A digital system stores text, photographs, music and money the same way: as binary digits, each one a 0 or a 1. Nothing in the machine knows which is which — the meaning comes from the software that reads it.
- Bit
- A single binary digit: 0 or 1. The smallest unit of data there is.
- Byte
- Eight bits, so 256 possible combinations — historically enough for one character of text.
- Binary
- A base-2 number system. Each place is worth twice the one to its right: 1, 2, 4, 8, 16…
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Using two states is not a style choice. It is about what a circuit can do reliably.
On and off are easy to tell apart: A circuit can always tell “on” from “off”, even when the voltage wobbles a little. Asking it to tell ten different voltage levels apart would make every part harder to build and every mistake more likely.
What follows from using bits
- Exact copies. A copy of a copy is identical, which is why piracy and backup are both easy.
- Compression. If data is numbers, patterns in it can be described more briefly than they can be listed.
- Encryption. Numbers can be transformed by a key and transformed back.
- Mixing. Text, sound and images share one storage and one network because they share one representation.
One byte, read four ways. The bits never change between the steps — only the software reading them does.
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A useful sense of scale: One byte holds a character. A thousand-word essay is a few kilobytes. A photograph is a few megabytes. An hour of video is a few gigabytes. Each step is about a thousand times the last.
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Binary representation sounds like a computer-science detail until you notice it is the reason two of this course's biggest arguments exist at all.
| Because data is bits… | …this becomes possible |
|---|---|
| Copies are exact and free | Perfect duplication of anything, including things people wanted kept |
| Numbers can be transformed | Encryption, and arguments about who may undo it: in 2016 the FBI asked a US court to make Apple unlock the San Bernardino attacker's iPhone |
| Patterns can be found in numbers | Machine learning on data collected for another purpose |
| Storage is cheap per bit | Collect now, decide why later — the habit behind most data dilemmas |
Real-world examples you can name
Apple v the FBI over the San Bernardino iPhone — February–March 2016
The FBI asked a court to compel Apple to write software that would disable the passcode limits on a locked iPhone. Apple refused, arguing that such a tool could not be confined to one device. The case ended when the FBI paid a third party to unlock the phone, leaving the legal question unresolved.
Who it affected: Every user of encrypted devices, and investigators who cannot read them.
How this is tested — the basics are tested as precise definitions, and then as the reason something else is possible. It comes up two ways:
Paper 1 — structured question
- Part a: define or state what a bit or a byte is
- Part b: explain why digital copies are identical
Paper 2 — source-based question
- Q1: read a figure in bits, bytes or megabytes off a source
- Q2: explain a technical term a source uses
The trap: a vague definition: “A bit is a small amount of data” earns nothing. “A bit is a single binary digit, 0 or 1” earns the mark. Definitions are the cheapest marks in the paper and the easiest to fumble.
Define the term byte.
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Explain one reason why representing data in binary makes encryption possible.
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