Units of Data (Edexcel GCSE Computer Science)
Revision Note
Written by: Robert Hampton
Reviewed by: James Woodhouse
Units of Data
What are units of data?
A unit of data is a term given to describe different amounts of binary digits stored on a digital device
These are the units you need to know for GCSE:
Unit | Symbol | Value |
|---|---|---|
Bit | b | 1 or 0 |
Nibble |
| 4 b |
Byte | B | 8 b |
Kibibyte | KiB | 1024 B (210) |
Mebibyte | MiB | 1,048,576 KB (220) |
Gibibyte | GiB | 1,073,741,824 MB (230) |
Tebibyte | TiB | 1,099,511,626,776GB (240) |
Megabyte vs Mebibyte
1 kibibyte = 1024 bits - binary prefixes (to the power of 2)
1 kilobyte = 1000 bits - decimal prefixes (to the power of 10)
Examiner Tips and Tricks
Remember, in the exam you are expected to know how to 'construct an expression' rather than do any calculations .
It is a non-calculator paper!
All expressions should use the binary prefixes (power of 2) so 1024 and not 1000!
Converting between units
It is often a requirement of the exam to be able to convert between different units of data, for example bytes to mebibytes (larger) or kibibytes to bytes (smaller)
This process involves division, moving up in size of unit and multiplication, moving down in size of unit
When dealing with all units bigger than a byte we use multiples of 1024 (210)
For example, 2000 kibibytes in mebibytes would be 2000 / 1024 = 1.95 MiB and 2 tebibytes in gibibytes would be 2 * 1024 = 2048 GiB
When dealing with bits and bytes the same process is used with the value 8 as there are 8 bits in a byte
For example, 24 bits in bytes would be 24 / 8 = 3 B and 10 bytes in bits would be 10 * 8 = 80 b
| Unit |
|
|---|---|---|
Multiply by 8 ⇑ | Bit | Divide by 8 ⇓ |
Byte | ||
Multiply by 1024 ⇑ | Kibibyte | Divide by 1024 ⇓ |
Mebibyte | ||
Gibibyte | ||
Tebibyte |
Calculate File Sizes
How do you calculate file sizes?
To calculate file sizes you must follow a simple formula, each type of file has it's own
In the exam you are expected to be able to construct expressions to calculate file sizes BUT there is no need to perform them
Expressions should be constructed for
Images
Sound
Images
Width x height x colour depth | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Sound
Sample rate x bit depth x duration | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Examiner Tips and Tricks
Given the file size and the values of any two of the variables, you may be expected to be able calculate the value of the missing one
Worked Example
An image uses a 24-bit colour depth. It is 150 pixels wide and 230 pixels high.
Construct an expression to calculate the file size of the image in MiB. You do not have to do the calculation [4]
How to answer this question?
You are not doing the calculation! just the expression
How do you calculate in bits? w x h x cd
Bits to bytes (/8)
bytes to kibibytes (x1024)
kibibytes to mebibytes (x1024)
Answer
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