GCSE OCR Computer science (1.2 Memory & storage)

Primary storage (main memory)

Refers to all the storage locations in a computer that can be directly accessed by the CPU. Includes both RAM and ROM.

Differences between volatile and non-volatile storage

Volatile: Loses all the data it contained when powered off.
Non-volatile: Data is not lost when the computer is powered off.

Secondary storage

Non-volatile storage- Encompasses many different types of devices used to permanently store data and applications. Can be internal or external.

Examples of secondary storage

Magnetic (hard) disks, solid state disks, optical discs

Advantages and disadvantages of using primary storage (as opposed to using secondary storage)

A: Has much quicker read/write speeds than secondary.
D: Has a lower capacity.

Read only memory (ROM)

Non-volatile storage- stores the boot sequence (BIOS) of the computer. The contents of ROM are set by the manufacturer (thus making it read only.)

Compare the characteristics of RAM and ROM.

RAM: Volatile, read/write, has the operating system.
ROM: Non-volatile, read only, contains boot instructions.

Virtual memory

Extra memory that is used to compensate for a lack of RAM. (Secondary storage is temporarily used as extra primary memory to store data not being used by RAM.)
Moves back to RAM when required by the CPU.

Advantages & disadvantages of virtual memory?

A: Frees up space in an otherwise full RAM.
D: Slows down the computer, as the CPU is forced to wait for data to be swapped between the virtual memory and RAM.

Flash memory [Same thing as SSD in this GCSE]

Non-volatile memory. Works by forcing electrons through a barrier with a strong electric current. This traps the electrons. No moving parts.

Advantages & disadvantages of flash memory/SSD?

A: No moving parts (so durable), fast access time, requires little power, portable.
D: Small capacity due to expensiveness, only can do so many limited read-write cycles before redundancy.

Magnetic storage

Different patterns of magnets store data. Sections are magnetised or demagnetised.

Advantages & disadvantages of magnetic storage?

A: Quite cheap, usually has very high capacities, not very susceptible to internal damage.
D: Not portable, susceptible to physical damage (e.g. if dropped), strong magnetic fields could corrupt data.

Optical storage

Works by shining a light onto a disk. The light reflects off pits burnt into the surface of the disk. When written to, these pits are burned into the disk.

Advantages & disadvantages of optical storage?

A: Very portable (easy to move lots of disks at once), low cost, no effect on performance from a power failure.
D: Prone to scratches which ruin data held, low capacity so sometimes many disks are needed.

Give 6 factors which affect the choice of storage device used.

Cost, capacity, speed of access, portability, durability, reliability.

What is a single binary digit also known as?

A bit.

Binary shift

Left shift- multiplies by 2, all digits move 1 place value to the left.
Right shift- divides by 2, all digits move 1 place value to the right.
Very prone to overflow errors.

4 bits is equal to ??? bytes?

4 bits = 0.5 of a byte (a nibble).

8 bits is equal to ??? bytes?

8 bits = 1 byte

1 kilobyte is equal to ??? bytes?

1 kilobyte = 1000 bytes

Arrange the following units in order, from smallest to largest:
terabyte, bit, kilobyte, petabyte, byte, gigabyte, megabyte

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

Character set

A set of symbols (characters) that can be represented by a computer system. Stored as binary digits.

What are 2 common character sets?

ASCII and Unicode.

Overflow error

Happens when a carry occurs in the 8th column. For 8 bit binary, this 9th value carried over cannot be stored and so the bit is lost.

Explain how ASCII works.

ASCII uses 7 bits (which gives a total of 128 unique characters.) Each character is given a different code for the computer to read and interpret. (e.g. upper case letter Q has he code 81 [or 1010001 in binary.])

Explain how Unicode works.

Unicode has 3 different encoding standards (UTF-8/16/32), which allows a range of 65,000+ characters.

What are the advantages of using Unicode over ASCII?

Unicode has 65,000+ characters compared to ASCII's 128, so Unicode is more internationally compatible. (However, Unicode requires more storage space.)

Colour depth

The range of colours an image has. More colours means more bits per pixel.

Image size (calculation)

Image size = Image height x Image width (in pixels) x Colour depth

Resolution

The clarity of an image (or how tightly packed the pixels are in said image.) Measured in pixels/dots per inch (ppi or dpi.)

Metadata

Data about data. Includes the image dimensions, file format, resolution, colour depth, & location among other things. Helps to organise and understand data.

Bitmap image

A type of image format that uses pixels, where each pixel has a binary code that represents a certain colour.

Difference between analogue and digital data?

An. : (In sound) Data recorded naturally- not synthesised by a computer. A continuous steam (e.g. sound waves.)
Dig. : Data digitised- a combination of 1s and 0s.

Why must computers use sampling to represent analogue data?

Computers cannot read analogue data and instead approximate it by sampling, converting the sound into binary numbers. (Turned into digital data.)

Sample (sound)

The conversion of an analogue signal to a digital sample.
-The computer regularly takes samples to approximate the original analogue sound.
-A unique bit pattern is assigned to each sample so that the computer is left with a sequence of 1s and 0s.

Sample rate

The number of times a sound wave is sampled (per second.) Measured in hertz (Hz)

What is the effect of increasing the sample rate?

File size increases, but sound quality also increases. Will use more memory when the file is played.

Bit depth

The number of bits used to record every sample. Like colour depth- higher bit depth means a higher file size & higher sound quality.

Size of sound file (calculation)

Sample rate x Duration x Bit depth x Number of channels
*Remember to convert to standard units (e.g. s, Hz, etc.)

Compression

Methods to reduce file size so that the file can be transmitted (sent/moved etc.) more easily.

What are the benefits of compression?

-Reduction in file size
-Files faster to download/transmit/upload
-Less storage space used up
-Less (mobile) data is used to send the file.

Lossy compression

Some data is permanently removed and discarded to reduce the file size. When uncompressed, approximations are made to fill in removed data.

What file types are most suitable for lossy compression?

Images & sound (the approximations made aren't detected by a human eye and so are acceptable.)

Advantages & disadvantages of lossy compression?

A: File size is smaller.
D: Destructive process (as in some data is permanently lost), many rounds of lossy can make the output unrecognisable from the original.

Lossless compression

No data is lost when the file is compressed. Data can be reconstructed.

What file types are most suitable for lossless compression?

Text files, source code of a program etc. (These files require 100% accuracy as 1 incorrect approximated character could change the meaning or cause errors.)

Advantages & disadvantages of lossless compression?

A: Not destructive (reversible), image & sound quality are retained.
D: Does not decrease the file size as much as lossy.