1.2 Memory and storage Flashcards
Why do we need primary storage
For faster access speed. Reading and writing from a hard disk is is a very slow compared to the speed of the processor so we use RAM to act as a temporary storage for programs and data while the program is being executed by the CPU
Key characteristics of ROM
- non-volatile
- used to store the bootstrap loader which is required to start up the computer
- memory can only be read NOT written to
Key characteristics of RAM
- Volatile
- Stores the user data/programs that are currently in use
- memory can be both read and written to
Why is virtual memory needed
Sometimes when RAM isn’t big enough to hold all the open programs and data in use, virtual memory will be used. This is a small section of the hard disk that is used like RAM. However, in order for the programs or data to be used, they have to be switched back over into RAM which is time consuming and can cause disk thrashing.
Why is secondary storage needed
Because ROM is read only and RAM is volatile
- storage of programs and data when the computer is turned off
- backup and archive of data files
What are the 3 common types of storage
- Optical
- Magnetic
- Solid State
What is the portability of secondary storage
How easily it can be unplugged and carried away, whether it fits in a pencil case or needs a larger bag
What is the durability of secondary storage
how easily it can be damaged
What is the reliability of secondary storage
how long it will last
Advantages and disadvantages of Magnetic storage (6)
- very large capacity
- faster than optical but slower than SSD
- not very portable as they are either built into the computer or would fit into a very large pocket
- not durable due to moving parts
- very reliable as they can write to the disk an infinite number of times
- relatively cheap per unit of storage
Advantages and disadvantages of Solid State storage (6)
- medium capacity
- very fast speed
- very portable (light and thin)
- Very durable
- very reliable
- considerably more expensive than other types
Advantages and disadvantages of Optical storage (6)
- Small capacity
- Quite slow
- very portable
- scratches can affect data (durability)
- reliable over medium terms but degrades over time
- medium cost
Units of data storage in order
- bit
- nibble
- byte
- kilobyte
- megabyte
- gigabyte
- terabyte
- petabyte
why must data be stored in binary
Because the computer understands the 1 to be on or true and the 0 to be off or false
Equation to work out sound file size
sound file size = sample rate x bit depth x duration (secs)
Equation to work out image file size
image file size = colour depth x image height(px) x image width(px)
Equation to work out text file size
text file size = bits per character x number of characters
what is an overflow error
when two binary numbers are added and the number is valued as more than 255 so the 8 bit binary can no longer store it
How are characters represented on a computer
as binary
What is a character set
all the different characters a computer can represent, encoded using a given number of bits
Number of bits per character in ASCII and therefore maximum number of characters in the set
7
128
Number of bits per character in Unicode and therefore maximum number of characters in the set
16
65,000+
Advantages and disadvantages of Unicode
- wide range of characters available
- larger amount of storage needed per character
How are images represented by binary
Every image is a series of pixels and each pixel is a specific colour, represented by a specific code
what is the colour depth of an image
the amount of bits per colour which affects how many colours in total can be represented
eg a bit depth of two would allow for 4 colours
what is the resolution of an image
the number of pixels that make up an image
What is Metadata
basically is data about the data and provides information about the image
What types of information does Metadata include
- file type
- time and date of creation
- file size
- creator of the data
and also for a digital image:
- height and width
- resolution
- colour depth
what effect does increasing colour depth and resolution have on the file size
also increases
what is the sample rate and what is it measured in
Sampling is a method of converting an analogue sound signal into a set of binary numbers that are then stored in a digital file.
Measured in Hertz (sample per second)
How are analogue sounds stored
- You speak into the microphone which records the sound in analogue.
- It is then passed through an analogue to digital conversion device (ADC) in the sound card which converts it into the digital version.
- The amplitude is measured at these regular intervals
- the values are rounded to a level (sample resolution)
- This is stored in binary
Reasons for compressing a file
IT REDUCES THE NUMBER OF BITS SO THAT:
- less storage space needed so more can be stored on a storage device
- faster downloads and uploads
- smaller file attachments for email
what is compression
a way of reducing the size of a file while still keeping as much of the original data as possible
How does lossless compression compress the file
It reduces the file size without any loss of data by looking for patterns
eg. four purple pixels in a row could be stored as
4 purple
rather than
purple purple purple purple
What file types would lossless compression be used for
- text files
- spreadsheets
- emails
- executable programs
Are lossy and lossless compression permanent
- lossless = no
- lossy = yes
How does lossy compression compress the file
Reduces the file size by throwing away as much of the original data as it can without noticeably affecting the quality of the picture.
eg. decreasing the colour depth by averaging out all the different colours
What file types would lossy compression be used for
- images
- sound
- video
3 types of lossy formats
- JPEG
- MPEG
- MP3
2 types of lossless formats
- GIF
what is unicode
the new standard for representing the characters of all the languages in the world such as Chinese, Arabic etc
what is sample resolution
the number of bits used to store each sample
what is the storage capacity of a single CD
640MB
what is the storage capacity of a single layer DVD
4.7GB
