1.2 Memory and storage (OCR GCSE J277)

Primary storage

"At GCSE level you can think of primary storage comprising of Random Access Memory (RAM) and Read Only Memory (ROM). It holds data and instructions which the CPU can much more easily and quickly access than from secondary storage devices."

RAM

Random Access Memory: "Volatile (data lost when power is off) Read and write. Purpose: temporary store of currently executing instructions and their data. E.g. applications and the operating system in use."

ROM

Read Only Memory: "Non-volatile (data retained when power is off) Read only. Purpose: stores instructions for starting the computer called the bootstrap."

Virtual memory

"Using part of the hard disk as if it were random access memory. Allows more applications to be open than physical memory could hold."

Secondary storage

"Permanent storage of instructions and data not in use by the processor. Stores the operating system, applications and data not in use. Read/write and non-volatile."

Optical storage

"CD/R, CD/RW, DVD/R, DVD/RW Use: music, films and archive files. Low capacity. Slow access speed. High portability. Prone to scratches. Low cost."

Magnetic storage

"Hard disk drive. Use: operating system and applications. High capacity. Medium data access speed. Low portability (except for portable drives). Reliable but not durable. Medium cost."

Solid state storage

"Memory cards & solid state hard drive (SSD). Use: digital cameras and smartphones. Medium capacity. High portability. Reliable and durable. No moving parts. Fast data access speed. High cost."

Storage capacity

"The amount of data a storage device is able to store. "

Storage speed

"The read/write access speed of a storage device."

Storage portability

"How easy it is to transport a given storage medium. E.g. Solid state and optical storage and designed to be highly portable, whereas more traditional magnetic storage is designed to stay in place."

Storage durability

"How resistant to damage and wear a tear a storage device is. Devices with low durability will wear out easily over time."

Storage reliability

"A relative measure of how confidant you can be that a given storage device will correctly allow you to write, read, delete and modify data."

Storage cost

"The relative price of a storage device e.g. per Megabyte of data"

Bit

"The smallest unit of storage in a computer system, represented by either a binary 1 or 0."

Nibble

"Half a byte / 4 bits."

Byte

"A collection of eight bits."

Kilobyte

"1 Kilobyte (KB) is 1024 Bytes. For the purpose of calculations in an exam you can assume 1000."

Megabyte

"1 Megabyte (MB) is 1024 Kilobytes (KB). For the purpose of calculations in an exam you can assume 1000."

Gigabyte

"1 Gigabyte (GB) is 1024 Megabytes (MB). For the purpose of calculations in an exam you can assume 1000."

Terabyte

"1 Terabyte (TB) is 1024 Gigabytes (GB). For the purpose of calculations in an exam you can assume 1000."

Petabyte

"1 Petabyte (PB) is 1024 Terabytes (TB). For the purpose of calculations in an exam you can assume 1000."

Denary numbers

"A numerical system of notation which uses 10 as its base. The 10 Decimal base digits are 0-9."

Binary numbers

"Binary describes a numbering scheme in which there are only two possible values for each digit: 0 and 1. The term in computing refers to any digital encoding system in which there are exactly two possible states. E.g. in memory, storage, processing and communications, the 0 and 1 values are sometimes called "low" and "high", respectively."

Binary arithmetic

"The process of adding together two of more positive 8-bit binary numbers (0-255)."

Overflow

"The generation of a number that is too large to be represented in the device meant to store it."

Hexadecimal

"A numerical system of notation which uses 16 rather than 10 as its base. The 16 Hex base digits are 0-9 and the letters A-F."

Binary shifts

"Allows you to easily multiple and divide base-2 binary numbers. A left shift multiplies by 2 and a right shift divides by 2.

Character set

"The set of symbols that may be represented in a computer at a particular time. These symbols, called characters, can be letters, digits, spaces or punctuations marks, the set includes control characters."

ASCII

"America Standard Code for Information Interchange: "A character set devised for early telecommunication systems but proved to be ideal for computer systems. ASCII codes use 7-bits giving 32 control codes and 96 displayable characters (the 8th bit is often used for error checking)."

Unicode

"Standard character set that replaces the need for all the different character sets. It incorporates characters from almost all the world's languages. It is a 16-bit extension of ASCII."

Pixels

"A pixel is the smallest unit of a digital image or graphic that can be displayed and represented on a digital display device. A pixel is represented by a dot or square on a computer monitor display screen."

Metadata

"A set of data that describes and gives information about other data."

Colour depth

"Also known as bit depth, is either the number of bits used to indicate the colour of a single pixel, in a bitmapped image or video frame buffer, or the number of bits used for each colour component of a single pixel."

Resolution

"The number of pixels (individual points of colour) contained on a display monitor, expressed in terms of the number of pixels on the horizontal axis and the number on the vertical axis."

Image quality

"The overall detail of an image, this is affected by the colour depth and resolution."

Image file size

"The file size of an image is increased when either its resolution (width & height in pixels) or its colour depth (number of bits needed to store a single pixel) increases."

File size of an image = colour depth x image height (px) x image width (px)

Sample rate

"The number of samples taken per second, measured in Hertz (Hz)."

Sample duration

"How many seconds of audio the sound file contains."

Sample bit depth

"The number of bits available to store each sample e.g. 16-bit"

Playback quality

"The finished quality of the digital sound file. This is effected by the sample rate and bit-depth. The higher the number the better the quality. The higher the number the larger the file size. CD quality is 44,100 samples per second."

Sound file size

"The overall size of a sound file is found by the following formula: Sample rate x duration (s) x bit depth"

Compression

"The process of reducing the size of a file in terms of its storage size."

Lossy compression

"A compression scheme where their generally involves a loss of resolution in parts of the image where experiences shows that it will be least noticed."

Lossless compression

"A compression scheme that allows the original images to be recreated."