memory and storage

Why do computers need primary storage?

So that data and instructions, which are copied from secondary storage into RAM, can be processed by the CPU.

What are the two main types of primary storage?

RAM and ROM

What is the purpose of RAM?

To store all the programs on the computer that are currently running.

What is the purpose of ROM?

To store the BIOS (boot program) which loads the operating system when the computer is switched on.

What are the key differences between RAM and ROM?

RAM is volatile - all the data stored in it is lost when the computer is switched off - whereas ROM is non-volatile.

RAM can be read from and written to but ROM can only be read from.

What is virtual memory?

A section of secondary storage used as an extension of RAM.

How does virtual memory work?

Unused data (data that hasn’t been used recently) is moved to a section of secondary storage (virtual memory), so that new data needed by the CPU can be transferred into RAM.

When the data in virtual memory is needed by the CPU again, it is transferred back into RAM.

Why do computers need secondary storage?

So that data can be stored even when the computer is powered off as it is non-volatile.

What are the three main types of secondary storage?

• Optical

• Magnetic

• Solid state

What is optical storage?

A type of secondary storage device that uses lasers to read and write data.

What are the common examples of optical storage?

CD, DVD and Blu-ray.

What are the advantages of optical storage?

• Very portable

• Lowest cost

What are the disadvantages of optical storage?

• Lowest capacity (particularly CDs + DVDs)

• Not durable as discs are easily scratched/broken

• Slowest access speed

What is magnetic storage?

A type of secondary storage that uses magnetic materials to store data.

What is the most common example of magnetic storage?

HDD (Hard Disk Drive).

What are the advantages of magnetic storage (HDD)?

• Highest capacity

• Low cost per GB (in comparison to solid state)

• Portable if external

• Relatively durable as long as not dropped/knocked when powered on due to the moving parts

What is the key disadvantage of magnetic storage (HDD)?

It has a slow access speed which can be attributed to its moving parts.

What is solid state storage?

A type of secondary storage device that uses electronic circuits to store data.

What are some common examples of solid state storage devices?

• SSD (Solid State Drive)

• USB flash drive

• Memory cards - e.g. SD card

What are the advantages of solid state storage?

• Very portable (external SSDs, USBs + memory cards)

• High capacity

• Fastest access speeds (as they don’t have any moving parts)

• Very durable

• Most reliable

What is the key disadvantage of solid state storage?

It is the most expensive per GB (in comparison to magnetic/optical).

Why does data need to be stored as binary on a computer?

Computers are made of transistors that have two states - on or off. Likewise, binary has only two values - 1 or 0 - so data can be fully represented by the state of the switches.

What are the main data storage units in order of increasing size?

• Bit

• Nibble (4 bits)

• Byte (8 bits)

• Kilobyte (1,000 bytes or 1 KB)

• Megabyte (1,000 KB)

• Gigabyte (1,000 MB)

• Terabyte (1,000 GB)

• Petabyte (1,000 TB)

What is the equation used to calculate the size of a sound file?

sample rate x duration (s) x bit depth

What is the equation used to calculate the size of a image file?

height (px) x width (px) x colour depth

What is the equation used to calculate the size of a text file?

number of characters x bits per character

What are the three number systems used in computer science?

• Binary (base-2) → 2 values, 0 or 1

• Denary (base-10) → 10 values, 0-9

• Hexadecimal (base-16) → 16 values, 0-9 and A-F

How can a denary number be converted to binary?

1. Divide the number by 2 until the quotient is 0.

2. For each division, record the remainder (which will be 0 or 1)

3. The number’s binary equivalent is the remainders left from the divisions, from the last division to the first.

EX: 178 into binary.

178/2 = 89 remainder 0

89/2 = 44 remainder 1

44/2 = 22 remainder 0

22/2 = 11 remainder 0

11/2 = 5 remainder 1

5/2 = 2 remainder 1

2/2 = 1 remainder 0

1/2 = 0 remainder 1

10110010

How can a binary number be converted into denary?

1. Multiply each bit by its place value.

2. Add these values together.

EX: 11110111 into denary.

128 × 1 = 128

64 × 1 = 64

32 × 1 = 32

16 × 1 = 16

8 × 0 = 0

4 × 1 = 4

2 × 1 = 2

1 × 1 = 1

128 + 64 + 32 + 16 + 4 + 2 + 1 = 247

How can a denary number be converted into hexadecimal?

1. Convert the denary number into binary.

2. Split the binary number into 2 nibbles, each containing 4 bits.

3. Convert each nibble back to denary.

4. Convert the denary number of each nibble into its corresponding hexadecimal digit.

EX: 240 into hexadecimal.

240/2 = 120 120/2 = 60 60/2 = 30 30/2 = 15

15/2 = 7 remainder 1 7/2 = 3 remainder 1

3/2 = 1 remainder 1 1/2 = 0 remainder 1

11110000

1111 → 15

0000 → 0

F0

How can a hexadecimal number be converted into denary?

1. Convert each digit into denary.

2. Convert the two denary values into 4-bit binary numbers.

3. Concatenate the two nibbles.

4. Convert the 8-bit binary number into denary.

EX: AB to denary.

A → 10

B → 11

10 → 1010

11 → 1011

10101011

128 + 32 + 8 + 2 + 1 = 171

What are the rules for binary addition?

0 + 0 = 0

0 + 1 = 1

1 + 0 = 1

1 + 1 = 0 (carry 1)

1 + 1 + 1 = 1 (carry 1)

What is an overflow error?

An overflow error is an error that occurs when the result of a binary calculation is too large to be stored using the bits available, resulting in the leftmost or most significant bit being lost.

How may an overflow error occur in binary addition?

If the result of the addition requires 9 bits, but there are only 8 available, the leftmost bit is lost.

What is the effect of shifting a binary number to the left?

It multiplies the binary number by 2 for each place that is shifted.

What is the effect of shifting a binary number to the right?

It divides the binary number by 2 for each place that is shifted.

How is a left shift carried out on a binary number?

Shift the digits of the binary number to the left for each place stated, adding zeros to the right for each place shifted.

How can a left shift cause an overflow error?

If there are not enough bits to store the result, the leftmost bits will be lost to the shift causing an overflow error.

How is a right shift carried out on a binary number?

Shift the digits of the binary number to the right for each place stated, adding zeros the the left for each place shifted.

How are characters represented in binary?

Each character is represented by a specific binary number, assigned by a character set.

How are character sets ordered?

Logically - the binary code for the letter ‘B’ will be one more than the binary code for the letter ‘A’.

The number of bits used to represent each character (in a character set) limit what?

The number of characters that can be represented.

What is ASCII?

American Standard Code for Information Interchange.

It is a character set that uses 8 bits per character, so can represent 256 (2⁸) different characters in total.

What is the key advantage of ASCII?

It takes up less memory than unicode (as it uses less bits).

What is the key disadvantage of ASCII?

It can only represent English characters, numbers and common symbols.

What is unicode?

A character set that uses 16 bits per character, so can represent 65,536 (2¹⁶) different characters in total.

What is the key advantage of unicode?

It can represent characters from multiple languages and many symbols.

What is the key disadvantage of unicode?

It takes up more memory than ASCII (as it uses more bits).

How is an image represented by binary?

An image is made up of a series of pixels, which each have a specific colour that is represented by a unique binary code.

What is colour depth?

The number of bits used to represent the colour of each pixel.

What is resolution?

The total number of pixels in an image (height x width).

How do colour depth and resolution affect image size and quality?

Increasing the colour depth and resolution of an image will increase the file size and quality of the image.

What is metadata?

Data about data.

EX: file type.

How is sound sampled?

The amplitude of the sound wave is measured at regular time intervals - each measurement is called a sample and is converted to a binary number.

Why does sound need to be sampled?

Analogue sounds must be stored in binary.

What is sample rate?

The number of times per second the sound wave is sampled.

It is measured in Hertz (Hz).

What is bit depth?

The number of bits available to store each sample.

How do sample rate and bit depth affect the playback quality?

Increasing the bit depth and sample rate increases the playback quality of the sound file.

This is because both enable the digital data used to store the sound to more closely resemble the sound wave.

How do sample rate, bit depth and duration affect the size of a sound file?

Increasing the sample rate, bit depth and duration increases the size of the sound file.

This is because more data is stored for each individual sample and there are more samples.

Why may compression be needed?

• To decrease the amount of memory a file takes up

• To increase the speed at which files are transferred

• To increase the speed at which files are read from and written to

What are the two types of compression?

• Lossy

• Lossless

What are the effects of lossy compression on a file?

It permanently removes data from the file to reduce its size.

What are the effects of lossless compression on a file?

It doesn’t permanently remove data from the file to reduce its size.

What are the advantages of lossy compression?

• It reduces file size significantly unlike lossless compression

• It can be used with image, video and sound files

What are the disadvantages of lossy compression?

• It removes data permanently so the file cannot be restored to its original form

• It isn’t suitable for text files

What are the advantages of lossless compression?

• It can be restored to its original form

• It’s suitable for text and executable files

What is the key disadvantage of lossless compression?

Files compressed using lossless compression are larger than those compressed using lossy compression.