1.2 Memory and Storage Flashcards
OCR GCSE Computer Science J277
Main Memory
where data and instructions are stored so they can be accessed directly by the CPU
Secondary Storage
used to permanently store data such as the operating system and user’s files
RAM (Random Access Memory)
holds all of the data and instructions that are currently being processed.
Memory Addresses
Inside the RAM, memory is split up into separate locations, each with a unique memory address. The processor uses these addresses to access data stored in RAM.
Volatile
Power is required to hold the data.
Read Only Memory (ROM)
typically used to store the boot sequence (BIOS) for a computer.
Non-volatile
will not lose its data when the power is switched off.
Basic Input/Output System (BIOS)
stored on ROM. It is a limited sequence of instructions which checks that the core components of the computer system (RAM, fundamental input/output devices, secondary storage) are connected and responding correctly.
Pages
When a system requires virtual memory, the operating system creates a set of virtual addresses. (RAM is separated into a set of physical addresses.) Data moved to virtual memory is stored as pages.
Virtual memory
used when there simply isn’t enough space in RAM for all the programs and data you are currently working on. Computers allocate a portion of secondary storage (HDD, SSD etc.) as virtual memory.
Optical
Secondary storage. Most common are CDs, DVDs, Blu-ray discs.
ROM Disc
Read-only and were primarily used to distribute data such as movies, music and software. However, high-speed internet connections have caused a steep decline.
R discs
Recordable. They can be written to one and then only read from that point on. Typically used to archive data.
RW discs
Rewritable. These discs can be written and read many times. Usually used to back-up files for later retrieval or rewriting.
Magnetic
Magnetic disk drives are often referred to as HDDs (Hard Disk Drives) or just Hard Drives.
HDD
Hard Disk Drive, magnetic storage with spinning platters that magnetise.
Flash memory
Type of solid state. Some microcontrollers use a memory chip to store instructions, called flash memory. Flash memory is non-volatile and can be written to and read from.
Solid State
Secondary storage that uses flash memory to save data electronically. No moving parts.
Types of Secondary storage
Optical, Magnetic and Solid State.
mean time to failure (MTTF)
Any storage mechanism can fail, and data can be corrupted. Disk reliability is usually measured in terms of the mean time to failure (MTTF).
binary
Base 2 number system. Two states, 0 = off, 1 = on
Bit
Binary Digit, a single 1 or 0
Nibble
4 bits (0000)
Byte
8 bits (000000000)
Kilobyte
1024 (or 1000) bytes
Megabyte
1024 (or 1000) kilobytes
Gigabyte
1024 (or 1000) megabytes
Terabyte
1024 (or 1000) gigabytes
Petabyte
1024 (or 1000) terabytes
Denary
Base 10 or decimal number system. Uses digits 0 to 9
Overflow
Occurs when you add 2 binary numbers together and the answer is bigger than the number of bits allowed.
Binary shift
Multiplying or dividing a binary number by 2.
Left Shift
A shift of one place to the left multiplies the binary number by 2
Right shift
A shift of one place to the right divides the binary number by 2
Precision Error
When odd numbers are divided with a right shift, the standard byte cannot represent fractional numbers, only integer (whole) numbers, so precision is lost.
Hexadecimal
Base-16. Uses 0 to 9 then A, B, C, D, E, F. Easier to read and interpret than binary, fewer digits (1 hex digit to 4 binary).
Colour Code
Hexadecimal code to represent colours. The first two digits are for red, third and fourth for green and last two for blue.
MAC Address
Media Access Control address, identifies a network interface controller (NIC). Written in hexadecimal like B2:27:EB
Memory Dumps
Appears on screen after a crash. Uses hexadecimal to reduce the length of the numbers on screen.
Bit Pattern
combinations of 1s and 0s used to represent data inside of a computer.
Character code
The bit-pattern used for each character becomes a numeric character code. Each character on the keyboard has a binary code which is transmitted to the computer each time a key is pressed.
Character
A character can be any of the following:
Letters (upper and lowercase letters have separate codes)
Punctuation e.g. ?/|\£$
Numbers 0-9
Non-printing commands (e.g. Enter, Delete, F1)
Character Set
A standardised collection of characters and the bit-patterns used to represent them is called a character set. 2 examples are ASCII and Unicode.
ASCII
Character Set consisting of 128 characters, each using 7 bits to uniquely represent them. American Standard Code for Information Interchange.
Extended ASCII
Character set consisting of 256 characters, each using 8 bits to uniquely represent them. Builds on standard ASCII. American Standard Code for Information Interchange.
Unicode
Character set consisting of 65,536 different characters, each using 16 bits to uniquely represent them. The first 128 match ASCII. Most widespread character set due to the world wide web.
UTF-8
The most common Unicode format is 8-bit. Characters can use as few as 8 bits, maximising compatibility with ASCII. However, UTF-8 also allows for variable-width encoding, expanding to 16, 24, 32, 40 or 48 bits when dealing with larger sets of characters.
UTF-16
Like UTF-8, 16-bit allows variable-width encoding, and can expand to 22 bits.
UTF-32
With 32-bit, each character uses exactly 32 bits; this is an example of fixed-width encoding.
Pixel
represents the smallest identifiable area of an image, each appearing as a square with a single colour.
Bitmap Image
Similar to a mosaic, a bitmap image (or bitmap graphic) is made up of picture elements or pixels
Colour Depth
The number of binary digits (bits) used to represent the colour of a pixel. Also called bit depth. 2 colours is 1 bit per pixel. 4 colours is 2 bits per pixel etc. The greater bit depth the better quality the image and the more colours can be represented.
Resolution
The size of the image, the height and width.
Metadata
Data about data. For instance, where a photo was taken, author name, colour depth, file format, date and time.
Analogue sound
Sound made by objects in real life, like voices.
Digital sound
Analogue sounds must be digitally recorded in binary. In order to record sound, the amplitude or height of the soundwave emitted must be measured and recorded at regular intervals.
Amplitude
Height of a wave.
Wave
Repeated pattern
Cycle
One completed repetition of a wave. Measured between two consecutive points, such as troughs or crests of the wave.
Time period
Time needed to complete a cycle, which is one complete repetition of a wave.
Frequency
The number of completed cycles per second of a sound wave. The higher it is, the higher the pitch of the sound.
1 Hertz (1Hz)
1 Hz = 1 cycle per second. And 1 sample per second in the sample rate.
Sample
The most common technique used to convert a sound from analogue to digital is sampling. When sampling a sound, the computer takes measurements of the analogue signal at regular time intervals. A unique bit pattern of binary numbers is assigned to each sample. The digitised sound can be stored and processed by a computer as a sequence of 1s and 0s.
Sample rate
How many samples taken per second of a sound wave. The more samples (higher the sample rate) the better the sound will resemble the analogue sound. Calculated by the number of samples multiplied by 1. So 8,000 samples a second is 8,000 Hertz.
Bit Depth / Sample Resolution
The sample resolution, also known as the bit depth, is the number of bits used to represent each sample. By increasing the sample resolution, more information on the original analogue wave will be taken. The higher the sample resolution, the more accurate (fidelity) a sound will be recorded.
Fidelity
Accuracy of sound. The higher the sample resolution, the more accurate (fidelity) a sound will be recorded.
Channel
However, if a sound is monophonic (mono) or stereophonic (stereo) that will also influence the audio quality and file size. A mono sound has one recording channel (also known as the level or track) A stereo sound contains at least two different recording channels to add an impression of position and direction to the sound recording.
Mono
One channel
Stereo
Two channels
Compression
Compression software uses algorithms to remove repeated or unnecessary data. This reduces the size of a file on disk, and of large files sent by email where there are maximum attachment limits.
Lossy Compression
JPG, MP3, WMV, MPG
Smallest file sizes
Least transmission time
Reduces Internet traffic and collisions
Detail is permanently lost
Music streaming
Online images and video
Image libraries on devices or in the cloud
Lossless Compression
TIF, PDF, GIF, PNG, MOV, ZIP
Original quality is preserved
No information or data is lost
Less significant reduction in file size
Text documents
Electronic books
High resolution print documents
