Inside your computer - Bettina Bair

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.