Unit 0 · What a computer actually does
How a computer stores a letter as a number
Underneath, a computer only has switches, so every letter is stored as a number. An agreed table maps numbers to characters: when you press B, the computer stores 66.
The mix-up
What people often think
“A computer understands words and letters directly.”
What the tutor says
Underneath, a computer only has switches, so every letter is stored as a number. When you type B, it stores 66.
If the computer only has switches, what has to happen when you press the B key?
How it works
A computer never sees the shape of a letter. When you press a key, the keyboard sends a code, and the computer keeps a number. A shared table, such as ASCII and the Unicode standard built on it, says which number stands for which character. In that table A is 65, B is 66 and H is 72.
Later letters do not need more space. One byte can hold any number up to 255, and B is only 66, so it fits in the same eight switches as A. Only the pattern changes.
Everything else a computer stores follows the same idea. Colours, sounds and photos are long runs of numbers, and a program decides what those numbers mean.
An example
H → 72 → 0 1 0 0 1 0 0 0
(64 + 8)Close cousins of this mix-up
The tutor watches for these in the same lesson.
“Later letters need more switches.”
One byte holds any number up to 255, and B is only 66. The same eight switches just need a different pattern.
Practise it with a tutor watching
Bits and switches
Turn a row of on and off switches into a number and a letter.
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