Data Representation · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 4% of your exam marks.
File size calculations and lossless vs lossy compression are regular 3 to 4 mark questions.
Every value held on a computer ultimately lives as bits. The units in the table below give names to bigger and bigger collections of bits, which is what file sizes are measured in.
| Unit | Symbol | Size | In bytes |
|---|---|---|---|
| Bit | b | a single 1 or 0 | 1/8 byte |
| Nibble | — | 4 bits | 1/2 byte |
| Byte | B | 8 bits | 1 byte |
| Kilobyte | kB | 1000 bytes | 1 000 |
| Megabyte | MB | 1000 kB | 1 000 000 |
| Gigabyte | GB | 1000 MB | 1 000 000 000 |
| Terabyte | TB | 1000 GB | 1 000 000 000 000 |
| Petabyte | PB | 1000 TB | 10¹⁵ |
The Cambridge syllabus uses the SI (decimal) convention where each prefix is a factor of 1000. This matches how storage manufacturers label disks (a "1 TB hard drive" really means 1 000 000 000 000 bytes).
In some contexts (particularly memory, RAM and operating-system file sizes), the older binary convention is used, where each prefix is a factor of 1024 (= 2¹⁰) instead of 1000. To avoid confusion, the binary units have different names:
| Binary unit | Symbol | Exact size |
|---|---|---|
| Kibibyte | KiB | 2¹⁰ bytes = 1 024 |
| Mebibyte | MiB | 2²⁰ bytes = 1 048 576 |
| Gibibyte | GiB | 2³⁰ bytes ≈ 1.07 × 10⁹ |
| Tebibyte | TiB | 2⁴⁰ bytes |
| Pebibyte | PiB | 2⁵⁰ bytes |
| Exbibyte | EiB | 2⁶⁰ bytes |
For exam questions, read the question carefully. If it says "1 kB = 1000 bytes" use 1000. If it says "1 KiB = 1024 bytes" use 1024. Both conventions appear in past papers.