How do computer parts connect

Connecting scattered parts

We've seen four parts so far.
The CPU that calculates, the memory that holds values, the storage that keeps things even when the power is off, and input and output that exchange values with us.
On their own, each just does its own job, but join them with one pathway and they finally become a single computer.

Scattered versus connected.

This pathway is called a bus.
Like passengers at a bus stop, many parts stand along the same road and pass values along it.
A computer is a group of parts joined by this pathway.

Passing values along the same road

The bus is a road everyone shares.
When the CPU says to memory "give me that value," the value rides this road over; when it says "store this," the value rides the road to storage.
Tap a part below and you'll see a value travel there.

Values riding back and forth along the pathway.

Because there's only one road, the parts can all reach each other.
The CPU talks to memory and to storage along the same road.
You don't need a separate wire for every pair of parts; one pathway is enough.

Take it out, load it up, calculate

Let's follow the path one value takes as it works.
It's taken out of storage and loaded into memory, the CPU picks it up and calculates, and the result goes back into memory.
This trip is possible because the parts are joined by the pathway.

One value passes through the parts.

Storage is big but slow, and memory is faster.
So the values you'll need are loaded into memory first, and the CPU takes them from there.
The memory layers we saw in Lesson 13 work together like this.

Taking turns

With only one road, several values can't travel at the same moment.
They go one at a time, taking turns.
A value from memory goes by, then a value from the CPU goes by.
It's so fast that to our eyes it looks simultaneous, but really they take turns.

Values passing along one pathway, one after another.

Deciding who uses the road first also happens inside the computer.
Thanks to that, values don't get tangled, and everyone can travel on a single pathway.

All working as one body

Now let's run one full lap.
When we type in a value (input), it's held in memory, the CPU picks it up and calculates, and the result passes through memory and appears on the screen (output).
Every part, joined by one pathway, works as a single computer.

Input → memory → CPU → memory → output.

This is the body of a computer.
Starting from a single switch, we built parts that calculate and remember, then joined them with one pathway into one machine.
In the last two lessons, we'll look at the clock that keeps every part in step and then look back over the whole road we've traveled.