The Bus and the 8-bit Word

The eighth intro ties eight separate wires into one concept — the bus. It starts with an experiment: build 77 out of eight lamps.

Experiment: eight lamps, one number

Now there are eight lamps — one per digit: 1, 2, 4, 8, 16, 32, 64 and 128. Building 77 means turning on the 64, 8, 4 and 1 lamps. In binary notation that is 01001101: the ones sit exactly at the places whose lamps are lit.

The rule: a bundle of wires is a bus

Eight wires carrying one number together are called a bus. Each wire carries its own digit: lit is 1, dark is 0. The notation is read digit by digit, left to right: 128, 64, 32, 16, 8, 4, 2, 1. Instead of drawing eight arrows, a bus is drawn as one thick arrow.

Transfer: read the bus

A bus carries the word 01001101. Read the digits: the ones sit at 64, 8, 4 and 1 — so the bus carries 77. A mistake at one place changes the number: it would read 78 if the one sat at the "2" place.

Where it is used

The bus is what the whole computer is made of. The 8-bit adder on this level adds two such words at once, not separate bits. Later, instructions and data will travel along buses: one thick arrow will stand for a whole number — from the op code to a memory address.