents colour of that pixel

• quality of a bitmap image depends on total amount of pixels = image resolution (height in px * width in px)

• scaling bitmap image up = visible loss of quality

• Most images are bitmaps, like photos and screenshots.

• File Size = Resolution x Colour Depth

• black&white image - bit depth 1 - only represents, 2¹ = 2 colours

• photos are 2^ 24 = 16mil colours

• Colour/bit depth = number of bits for each px

Vector images

• drawn by the computer following precise mathematical instructions to create lines and objects

• usually smaller in file size compared to bitmaps because each pixel in a bitmap is stored as an individual binary value

• can be scaled up without loss of quality and are typically used for logos and animations

Metadata

• additional data about a file

• e.g: Height/Width in pixels, Colour depth, Resolution, Geolocation, Date created, Last edited, File type, Author details

• important - dimensions must be known so image can be displayed correctly

Converting Analogue sound to binary

• Analogue sound waves must be digitally recorded and stored in binary

• To record the sound, amplitude (height) of analogue sound wave is measured and recorded in binary at specific intervals

Sampling an Analogue Sound Wave

• Digital sampling is discrete (separate) and not continuous like analogue waves

• To get highest quality sound, many samples are taken to recreate analogue wave as closely as possible

Sample rate

• number of times per second amplitude of the sound wave is measured - number of samples recorded in a second

• measured kilohertz (kHz), e.g CD quality is 44.1 kHz

• higher sample rate, better audio quality as the digital data more closely resembles an analogue wave

• but, higher sample rates = larger file sizes because more data is stored for each individual sample

Bit Depth

• number of bits available to represent each sample

• sample with bit depth 4 could be 0101 or 0111

• common bit depth is 16 bits

• higher bit depth = more bits available for each sample = quality higher as wave more closely resembles analogue wave

• file size larger if bit depth higher, as each sample stores additional bits

Bit rate

• amount of data processed per second

• sample rate x bit depth = in bits per second

• Higher bit rate = higher quality sound, higher file size

sound file size equation

sound file size = sample rate x bit depth x duration

• shows the total number of bits in a sound

record an audio file/sound steps

1. input device e.g. microphone picks up sound, converts into electrical signal

2. signal used by ADC (analogue-digital converter) and sampled at regular intervals - directly represent sound wave’s traits, like amplitude.

3. represented as binary sequence and saved to file. raw data of converted signal is uncompressed

What is compression?

• To compress a file means to make its size smaller, less bits, lower storage capacity

• Benefits:

• Files take up less storage space (more files can be stored on a storage device)

• Files can be transferred quicker (as they are smaller)

• Files can be read from or written to quicker

Lossy compression

• permanently removing data from file - limits number of bits the file needs = reduces size and quality = sometimes seen by humans but may be unnoticeable

• commonly used - many software can read it

• less bandwidth so downloaded and transmitted faster

• great reduction in file size = more files can be stored on storage device

• can’t be used on text files as they become corrupted, or executable files

• good for JPEG (image), AAC and MP3 (audio)

Lossless compression

• Files need to be reduced in size temporarily - without loss of data, just encoded differently

• bigger than lossy, even when compressed

• can be put back to original (decompressed)

• can be used on all types of data

• best for texts and executable files

• FLAC (audio), TIFF, PNG (image)