Memory and Storage

Primary storage consists of ram and rom

RAM (Random Access Memory)

• Main memory that holds the data for programs while they are running.

• RAM is volatile (power off = the data is lost)

• The CPU will fetch instructions from the RAM in the FDE cycle.

• When the RAM is full the computer uses VIRTUAL MEMORY. It uses the secondary storage as temporary RAM so that the computer can continue running (but slowly).

ROM (Read Only Memory)

• ROM contains the BIOS (boot up sequence for the computer)

• ROM is non-volatile (the data is still stored after the power is off)

Secondary Storage: long term storage

• Non-Volatile (the data still stored after the power is turned off)

• Types of storage: Magnetic, Solid State or Optical

Magnetic (Hard Disk Drive): moving parts, large capacity, lower cost

Solid State (Solid State Drive, USB memory stick, memory card): Flash memory, no moving parts, more robust, faster read/write speeds and more expensive than magnetic.

Optical(CD and DVD): Low cost, portable, not very robust

Denary: uses the number 0-9 and the column headings go up in powers of 10

Binary: uses the numbers 0 and 2. the column headings go up in power of 2

Hexadecimal: uses 0-F (a=10, b=11, c=12, d=13, e=14, f=15)

Characters

• Character sets : the characters that are recognised or represented by a computer science

• ASCII : each character is represented by an 8 bit binary number ( 256 possibilities )

• Unicode : each letter is represented by a 16 - bit or 32-bit binary number . this gives more character options than ASCII and allows the character set to represent charcters and symbols from all languages

Images

Images are made up of pixels. The colour of each pixel is represented by a binary number. If an image uses 1 bit to represent each colour then it will only have 2 colours: This is a 1-bit image 50 it uses 2 colours.

Colour depth = the number of bits used for each pixel

Resolution = how many pixels per inch (dpi). If there are more dots per inch then there are more pixels in the image so it will have a higher resolution and a better picture quality. The higher the resolution or the colour depth, the more bits used, so the bigger the file size.

Metadata = information about the image file that is stored within it. This makes sure the file is displayed correctly. It can include: the height, width, colour depth, resolution, time/date and file format.

Sound

When sound is recorded it is an analogue signal (waves). It has to be converted to a digital signal so that it can be stored by a computer. This is done by sampling.

Sampling: The amplitude (height) of the wave is measured at regular intervals which creates a digital representation of the wave. If samples are taken more frequently then you end up with a more accurate sound file but a larger file size.

Sample rate is measured in Hertz (Hz). A larger sample rate = a larger files size but higher quality.

Duration is how many seconds of audio the sound file contains. A longer duration = a larger file size.

Bit depth is the number of bits available to store each sample (e.g. 16-bit). Higher bit depth = a better quality but higher file size.

Compression

Compression is used to make file sizes smaller. Smaller file sizes means that data will be faster to send, quicker to download (so webpages will load faster) and it will take up less storage space.

Lossy Compression: permanently removes some of the data from a file to make the file size smaller. The file - eg: an image or sound track - will be a lower quality than the original.

Lossless Compression: data is temporarily removed from the file and then put back together when it is opened. This is good for program files or documents where you do not want to lose any content but the files can only be made a little bit smaller.