Units of information study guide

1. Start with the representation unit

When a question refers to a bit, remember that the bit has exactly two possible values: 0 and 1. If a representation contains n bits, each position can independently be one of these two values. Therefore the total number of configurations is 2^n.

For example, with 3 bits, the first position has 2 choices, the second has 2 choices and the third has 2 choices. The total is 2 × 2 × 2 = 2^3 = 8. Listing the configurations confirms this: 000, 001, 010, 011, 100, 101, 110 and 111. This is the reasoning behind the formula, rather than a fact to apply without checking the number of bits.

A byte must be distinguished from a bit: a byte contains 8 bits. If a question says that a value uses one byte, the relevant number of bits is 8, so the number of possible configurations is 2^8. If it says that a value uses 3 bits, do not substitute 8 merely because a byte is a familiar unit.

2. Identify the prefix before calculating

The main application skill for byte quantities is recognising whether the prefix is binary or decimal. Binary prefixes represent powers of 2:

• 1 KiB = 2^10 B
• 1 MiB = 2^20 B
• 1 GiB = 2^30 B
• 1 TiB = 2^40 B

Decimal prefixes represent powers of 10:

• 1 kB = 10^3 B
• 1 MB = 10^6 B
• 1 GB = 10^9 B
• 1 TB = 10^12 B

The capitalisation and spelling matter. Ki, Mi, Gi and Ti identify binary prefixes, while k, M, G and T identify decimal prefixes. Thus, 1 KiB and 1 kB describe different quantities: the first is 2^10 B, while the second is 10^3 B.

3. Exam application process

Use a three-stage method. First, copy the unit exactly as given, including capital letters. Second, replace the prefix with its specified power of 2 or 10. Third, show the resulting expression in bytes. For instance, a question asking for the meaning of 1 GiB should be answered as 1 GiB = 2^30 B, whereas 1 GB should be answered as 1 GB = 10^9 B.

Do not silently convert a binary prefix into a decimal prefix. Also avoid assuming that the word “kilobyte” alone settles the interpretation: historically, terms such as kilobyte and megabyte have often been used when kibibyte and mebibyte were meant. Prefer the explicit symbol where one is supplied, and explain the power used.

4. Self-check

Before finishing an answer, ask: Have I identified whether the quantity is measured in bits or bytes? If bits are involved, have I used 2^n with the correct value of n? If bytes and a prefix are involved, have I selected the correct binary or decimal table? Have I preserved the prefix symbol and written the answer in bytes where required? These checks target the main distinctions in this topic.