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Do you remember when you first realized

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that your computer was more

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than just a monitor and keyboard?

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That between the mouse click
and the video playing,

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there was something
that captured your intention,

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understood it,

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and made it real?

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What is that something?

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Is it gremlins?

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Let's imagine that we can shrink down

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to the size of an electron

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and inject ourselves
into a click of a mouse.

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If you took your mouse apart,

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

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It has a couple buttons

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and a system for detecting
motion and distance.

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You might have an optical mouse

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that makes these measurements
with lights and sensors,

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but older ones did this
with a hard rubber ball

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and some plastic wheels.

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Same concept.

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When you click the button on your mouse,

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it sends a message to the computer

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with information about its position.

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When your mouse click is received,

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

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This subsystem acts like the eyes and ears

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and mouth and hands of the computer.

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Basically, it provides
a way for the computer

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to interact with its environment.

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But it also acts like a buffer

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to keep the CPU from being
overwhelmed by distractions.

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In this case, the I/O subsystem decides

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that your mouse click is pretty important

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so it generates an interrupt to the CPU.

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"Hey, CPU! Got a click here."

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The CPU, or central processing unit,

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is the brains of the whole computer.

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

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the CPU doesn't take
up the whole computer,

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but it runs the show all the same.

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And the CPU's job, its whole job,

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is fetching instructions from memory

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and executing them.

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

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maybe really fast,

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like 60 words a minute,

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the CPU is fetching and executing

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billions of instructions a second.

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Yes, billions every second:

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instructions to move your mouse
around on the screen,

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to run that clock widget on your desktop,

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play your internet radio,

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

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and much, much more.

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

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"But oh my gosh

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there's a very important mouse click

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coming through now!

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

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There are programs for everything

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that the CPU does.

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A special program for the mouse,

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for the clock widget,

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for the internet radio,

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and for dealing with letters
sent by the keyboard.

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Each program was initially
written by a human

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in a human-readable programming language,

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like Java,

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C++,

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or Python.

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But human programs take up a lot of space

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and contain a lot of unnecessary
information to a computer,

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so they are compiled and made smaller

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and stored in bits of ones
and zeros in memory.

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The CPU realizes that it
needs instructions

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for how to deal with this mouse click,

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so it looks up the address
for the mouse program

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and sends a request
to the memory subsystem

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for instructions stored there.

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Each instruction
in the mouse device driver

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is duly fetched and executed.

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

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Because the CPU learns
that the mouse was clicked

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when the cursor was over a picture

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of a button on the monitor screen,

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and so, the CPU asks memory
for the monitor program

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to find out what that button is.

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And then the CPU has to ask memory

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for the program for the button,

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which means that the CPU needs

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the monitor program again

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to show the video
associated with the button,

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and so it goes.

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

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before you even see
the button on the screen

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light up when you clicked it.

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So, just the simple task
of clicking your mouse

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means visiting
all of the critical components

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of your computer's architecture:

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peripherals,

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the basic input-output system,

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the CPU,

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programs,

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and memory,

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and not one gremlin.