Inside your computer - Bettina Bair

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How does a computer work? The critical components of a computer are the peripherals (including the mouse), the input/output subsystem (which controls what and how much information comes in and out), and the central processing unit (the brains), as well as human-written programs and memory. Bettina Bair walks us through the steps your computer takes with every click of the mouse.

Lesson by Bettina Bair, animation by Flaming Medusa Studios.

Do you remember when you first realized

that your computer was more

than just a monitor and keyboard?

That between the mouse click
and the video playing,

there was something
that captured your intention,

understood it,

and made it real?

What is that something?

Is it gremlins?

Let's imagine that we can shrink down

to the size of an electron

and inject ourselves
into a click of a mouse.

If you took your mouse apart,

you'd see that it's really
a very simple machine.

It has a couple buttons

and a system for detecting
motion and distance.

You might have an optical mouse

that makes these measurements
with lights and sensors,

but older ones did this
with a hard rubber ball

and some plastic wheels.

Same concept.

When you click the button on your mouse,

it sends a message to the computer

with information about its position.

When your mouse click is received,

it's handled by the basic
input/output subsystem.

This subsystem acts like the eyes and ears

and mouth and hands of the computer.

Basically, it provides
a way for the computer

to interact with its environment.

But it also acts like a buffer

to keep the CPU from being
overwhelmed by distractions.

In this case, the I/O subsystem decides

that your mouse click is pretty important

so it generates an interrupt to the CPU.

"Hey, CPU! Got a click here."

The CPU, or central processing unit,

is the brains of the whole computer.

Just like your brain doesn't
take up your whole body,

the CPU doesn't take
up the whole computer,

but it runs the show all the same.

And the CPU's job, its whole job,

is fetching instructions from memory

and executing them.

So, while you're typing, typing, typing,

maybe really fast,

like 60 words a minute,

the CPU is fetching and executing

billions of instructions a second.

Yes, billions every second:

instructions to move your mouse
around on the screen,

to run that clock widget on your desktop,

play your internet radio,

manage the files you're
editing on the hard drive,

and much, much more.

Your computer's CPU
is one heck of a multitasker!

"But oh my gosh

there's a very important mouse click

coming through now!

Let's drop everything now
and deal with that!"

There are programs for everything

that the CPU does.

A special program for the mouse,

for the clock widget,

for the internet radio,

and for dealing with letters
sent by the keyboard.

Each program was initially
written by a human

in a human-readable programming language,

like Java,

C++,

or Python.

But human programs take up a lot of space

and contain a lot of unnecessary
information to a computer,

so they are compiled and made smaller

and stored in bits of ones
and zeros in memory.

The CPU realizes that it
needs instructions

for how to deal with this mouse click,

so it looks up the address
for the mouse program

and sends a request
to the memory subsystem

for instructions stored there.

Each instruction
in the mouse device driver

is duly fetched and executed.

And that's not nearly
the end of the story!

Because the CPU learns
that the mouse was clicked

when the cursor was over a picture

of a button on the monitor screen,

and so, the CPU asks memory
for the monitor program

to find out what that button is.

And then the CPU has to ask memory

for the program for the button,

which means that the CPU needs

the monitor program again

to show the video
associated with the button,

and so it goes.

And let's just say there
are a lot of programs involved

before you even see
the button on the screen

light up when you clicked it.

So, just the simple task
of clicking your mouse

means visiting
all of the critical components

of your computer's architecture:

peripherals,

the basic input-output system,

the CPU,

programs,

and memory,

and not one gremlin.