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[Inaudible Background
Speaker and Music]

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>> On January 24, Apple Computer
will introduce Macintosh,

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and you'll see why 1984
won't be like 1984.

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>>David: So that commercial
aired once in 1984,

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during the Super Bowl and
this was 25 years ago now,

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and it was Apple first debuted
the Macintosh, and at the time,

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they were facing Big Blue,
IBM, and those IBM PC's,

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and most of you probably
haven't even used

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in their original form,
but it was a big deal

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as to what the Apple had begun
do in 1984 with the industry

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and with computers
as we now know it.

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And, today is focused
on precisely

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that general world of hardware.

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We've spent much of the
semester working our way on up,

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we went through C,
we went through PHP,

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a little Java script and AJAX,
and HTML, and all of that,

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and then today, finally,
we'll come full circle

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and actually look underneath
the hood as to how all

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of this magic has
been happening.

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When you compile something with
GCC, when you run something

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with PHP, what is, in fact,
happening underneath the hood?

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But a few announcements first.

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So, one little copy paste
fail, this handout is indeed

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about Quiz 1, not
about Quiz zero,

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which the first sentence
says, my apologies, but yes,

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Quiz 1 is in, on Wednesday 11/18
and do peruse the handout here

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for all of the available
resources coming up at you,

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including review sessions
and quiz review in sections.

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Also, take note that on Monday,

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we're going to be having
a few guest lecturers

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from the Computer
Science faculty.

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Each will talk a little bit

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about Computer Science beyond
CS50 and specifically some

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of the courses you can take.

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It's actually much more exciting
taking Computer Science courses

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here these days, than in
my day, where pretty much

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after you took 50, you took
51 and after 51, you took 121,

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and after 121, you took 124,

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like there was a very
clearly defined path,

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but there wasn't much
digression from it.

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Now, there's so many
more faculty and courses,

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that after CS50, you guys will
be able to, if you so chose

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for reasons of majoring,
minoring, or just for fun, CS51,

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which is about functional
programming, and abstraction,

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which you'll hear a little
more about on Monday.

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CS61, which is about systems
programming, it's in that course

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where you diffuse
the binary bomb,

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that piece that I
described a while back.

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You can take 121 next fall,
which is a theory course,

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and speaks to the fundamentals

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of what computers
can and can't do.

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CS124, which is data
structures and algorithms,

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which picks up essentially
where we left off

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with hash tables,
tables, and tried.

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CS171, which is about
visualization

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and taking interesting large
data sets and presenting it

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to human in ways
they can understand.

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CS105, privacy and
security, which is more

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of a seminar course, where you
talk about the implications

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of various technologies
on society,

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and its privacy and security.

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CS179, which is a new course,

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in which you will actually do
iPhone development and focus

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on the design of
good, user interfaces,

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so there's quite the
platter of courses

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from which you'll be able

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to chose this spring,
this coming fall.

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And realize, too, at
the end of this course,

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CS50 is a terminal
course for many students.

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Usually about half of
the students in 50,

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don't take Computer Science,
and that perfectly fine,

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because the course is
meant to provide you

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with enough background, enough
skill set to actually go off

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and do interesting things,
even beyond the course,

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but realize that, if you
are interested in minoring

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in Computer Science, the
department is quite eager

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to entice to all the
more students into it,

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and you have many different
things to chose from these days.

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Majoring, all the better, but
even if you just want to dabble

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in Computer Science, realize
that this course, in particular,

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opens a lot of doors, within
the engineering school,

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so do keep that in mind.

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We'll have a little cheat
sheet, probably on the website

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or on paper on Monday, just
to give you a more clear sense

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than this verbal tour,
as to what awaits you.

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So, one pitch, before
I turn things

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over to a couple of other folks.

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The first is, this one
here, so the website is now

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up as promised, we do
indeed want some TF's

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and CA's and graders.

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What does this mean?

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A teaching fellow for this
course, as you may have inferred

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from the person you've
been working with all term,

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someone who leads sections,
who grades problem sets,

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who works with students
one on one in office hours,

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fields questions on
the bulletin boards

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and does all those things
you're familiar with TFN.

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I would say that TFN50 is
a fairly unique experience.

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It's definitely intense, perhaps
as intense as taking the course,

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but so much fun, and dare
say, so much more fun being

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on this side of things,
than on that side of things,

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and it's a nice way to
main, to remain part

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of the community
that you might feel.

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CA's are volunteer's.

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CS50 alumni who offer 2 hours
of office hours per week,

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it's largely self-scheduled,
it's meant to be very flexible

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so that we can keep as many
alumni involved in the course,

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and to help their successors in
the course and the computer lab.

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And then finally graders,

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I'll spend a few hours a
week hours a week helping

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out with key sets.

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And finally, that's enough
from me for the moment,

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let me invite an
alumnus from CS50

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and a former TFN [inaudible]
man, and now President

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of HCS, Kato Yuchiama.

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Oh sure, yeah.

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Let me get him a microphone,
which is over here, okay.

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All yours.

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>> Kato Yuchiama: Hey.

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My name's Kato, I'm President
of Harvard Computer Society.

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Today I'm here to
invite you to an event

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that we are holding Friday
in conjunction with the C's

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and CS department faculty.

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We're holding CS
concentration cider and pie,

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so this is your opportunity
to eat cider and pie.

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Well, in fact, it's actually
for people interested in CS,

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people already concentrating
in, people thinking

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of [inaudible] fielding
in CS, you know,

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if you want to take
any of those courses

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that David just mentioned,
it's your opportunity

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to meet CS professors,
and also upper classmen

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who are already concentrating in
CS, so it's a great opportunity

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to ask questions, and it's
not just the CS professor kind

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of thing because now you can ask
people whether that class is,

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you know, is 124
actually a good class?

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You know, professors won't
tell you the real answer,

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but upper classmen will.

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So, as it says here,
it's a great opportunity

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to meet faculty, learn about
cool classes, you can eat,

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not eat, meet CS undergrads,
eat the food, win great prizes,

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we're still deciding
that, but we are sure

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that there will be great
prizes that you can win.

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And, also, its, you
know, it's in a kind,

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we've never had a
opportunity like this,

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at least in the 3 years
that I've been here,

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so it should be great,
should be great fun.

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So, that CS Concentration Cider
and Pie, we asked that you RSVP

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on our Facebook event, if
you're, if you want to RSVP

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at CS, go to
HCS.Harvard.edu/cider,

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that's Harvard Computer
Society.Harvard.edu/cider,

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you don't need to RSVP, so
you can show up, its Friday 3

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to 4:30 in Maxwell's Dworkin
lobby, and if you don't know

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where Maxwell Dworkin is,

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you can use David's awesome
maps.cs50.net tool to look it

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up on the map, so thank
you, and have a great time.

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[Applause]

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>>David: Thanks very much, I'll
give this back to Jennifer.

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All right, so one
pitch, if you didn't pick

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up your shuttle boy
card, we have some

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with the teaching fellows
on your way out today.

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It was kind of funny,
we were looking over,

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so we have this form on
the courses website now,

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as some of you may have noticed,
and we wrote a little script

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in PHP that inserts
registrations

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for shuttle boy card into our
database, and what was funny was

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to see all the interest in
this card on like Thursday,

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and Friday, after
Wednesdays lecture,

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when we actually handed
them out in person,

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so this little request form
began this variable list

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of who was not here
on Wednesday.

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The irony is, they're
probably still not here today,

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but that's okay, we
send them our best, and,

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we actually some
400 plus response,

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so we'll start mailing
those out to your friends

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who probably filled out
that form this week.

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A big shout out to our own
Drew Rob, who is completely,

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trounced everyone
on the big board.

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I looked up the word for this,
its 113 quintillion dollars

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that he has made atop the
big board, really putting

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to shame the leaders from last
week, so this was all just

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for fun, clearly there was some
opportunities to take advantage

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of here, but congrats to Drew
and all of you nonetheless.

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So, without further
ado, coming at you now,

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last advertisement
today is store.cs50.net.

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The CS department actually has
a tradition of having silly T-

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shirts and what-not at
the end of the semester,

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I still remember my I
Survived CS51 T-shirt

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from many years ago, and
so we've continued on in

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that tradition, and thanks
to this fancy website,

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can we sell all sorts of
things to ourselves and such,

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so feel free to take that out,
but allow me to indulge us

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in a little display perhaps
of what's been designed

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by our own TF's this year.

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[ Music and Applause ]

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[ Inaudible Speaker ]

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[ Applause & Music ]

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>>David: So, as you know,
final projects are coming

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up as is the CS50 fair, and we,
CS50, were particularly proud

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to see very recently that some

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of our own CS50 alumni
turned teaching fellows,

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some of them now, have been
awarded a wonderful prize

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from AT & T, some of you
may have seen this float

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around the house list, some of
you may have on your own iPhone

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and iPods, the Rover App,
by Harvard Student Agencies,

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and this actually grew out of
CS50 final project a couple

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of years ago, and I thought
I would give them a moment,

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give them a little bit
of CS50 shout out here

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by showing this few minute
video that they put together

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with AT & T. So, if you
can think of something

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as magical yourselves
to do this year,

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perhaps it will be your video
we show at next year's class.

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Here we go.

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[ Silence ]

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[Music]

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>>Hi, my name is
Winston, I'm from Virginia

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00:12:01,746 --> 00:12:04,686
and I studied Physics here
at Harvard, but I just really

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like working with my
hands and making things.

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>>I'm Joy.

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00:12:14,636 --> 00:12:15,826
I'm from San Diego,
but I'm Canadian.

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I'm majoring in Computer
Science and minoring

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in Visual Environmental
Studies, and I like good designs

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and intuitive [inaudible].

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>>I'm Cameron, I'm
from Colorado,

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and [inaudible] Computer
Science concentrator here.

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>>Okay, my name is
Drew, and [inaudible]

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and study Physics [inaudible]
enjoyed programming.

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>>My name is Alex, I'm
an Engineering major

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from New Jersey, and for a very
long time I've been interested

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in making [inaudible]
devices more useful.

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[Music]

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>>Joy: Rover is a
mobile platform

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to connect students
with the community.

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The [inaudible] version, we
implemented a student guidebook

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into a mobile format
that had [inaudible]

235
00:13:00,376 --> 00:13:02,256
as well [inaudible] platform for
version [inaudible] to be able

236
00:13:02,286 --> 00:13:02,976
to connect to the student.

237
00:13:03,526 --> 00:13:07,876
>>We also helped to connect
students to their peers

238
00:13:08,356 --> 00:13:10,396
through student-to-student
interactions.

239
00:13:10,396 --> 00:13:12,526
For example, student
groups are often interested

240
00:13:12,526 --> 00:13:13,876
in getting more students
to come to their events,

241
00:13:13,906 --> 00:13:15,196
so instead of posting
fliers on a bulletin board,

242
00:13:15,226 --> 00:13:15,976
we can post fliers on Rover.

243
00:13:16,786 --> 00:13:19,506
Also, for example, you can
have a free [inaudible],

244
00:13:19,796 --> 00:13:21,256
somewhat similar
to the [inaudible],

245
00:13:21,256 --> 00:13:24,076
where anytime there's an event
on campus that has free food,

246
00:13:24,346 --> 00:13:28,886
students can take
advantage of that.

247
00:13:28,886 --> 00:13:30,116
>>Joy: And this platform
is also a lot of potential

248
00:13:30,146 --> 00:13:31,076
for administration
to communicate

249
00:13:31,106 --> 00:13:31,976
with students in
a timely fashion,

250
00:13:33,606 --> 00:13:36,486
almost everyone has
a mobile device,

251
00:13:36,916 --> 00:13:41,686
and so the platform will
be a quick way to reach

252
00:13:41,896 --> 00:13:48,466
out to student, also for student
to professor communication,

253
00:13:49,046 --> 00:13:50,116
[inaudible] great deal.

254
00:13:52,286 --> 00:13:53,996
>>All of this [inaudible]
social networks.

255
00:13:54,306 --> 00:13:56,886
So, for example, if a
student, who's a friend

256
00:13:56,916 --> 00:13:58,386
of yours is taking advantage
of lots of free food deals,

257
00:13:58,416 --> 00:13:59,586
or is visiting a local
merchant that they like

258
00:13:59,616 --> 00:14:00,876
or has had a great
experience with a professor,

259
00:14:00,906 --> 00:14:02,316
we want to broadcast that to
all of their other friends

260
00:14:02,346 --> 00:14:02,976
who are also using Rover.

261
00:14:03,736 --> 00:14:08,836
>>And, lastly, at the back end
of all of this is analytics

262
00:14:08,836 --> 00:14:10,876
for our partners and merchants

263
00:14:10,986 --> 00:14:14,396
so that we can know exactly
who's clicking on what,

264
00:14:14,716 --> 00:14:16,816
and when, and this really
provides business intelligence

265
00:14:16,846 --> 00:14:17,656
and usage statistics for us,

266
00:14:17,686 --> 00:14:18,976
so that we know what's
mostly useful for student.

267
00:14:20,256 --> 00:14:23,156
>>Rover is unique because it's
an application that's written

268
00:14:23,326 --> 00:14:26,826
for students, entirely by
students, with students in mind.

269
00:14:26,826 --> 00:14:29,496
All of the content on Rover
is produced by students,

270
00:14:29,846 --> 00:14:32,506
all the deals are things
that students really want.

271
00:14:32,536 --> 00:14:33,976
What student doesn't want a
free burrito, for example.

272
00:14:34,516 --> 00:14:41,546
[ Music ]

273
00:14:42,046 --> 00:14:44,946
>>[Inaudible speaker comment]
and we found out about how

274
00:14:45,096 --> 00:14:50,876
AT & T Big Mobile on
campus helped them expand

275
00:14:50,986 --> 00:14:53,916
from just one campus to a
number of campuses nationwide,

276
00:14:53,986 --> 00:14:55,836
and so by entering
Big Mobile on campus,

277
00:14:56,396 --> 00:14:57,986
it really helped us think

278
00:14:58,306 --> 00:15:00,326
about how can we take
what we've done here,

279
00:15:00,356 --> 00:15:01,976
in the Harvard Square community
and extended it nationwide.

280
00:15:02,286 --> 00:15:04,376
>>[Inaudible] fantastic
experience to work

281
00:15:05,196 --> 00:15:08,486
with AT & T people,
to really bring out,

282
00:15:08,906 --> 00:15:15,276
to expose our application to,
perhaps, a larger audience

283
00:15:15,886 --> 00:15:19,976
than we ever could have
pitched it to without AT & T,

284
00:15:20,006 --> 00:15:21,566
so it's just a tremendous
opportunity for us, and the fact

285
00:15:21,596 --> 00:15:22,976
that we have some nice prizes
too, doesn't hurt as well.

286
00:15:24,046 --> 00:15:24,566
>>There's hundreds

287
00:15:24,786 --> 00:15:28,246
of applications being
built every day,

288
00:15:28,246 --> 00:15:34,886
and it's certainly sometimes
hard to distinguish yourself

289
00:15:34,986 --> 00:15:36,236
from all [inaudible]
applications

290
00:15:36,236 --> 00:15:41,756
that are being developed,
so this is huge help.

291
00:15:41,756 --> 00:15:45,126
>>I really like making things

292
00:15:45,126 --> 00:15:46,006
that our friends
are able to use.

293
00:15:46,036 --> 00:15:47,026
I like making things
that I like to use,

294
00:15:47,056 --> 00:15:47,956
and I know this application
was made

295
00:15:47,986 --> 00:15:48,826
with Rover is something
I consult,

296
00:15:48,856 --> 00:15:49,636
you know four or
five times a day.

297
00:15:49,666 --> 00:15:50,986
>>Just to connect students
and like mobile business,

298
00:15:51,016 --> 00:15:52,336
or like to get students
into your local theatres,

299
00:15:52,366 --> 00:15:54,016
we have local arts and it's
just the community, and we think

300
00:15:54,046 --> 00:15:54,976
that connection can
be really powerful.

301
00:15:55,516 --> 00:15:57,546
[Music]

302
00:15:58,046 --> 00:15:59,996
>>David: So, and there, if you
have an iPhone or iPod touch,

303
00:15:59,996 --> 00:16:02,546
and congrats to our alumni
and where they've taken that.

304
00:16:02,546 --> 00:16:05,526
It's a, it's worth noting,
lest you be quite awe struck

305
00:16:05,566 --> 00:16:07,096
by what these guys
have all pulled off,

306
00:16:07,256 --> 00:16:10,366
that there's this kind of
unspoken rule in programming,

307
00:16:10,846 --> 00:16:12,516
where anything you
might ever want to do,

308
00:16:12,516 --> 00:16:15,206
and you've probably experienced
this, probably takes twice

309
00:16:15,206 --> 00:16:18,096
as long, three times as long
as you actually expect it to,

310
00:16:18,096 --> 00:16:20,786
so realize that this product
is actually the work now

311
00:16:20,786 --> 00:16:24,636
of a couple of years of ongoing
development, so realize that,

312
00:16:24,636 --> 00:16:28,066
don't worry that the
bar has been set so high

313
00:16:28,066 --> 00:16:30,296
for your own final projects,
what these guys essentially did

314
00:16:30,296 --> 00:16:32,866
for their first version
was a very functional,

315
00:16:32,866 --> 00:16:36,136
and very nice prototype, sort
of version 1 of this all,

316
00:16:36,286 --> 00:16:37,986
and they've continued to
build on that as well,

317
00:16:37,986 --> 00:16:39,816
so we hope you too will
dive into something

318
00:16:39,816 --> 00:16:43,176
that interests you, if
not, for just this duration

319
00:16:43,176 --> 00:16:45,836
of this semester, even beyond,
so congrats to those guys.

320
00:16:46,096 --> 00:16:52,306
All right, so today's goal is to
focus a bit on what's been going

321
00:16:52,306 --> 00:16:53,636
on underneath the hood.

322
00:16:53,686 --> 00:16:54,806
So, what do we mean by that?

323
00:16:54,806 --> 00:16:57,276
Well, we took for
granted, for some time,

324
00:16:57,276 --> 00:17:00,336
that you can write programming
code in a language called C,

325
00:17:00,706 --> 00:17:03,746
you can then run a
compiler called GCC,

326
00:17:03,746 --> 00:17:06,786
and out of that comes a
program, and just in English,

327
00:17:07,016 --> 00:17:08,726
what does it mean
to compile a program

328
00:17:08,726 --> 00:17:12,286
from source code
into object code?

329
00:17:12,486 --> 00:17:13,146
Anyone at all?

330
00:17:13,146 --> 00:17:16,226
Yeah, in the back.

331
00:17:16,226 --> 00:17:16,426
>>[Inaudible Speaker Comment]

332
00:17:16,426 --> 00:17:18,516
>> David: Okay, so take
text and interpret it,

333
00:17:18,516 --> 00:17:20,956
or let me tweak your
words, convert it to binary.

334
00:17:21,016 --> 00:17:23,436
So, take something that's sort
of pseudo English, pseudo code,

335
00:17:23,476 --> 00:17:25,226
or actual code, I'll run it

336
00:17:25,226 --> 00:17:27,456
through this program called
the compiler, and the output

337
00:17:27,456 --> 00:17:30,076
of that is not just ascii
characters that humans can read,

338
00:17:30,076 --> 00:17:33,256
but its zero's and one's
that actual CPU's understand.

339
00:17:33,486 --> 00:17:36,686
So, what does it mean when
Intel says, "Intel is inside"

340
00:17:36,686 --> 00:17:38,506
or you see this little
sticker on PC's.

341
00:17:38,726 --> 00:17:41,016
Well, that means that the
CPU, inside of the computer,

342
00:17:41,016 --> 00:17:43,816
was made by this company called
Intel, now what does that mean?

343
00:17:43,816 --> 00:17:47,526
That means that they
decided, some years ago now,

344
00:17:47,816 --> 00:17:51,016
that every CPU, the
brains of a computer,

345
00:17:51,246 --> 00:17:52,856
would understand
certain instructions,

346
00:17:53,056 --> 00:17:55,596
and what's rather mind blowing
is that the instructions

347
00:17:55,686 --> 00:17:58,926
that all of our programs,
whether p-set 1

348
00:17:58,926 --> 00:18:02,806
or p-set 8 reduced to, are
basic things like addition

349
00:18:02,886 --> 00:18:05,876
and subtraction and
multiplication, moving something

350
00:18:05,876 --> 00:18:08,846
from memory from here to here,
loading something from disc

351
00:18:09,126 --> 00:18:12,156
or from here to here,
these very basic primitives

352
00:18:12,156 --> 00:18:14,266
that we completely
take for granted now,

353
00:18:14,596 --> 00:18:16,216
are really what's composed all

354
00:18:16,216 --> 00:18:17,896
of the programs you
guys have been writing.

355
00:18:18,156 --> 00:18:20,066
So, just to put this
into some perspective,

356
00:18:20,066 --> 00:18:22,726
let me go over to a terminal
window here, and I'm going

357
00:18:22,726 --> 00:18:24,936
to open just a file
called, Hello dot C,

358
00:18:24,936 --> 00:18:27,506
I didn't bother printing this
out because it's pretty familiar

359
00:18:27,506 --> 00:18:29,496
by now, and that all
there is to this.

360
00:18:29,496 --> 00:18:31,816
This is our canonical
Hello World program.

361
00:18:32,116 --> 00:18:35,836
Well, this language C was
actually an enhancement some

362
00:18:35,836 --> 00:18:38,996
years ago over a language
called Assembly language, so,

363
00:18:39,206 --> 00:18:41,826
what you're about to see is
a language that's much closer

364
00:18:41,826 --> 00:18:43,656
to the hardware, it's
much more arcane,

365
00:18:43,836 --> 00:18:46,916
and it's certainly not nearly
as much fun, if I'm even allowed

366
00:18:46,916 --> 00:18:48,986
to use that word in this
context, than programming

367
00:18:48,986 --> 00:18:52,106
in C. So, what I'm going to
do is run the compiler, GCC,

368
00:18:52,206 --> 00:18:54,416
but I'm going to use
perhaps an unfamiliar flag,

369
00:18:54,416 --> 00:18:57,596
I'm going to say "dash capital
S", and then I'm going to say

370
00:18:57,596 --> 00:19:01,596
"Hello dot C" and what I'm going
to get as output, per LS here,

371
00:19:01,836 --> 00:19:04,966
is a hello dot S, so this
is an intermediate file

372
00:19:04,966 --> 00:19:07,836
that we actually never
really see, not by default,

373
00:19:08,026 --> 00:19:11,526
but inside of here is
what the compiler is doing

374
00:19:11,526 --> 00:19:13,216
for this very basic program.

375
00:19:13,216 --> 00:19:16,806
So, this is an intermediate
step between C source code,

376
00:19:16,806 --> 00:19:19,406
and what's called object
code, zeros, and ones.

377
00:19:19,406 --> 00:19:20,726
Well, what do I mean by that?

378
00:19:20,726 --> 00:19:22,966
Well, let's see, there's
really not much to this file.

379
00:19:22,966 --> 00:19:24,796
It might look a little
cryptic, let me shrink

380
00:19:24,796 --> 00:19:27,166
down the font a little
bit, and re-open it,

381
00:19:27,166 --> 00:19:30,886
there's really not much
too it, 20, 30 lines here,

382
00:19:30,886 --> 00:19:32,696
and there's some
new syntax here,

383
00:19:32,696 --> 00:19:35,906
and all of this is actually
specific to Intel Inside.

384
00:19:36,186 --> 00:19:38,546
Because I'm running this
program on a computer

385
00:19:38,546 --> 00:19:40,456
that has Intel hardware
inside of it,

386
00:19:40,666 --> 00:19:43,056
these are literally
the instructions

387
00:19:43,236 --> 00:19:45,256
that that CPU understands.

388
00:19:45,256 --> 00:19:46,876
Well, let's see if we can't
make sense of some of this.

389
00:19:47,286 --> 00:19:50,456
So, at the top here, this seems
to be somewhat uninteresting,

390
00:19:50,456 --> 00:19:52,846
apparently the original
name of the file.

391
00:19:52,916 --> 00:19:56,116
Here's a dot section directive,
whatever that means, dot RO.

392
00:19:56,116 --> 00:19:58,336
What might this mean, dot RO?

393
00:19:58,886 --> 00:20:01,596
So, read-only, so
read-only data, apparently,

394
00:20:01,596 --> 00:20:04,126
comes here at the top, now
why does that make sense?

395
00:20:04,126 --> 00:20:06,716
Well, just below this
read-only data section,

396
00:20:06,936 --> 00:20:08,086
there's this cryptic string,

397
00:20:08,086 --> 00:20:10,916
but then something a little more
familiar, namely the string,

398
00:20:10,916 --> 00:20:13,656
Hello World, which I
hardcoded into my program,

399
00:20:13,876 --> 00:20:15,966
and is therefore itself
a read-only string.

400
00:20:15,996 --> 00:20:19,046
I could change what, I could
add strings to this program

401
00:20:19,046 --> 00:20:21,076
and such, but this is the
one I hardcoded in here,

402
00:20:21,436 --> 00:20:23,716
so there we have
this piece of data,

403
00:20:24,056 --> 00:20:25,766
dot text, what did this denote?

404
00:20:26,546 --> 00:20:27,756
If you can think
back a few weeks?

405
00:20:27,756 --> 00:20:30,216
What is the text
section of your program?

406
00:20:32,296 --> 00:20:33,746
Anyone recall?

407
00:20:33,746 --> 00:20:34,046
>>[Inaudible Speaker Comment]

408
00:20:34,046 --> 00:20:36,296
>>David: Say it a little louder.

409
00:20:36,296 --> 00:20:37,536
>>[Inaudible Speaker Comment]

410
00:20:37,536 --> 00:20:38,716
>>David: So, what
you were coding,

411
00:20:38,716 --> 00:20:41,216
so this was the actual program,

412
00:20:41,216 --> 00:20:42,656
the code that you
actually wrote.

413
00:20:42,836 --> 00:20:44,576
The review sections,
remember this rectangle?

414
00:20:44,576 --> 00:20:47,956
We had the heap, we had the
stack, we had initialized data,

415
00:20:47,956 --> 00:20:50,036
uninitialized data, and
those last two related

416
00:20:50,036 --> 00:20:52,446
to global variables, but
then above everything

417
00:20:52,446 --> 00:20:54,726
in that picture was
the text section,

418
00:20:54,976 --> 00:20:56,406
and text just meant
your program,

419
00:20:56,616 --> 00:20:58,646
so this actually makes
sense, because below text,

420
00:20:58,646 --> 00:21:01,046
dot text here, here
comes our program.

421
00:21:01,316 --> 00:21:03,916
So, this looks like, here comes
a functions that's apparently

422
00:21:03,916 --> 00:21:06,726
called Main, all right,
here is the main function,

423
00:21:06,726 --> 00:21:07,766
what goes on here?

424
00:21:07,766 --> 00:21:10,666
Well, these are these very
basic primitive that I alluded

425
00:21:10,666 --> 00:21:12,536
to earlier, and some
of them we can infer.

426
00:21:12,686 --> 00:21:16,236
Looks like there's a subtract
operation, an and operation,

427
00:21:16,276 --> 00:21:20,236
a move operation to move things
around in memory, add is in here

428
00:21:20,236 --> 00:21:22,366
as well, and L usually
denotes long,

429
00:21:22,366 --> 00:21:24,886
so these are actually
manipulating 32 bit values,

430
00:21:24,886 --> 00:21:26,876
in this case here,
but that's it.

431
00:21:26,876 --> 00:21:31,136
That's all it takes to implement
Hello World, and some years ago,

432
00:21:31,136 --> 00:21:32,886
if you wanted to
write Hello World,

433
00:21:33,236 --> 00:21:35,196
this was the stuff
that you wrote.

434
00:21:35,536 --> 00:21:37,646
Rewind a little further,
in what you actually wrote,

435
00:21:37,646 --> 00:21:40,116
were punch cards, right,
things with holes and what-not,

436
00:21:40,116 --> 00:21:42,046
that you actually fed
physically into a machine

437
00:21:42,046 --> 00:21:45,276
and it would parse it, so we've
come a long way, and certainly,

438
00:21:45,276 --> 00:21:47,236
if you think now to
implementing in PHP,

439
00:21:47,236 --> 00:21:50,076
like you did this past week
and will this next week,

440
00:21:50,336 --> 00:21:52,556
we've come a significantly
far way here,

441
00:21:52,826 --> 00:21:55,636
but it's all the same stuff
that's been around for 20,

442
00:21:55,696 --> 00:21:58,836
30 years, even since Apple
debuted that particular video.

443
00:21:59,056 --> 00:22:00,276
So, what's really going on?

444
00:22:00,276 --> 00:22:03,126
Well, let's take a look at some,
let's come back to this picture

445
00:22:03,126 --> 00:22:04,456
in a moment, and consider this.

446
00:22:05,026 --> 00:22:06,386
So, when you compile something,

447
00:22:06,386 --> 00:22:07,926
we've kind of been
waving our hands,

448
00:22:07,926 --> 00:22:09,836
or taking for granted some
of the things that go on,

449
00:22:09,836 --> 00:22:11,876
none of them are very
complicated, but we just,

450
00:22:11,876 --> 00:22:13,226
they all seem to happen at once.

451
00:22:13,576 --> 00:22:17,166
So, when you run GCC,
there's five steps that happen

452
00:22:17,166 --> 00:22:19,386
between the time you write
your code and run your code.

453
00:22:19,606 --> 00:22:21,436
So, the first step is
called pre-processing.

454
00:22:22,166 --> 00:22:24,996
So pre-processing, we've
actually used ourselves.

455
00:22:24,996 --> 00:22:29,436
Anytime you use slash
include or slash define,

456
00:22:29,436 --> 00:22:31,836
if you say that in certain
pieces of code we gave you,

457
00:22:32,076 --> 00:22:35,356
that's actually pre-processor
directives,

458
00:22:35,656 --> 00:22:39,016
and GCC first does a pass from
top to bottom, left to right

459
00:22:39,016 --> 00:22:41,516
over all of your code, and
anywhere it sees, sorry,

460
00:22:41,516 --> 00:22:46,466
I think I might have said slash,
sharp symbol, the sharp symbol

461
00:22:46,466 --> 00:22:48,996
and then include, or
sharp define, it says "Oh,

462
00:22:49,156 --> 00:22:50,666
this is a pre- processor
directive,

463
00:22:50,926 --> 00:22:52,686
let me do something with this.

464
00:22:52,686 --> 00:22:55,316
If its sharp define,
what do I want to do?

465
00:22:55,316 --> 00:22:57,076
I want to do a find
and replace for all

466
00:22:57,076 --> 00:22:59,186
of these global constants,
and replace it

467
00:22:59,186 --> 00:23:00,916
with the actual value
that's been defined.

468
00:23:01,136 --> 00:23:04,406
If it says sharp include, I want
to go grab the foo dot H file,

469
00:23:04,476 --> 00:23:08,016
open it, and literally paste
its contents into this location

470
00:23:08,016 --> 00:23:11,126
in the file, and then
proceed with step 2.

471
00:23:11,406 --> 00:23:14,836
Its step 2 when your code is
actually compiled, so we've kind

472
00:23:14,836 --> 00:23:16,026
of clumped all of these steps

473
00:23:16,026 --> 00:23:17,786
into just this one
big word compiled,

474
00:23:18,116 --> 00:23:19,826
but compiling is
really the second step.

475
00:23:20,166 --> 00:23:21,816
You've now got a
big piece of code,

476
00:23:21,816 --> 00:23:23,416
all of these sharp includes

477
00:23:23,416 --> 00:23:27,646
and sharp defines have actually
been pre-processed, so now,

478
00:23:27,646 --> 00:23:31,556
the compiler, GCC, takes all
of that code and converts it

479
00:23:31,556 --> 00:23:36,166
from your source code,
into assembly code,

480
00:23:36,166 --> 00:23:38,626
the stuff that we just saw
on the board a moment ago.

481
00:23:38,876 --> 00:23:40,986
And, then there's another step
that's happening behind the

482
00:23:40,986 --> 00:23:44,086
scenes, all of that assembly
code is then converted

483
00:23:44,086 --> 00:23:46,936
or assembled into object code,

484
00:23:47,246 --> 00:23:50,196
so object code is
ultimately the zeros and ones,

485
00:23:50,196 --> 00:23:53,766
so anytime you've seen, in your
directory's those dot O files,

486
00:23:54,126 --> 00:23:56,086
some of them you didn't
create yourself, right,

487
00:23:56,086 --> 00:23:58,856
and odds are you just wondered
what they were, on first glance,

488
00:23:58,856 --> 00:24:00,476
make clean, usually
deleted them,

489
00:24:00,616 --> 00:24:02,746
well what were all
those dot O files?

490
00:24:03,046 --> 00:24:05,056
Those were object files,

491
00:24:05,056 --> 00:24:07,746
those were the binary files
containing zeros and ones

492
00:24:07,746 --> 00:24:10,276
that were the result of
compile speller dot C,

493
00:24:10,536 --> 00:24:14,236
or dictionary dot C, or any
number of other dot C files

494
00:24:14,236 --> 00:24:15,136
that you happen to write.

495
00:24:15,566 --> 00:24:17,916
But, then there's one last
step, and some of you tripped

496
00:24:17,916 --> 00:24:20,526
over this early on, if you
didn't know what flags to use,

497
00:24:20,796 --> 00:24:24,036
sometimes you had to add that
dash L flag to your make file,

498
00:24:24,036 --> 00:24:27,806
or dash L to the end of you
GCC statement, and what did

499
00:24:27,806 --> 00:24:31,056
that mean or what's an
example of having to use dash L

500
00:24:31,406 --> 00:24:32,896
on the GCC command line?

501
00:24:33,396 --> 00:24:36,726
It's kind of a hint up
here in a big font, but.

502
00:24:36,726 --> 00:24:37,876
>>[Inaudible Speaker Comment]

503
00:24:37,876 --> 00:24:40,736
>>David: So, linking CS50, so
anytime you use get [inaudible]

504
00:24:41,106 --> 00:24:46,016
or get float, or get string, you
had to link N with dash L CS50,

505
00:24:46,016 --> 00:24:47,906
and it was kind of
stupid, but you always had

506
00:24:47,906 --> 00:24:50,876
to put the dash L
something at the very end

507
00:24:50,876 --> 00:24:53,336
of the command line, but
that's actually consistent

508
00:24:53,336 --> 00:24:56,406
with the reality that his
linking step is very much

509
00:24:56,406 --> 00:24:57,876
at the end of the whole process,

510
00:24:57,876 --> 00:25:00,096
it's sort of the last
thing GCC needs to know.

511
00:25:00,376 --> 00:25:01,226
So, what does that mean?

512
00:25:01,226 --> 00:25:05,186
Well, when you compile your
code, like let's take an example

513
00:25:05,186 --> 00:25:09,496
like Mario dot C. So when you
have Mario dot C and you compile

514
00:25:09,496 --> 00:25:13,326
that with GCC, you ultimately
get a Mario dot O file,

515
00:25:13,326 --> 00:25:16,036
but if you also use the
get [inaudible] function

516
00:25:16,036 --> 00:25:18,436
from CS50's library, that means

517
00:25:18,436 --> 00:25:22,036
that you'd had sharp
include CS50 dot H. So,

518
00:25:22,036 --> 00:25:23,966
the pre-processor
copied and pasted all

519
00:25:23,966 --> 00:25:25,506
of the dot H files contents

520
00:25:25,506 --> 00:25:27,406
and in there were the
function declarations,

521
00:25:27,406 --> 00:25:29,136
some global constants,
stuff like that,

522
00:25:29,646 --> 00:25:32,416
but behind the scenes what
was happening was GCC had

523
00:25:32,416 --> 00:25:37,026
to go compile and find CS50
dot C, and this is a bit

524
00:25:37,026 --> 00:25:40,116
of a white lie because we
optimized away this particular

525
00:25:40,116 --> 00:25:42,016
need, but essentially at
this point in the story,

526
00:25:42,016 --> 00:25:44,526
you have Mario dot C
compiled to Mario dot O,

527
00:25:44,686 --> 00:25:48,046
you had CS50 dot C somewhere
on the server compiled

528
00:25:48,046 --> 00:25:52,076
to CS50 dot O, so now you have
these two independent files,

529
00:25:52,126 --> 00:25:54,146
both of which contain a
bunch of zeros and ones,

530
00:25:54,336 --> 00:25:57,636
object code that Intel
CPU's understand.

531
00:25:57,866 --> 00:26:00,816
You now need to literally
merge these things together,

532
00:26:00,896 --> 00:26:04,976
link them together, so the last
step that GCC does for you,

533
00:26:05,196 --> 00:26:07,946
is it takes this dot O file,
it takes this dot O file,

534
00:26:07,946 --> 00:26:10,196
and it smashes them
together and lays them

535
00:26:10,196 --> 00:26:13,076
out in an appropriate way
in memory and on disc,

536
00:26:13,356 --> 00:26:17,016
so that what you have at the
end is just one executable file,

537
00:26:17,076 --> 00:26:19,716
called Mario, or
called Mario dot exe,

538
00:26:19,716 --> 00:26:21,466
or whatever the case
may be, and then you,

539
00:26:21,466 --> 00:26:23,416
the user, can finally run that.

540
00:26:23,606 --> 00:26:25,156
So, just to put a
picture to this,

541
00:26:25,156 --> 00:26:28,436
if we have the same
example, Hello dot C,

542
00:26:28,696 --> 00:26:30,086
it's a little blurry
on the screen here,

543
00:26:30,086 --> 00:26:31,776
but here are three
of those steps.

544
00:26:31,776 --> 00:26:35,486
If I compile Hello dot C,
here is that source code in C

545
00:26:35,746 --> 00:26:38,096
at the very top, that's, again,
my so-called source code,

546
00:26:38,346 --> 00:26:41,206
I compile it, what
that give me then,

547
00:26:41,206 --> 00:26:44,916
is this stuff called assembly
language, which is much closer

548
00:26:44,916 --> 00:26:47,216
to the CPU's language.

549
00:26:47,216 --> 00:26:50,076
It's much clearer to the
CPU what all this is,

550
00:26:50,296 --> 00:26:51,806
but it's much less
clear, dare say,

551
00:26:51,806 --> 00:26:53,726
to the human, what
all of this is.

552
00:26:53,776 --> 00:26:55,396
But, it's the same
code converted

553
00:26:55,396 --> 00:26:58,426
to the basic building
blocks; add, subtract, move,

554
00:26:58,696 --> 00:27:02,306
that the CPU itself understands,
but it's still more English

555
00:27:02,306 --> 00:27:04,676
or pseudo code than it
is actual zeros and ones,

556
00:27:04,956 --> 00:27:08,906
so the final step of assembling
is what actually generates,

557
00:27:09,096 --> 00:27:11,806
during the assembly process,
those zeros and ones,

558
00:27:11,806 --> 00:27:14,416
and these zeros and ones
are literally what happens,

559
00:27:14,576 --> 00:27:17,386
when you compile
Hello World on an X86,

560
00:27:17,486 --> 00:27:19,756
on an Intel computer, using GCC.

561
00:27:19,756 --> 00:27:21,356
Eventually, you get
those zeros and ones.

562
00:27:21,356 --> 00:27:23,716
You actually get a lot more
zeros and ones that would fit

563
00:27:23,716 --> 00:27:26,066
on the slide, but you
get patterns of zeros

564
00:27:26,066 --> 00:27:27,466
and ones just like that.

565
00:27:27,856 --> 00:27:30,366
So, how does the CPU go
about executing this program?

566
00:27:30,666 --> 00:27:33,826
Well, essentially it gets
fed piece by piece by piece.

567
00:27:33,826 --> 00:27:36,366
And even though we, the humans,
can't really see what's going

568
00:27:36,366 --> 00:27:39,456
on here, these patterns of
zeros and ones here are laid

569
00:27:39,456 --> 00:27:43,766
out in sequences of 8
bits, and you may recall

570
00:27:43,766 --> 00:27:45,186
that most CPU's we've talked

571
00:27:45,186 --> 00:27:47,996
about in this class
are 32 bits long, so,

572
00:27:47,996 --> 00:27:50,526
or are 32 bit architectures,
although when we moved

573
00:27:50,526 --> 00:27:52,886
to the Cloud, it became 64
bits, but for the most part,

574
00:27:53,136 --> 00:27:56,166
32 bit computers are
still very much in vogue,

575
00:27:56,536 --> 00:27:57,556
so what does this mean?

576
00:27:57,556 --> 00:28:00,786
Well this means a CPU can
understand 32 bits at a time,

577
00:28:01,106 --> 00:28:03,586
so when you run a program,
when you double-click its icon,

578
00:28:03,586 --> 00:28:07,146
run it at the command line,
32 bits at a time are fed

579
00:28:07,146 --> 00:28:11,086
from your program into the
CPU, inside of your computer,

580
00:28:11,086 --> 00:28:13,676
and some of those bits,
usually the first several bits,

581
00:28:14,016 --> 00:28:16,246
tell the CPU what
instruction to use,

582
00:28:16,456 --> 00:28:18,216
whether its add or
subtract or move.

583
00:28:18,506 --> 00:28:21,986
And then the last 8 bits and the
last 8 bits, after you have 8

584
00:28:21,986 --> 00:28:24,946
or 16 at the beginning, the last
bits usually reference these

585
00:28:24,946 --> 00:28:26,566
things called registers.

586
00:28:26,906 --> 00:28:29,516
So registers, we actually said
very early on in the course,

587
00:28:29,516 --> 00:28:31,846
are the smallest units
of memory that are use,

588
00:28:31,896 --> 00:28:34,266
that are actually useful
to a typical computer.

589
00:28:34,266 --> 00:28:38,306
Inside of the CPU, are
generally 32 or 64,

590
00:28:38,586 --> 00:28:41,656
things called registers, and
these are very small chunks

591
00:28:41,656 --> 00:28:43,826
of memory, it's kind of
like RAM, but much smaller,

592
00:28:44,116 --> 00:28:49,856
you have 32 bits of memory that
is, ultimately, the location

593
00:28:50,246 --> 00:28:52,156
that your operands are stored.

594
00:28:52,156 --> 00:28:52,966
What do I mean by that?

595
00:28:52,966 --> 00:28:54,206
Well, CPU, at the
end of the day,

596
00:28:54,206 --> 00:28:55,506
when it adds two
things together,

597
00:28:55,506 --> 00:28:57,876
it literally adds this
thing to this thing,

598
00:28:57,876 --> 00:29:01,126
it doesn't string 10 different
values together and add them all

599
00:29:01,126 --> 00:29:03,916
at once, it generally operates
on two inputs at a time,

600
00:29:04,226 --> 00:29:06,006
so usually two registers
are used,

601
00:29:06,006 --> 00:29:08,286
two little tiny storage
areas in memory,

602
00:29:08,556 --> 00:29:11,396
the CPU adds those values
together, puts the results

603
00:29:11,396 --> 00:29:14,776
in another register,
and the process repeats.

604
00:29:14,876 --> 00:29:17,996
So, in short, it's a huge
leap to make, mentally,

605
00:29:17,996 --> 00:29:21,716
from a PHP program, or even a
C program, down to this level

606
00:29:21,716 --> 00:29:24,656
of zeros and ones, but there's
been many years of development

607
00:29:24,696 --> 00:29:28,616
from this low level to punch
cards, to assembly code, to C,

608
00:29:28,616 --> 00:29:31,166
to PHP, so that everything
actually has been built

609
00:29:31,166 --> 00:29:32,826
on top of prior work.

610
00:29:32,826 --> 00:29:35,536
So, if you like sort of what
you're sensing is underneath the

611
00:29:35,536 --> 00:29:38,246
hood, the course in question,
here at Harvard at least,

612
00:29:38,246 --> 00:29:40,676
would be CS141, this
is computer hardware,

613
00:29:40,876 --> 00:29:42,606
and you can even get a
taste of this kind of stuff

614
00:29:42,606 --> 00:29:45,736
in like Physics 123, which is
a fun, hands-on course as well.

615
00:29:46,026 --> 00:29:47,716
So, there's a lot going
on there, but at the end

616
00:29:47,716 --> 00:29:51,156
of the day, it reduces to
these very basic principles.

617
00:29:51,526 --> 00:29:53,266
All right, so here's a
more concrete example

618
00:29:53,266 --> 00:29:56,156
where we actually have
multiple dot O files,

619
00:29:56,156 --> 00:29:57,366
or multiple dot C files.

620
00:29:57,366 --> 00:30:00,556
So, on the left here is just
a picture of compiling, again,

621
00:30:00,556 --> 00:30:03,446
something like Hello dot C,
but Hello dot C, remember,

622
00:30:03,486 --> 00:30:07,576
uses Print F, so that
previous, this pictorial,

623
00:30:07,756 --> 00:30:10,286
was a bit misleading, because
we didn't actually acknowledge

624
00:30:10,286 --> 00:30:13,306
that there was Print F and in
what library is Print F defined,

625
00:30:14,636 --> 00:30:16,236
and what header file
is it declared?

626
00:30:16,236 --> 00:30:17,696
>>[Inaudible Speaker Comment]

627
00:30:17,696 --> 00:30:19,026
>>David: Yeah, it's
a standard IO dot H,

628
00:30:19,026 --> 00:30:20,426
so that's because someone else,

629
00:30:20,426 --> 00:30:22,426
the people who wrote
the C compiler,

630
00:30:22,696 --> 00:30:25,386
wrote a file called
Standard IO dot C,

631
00:30:25,426 --> 00:30:27,916
and Standard I dot O dot H,

632
00:30:28,126 --> 00:30:31,296
and inside the dot C file is
the implementation of Print F,

633
00:30:31,296 --> 00:30:34,236
so I'm writing a program
called Hello dot C,

634
00:30:34,236 --> 00:30:36,956
and I'm using Print
F, yes, I compile it,

635
00:30:37,266 --> 00:30:40,346
that gives me assembly code,
yes, I then assemble it, and,

636
00:30:40,346 --> 00:30:42,796
again, all of this happens
for you with modern compilers,

637
00:30:42,796 --> 00:30:45,606
like GCC, and out of that
comes zeros and ones,

638
00:30:45,806 --> 00:30:47,566
well there's actually
another step here.

639
00:30:47,816 --> 00:30:51,996
In this first step, recall that
I included Standard IO dot H,

640
00:30:52,236 --> 00:30:54,566
so the fact that that
gets copied and pasted

641
00:30:54,566 --> 00:30:57,426
into my header file is a clue
to the compiler that, oh,

642
00:30:57,426 --> 00:31:01,206
I actually need the
functions that,

643
00:31:01,206 --> 00:31:04,736
the code that implements
Print F. Well, where is that?

644
00:31:05,016 --> 00:31:06,356
Typically, it's going
to be in a file

645
00:31:06,356 --> 00:31:10,026
like Standard IO dot C. This
lives elsewhere on the system,

646
00:31:10,026 --> 00:31:11,926
you don't even know
or care where it is,

647
00:31:12,296 --> 00:31:14,226
but what this inclusion,

648
00:31:14,226 --> 00:31:17,246
sharp include tells the compiler
essentially is, oh, you're going

649
00:31:17,246 --> 00:31:20,906
to need to use this code, so
let me find Standard IO dot C,

650
00:31:21,146 --> 00:31:23,686
let me compile it, or make
sure it's already compiled,

651
00:31:23,686 --> 00:31:26,966
that gives me assembly code,
then I need to assemble that,

652
00:31:26,966 --> 00:31:29,476
which gives me the zeros and
ones, and then that final step,

653
00:31:29,476 --> 00:31:33,246
linking, is to actually merge
my bits with the standard bits,

654
00:31:33,656 --> 00:31:36,006
merge them together
and get one executable,

655
00:31:36,006 --> 00:31:39,296
the Hello binary,
or Hello dot exe.

656
00:31:39,596 --> 00:31:44,686
So, why, though, have you
never had to do dash L STD IO?

657
00:31:45,786 --> 00:31:48,156
Since it seems that that
was the necessary magic

658
00:31:48,186 --> 00:31:50,176
to make this work
for CS50's library.

659
00:31:50,176 --> 00:31:55,156
Why do you not need to
type dash L Standard IO?

660
00:31:55,246 --> 00:31:56,856
Feels like there's an
inconsistency here.

661
00:31:57,356 --> 00:32:00,966
>>[Inaudible Speaker Comment]

662
00:32:01,466 --> 00:32:02,696
>>David: Yeah, it's
common enough

663
00:32:02,696 --> 00:32:04,216
that the compiler
already links it in.

664
00:32:04,216 --> 00:32:04,966
So, what does that mean?

665
00:32:05,176 --> 00:32:06,786
Well, they're standard
for a reason.

666
00:32:06,786 --> 00:32:09,576
The fact that it's
Standard IO, Standard LIB,

667
00:32:09,836 --> 00:32:12,386
means that these are standard
enough that you just don't have

668
00:32:12,386 --> 00:32:14,956
to provide that linker flag,
it's essentially provided

669
00:32:14,956 --> 00:32:16,246
for you automatically.

670
00:32:16,466 --> 00:32:19,006
So that's all that's really been
happening underneath the hood

671
00:32:19,006 --> 00:32:23,976
there, is this whole process,
but largely unbeknownst to you,

672
00:32:23,976 --> 00:32:26,646
and just as an aside,
there's one detail that's kind

673
00:32:26,646 --> 00:32:29,106
of interesting to realize,
especially if you're a PC user,

674
00:32:29,106 --> 00:32:32,766
those of you with PC's, have you
ever seen or heard of DLL files?

675
00:32:33,566 --> 00:32:34,796
Anyone? Kind of sort of?

676
00:32:34,796 --> 00:32:39,206
Okay, so what are they,
if you've all seen them?

677
00:32:39,206 --> 00:32:39,276
>>[Inaudible Speaker Comment]

678
00:32:39,276 --> 00:32:41,186
>>David: Okay good,
[inaudible] pedagogical answer.

679
00:32:41,186 --> 00:32:41,666
What are they?

680
00:32:41,956 --> 00:32:45,346
So, what is a DLL file, so that
stands for dynamic link library,

681
00:32:45,586 --> 00:32:48,596
and without going into too
much WIN32 or Windows detail,

682
00:32:48,866 --> 00:32:50,936
that's the same idea as
what's going on here.

683
00:32:50,936 --> 00:32:53,916
There's two ways to
link programs together.

684
00:32:54,196 --> 00:32:55,916
You can either statically
link them,

685
00:32:56,006 --> 00:32:57,496
or you can dynamically
link them.

686
00:32:57,496 --> 00:32:58,286
Well, what does that mean?

687
00:32:58,576 --> 00:33:02,156
This picture here is describing
static linking, and in fact,

688
00:33:02,156 --> 00:33:04,776
it's kind of a white
lie depiction

689
00:33:04,776 --> 00:33:06,586
of what you guys have been
doing, because generally

690
00:33:06,586 --> 00:33:08,696
if you want this
picture to happen,

691
00:33:09,036 --> 00:33:11,536
you have to pass GCC
the dash static flag,

692
00:33:11,796 --> 00:33:14,896
and say statically compile
this, but that's okay here,

693
00:33:15,436 --> 00:33:18,646
because the take away is that
this means static compiling,

694
00:33:18,646 --> 00:33:21,516
which means, when you
get this executable file,

695
00:33:21,556 --> 00:33:24,986
your Mario program, you Hello
program, in theory, you can move

696
00:33:24,986 --> 00:33:28,336
that program from computer to
computer to computer and so long

697
00:33:28,336 --> 00:33:30,806
as you move it to another
computer that has the same CPU,

698
00:33:30,996 --> 00:33:33,156
you can run at the command
line, or double-click it there,

699
00:33:33,156 --> 00:33:34,716
and it will simply work.

700
00:33:35,006 --> 00:33:37,926
By contrast, if you
dynamically link a program,

701
00:33:38,156 --> 00:33:39,586
it's not necessarily
going to work

702
00:33:39,586 --> 00:33:40,956
if you move it to
another computer.

703
00:33:40,956 --> 00:33:42,146
Any of you who have ever tried

704
00:33:42,146 --> 00:33:44,626
to copy programs you've
installed on your computer,

705
00:33:44,916 --> 00:33:48,056
Microsoft Word or Photoshop
or AOL instant messenger,

706
00:33:48,366 --> 00:33:52,456
its generally not sufficient to
click and drag the dot exe file,

707
00:33:52,456 --> 00:33:54,056
and put it on your
other computer,

708
00:33:54,056 --> 00:33:55,596
or put it on our
friends computer,

709
00:33:55,786 --> 00:33:58,526
because there's usually a
lot more files that comprise

710
00:33:58,686 --> 00:34:01,626
that actual program, many
of them these dot DLL files.

711
00:34:01,916 --> 00:34:05,256
So, the other way to link
programs together is not

712
00:34:05,296 --> 00:34:09,306
to create one binary that is
the result of combining bits

713
00:34:09,306 --> 00:34:11,176
from the left with
bits from the right,

714
00:34:11,616 --> 00:34:14,516
but rather you just take
your own bits from the left,

715
00:34:14,516 --> 00:34:17,026
and you include inside
of the binary,

716
00:34:17,026 --> 00:34:21,566
inside of the executable,
essentially the path to files

717
00:34:21,566 --> 00:34:25,426
on the local hard drive that
contain Standard dot IO,

718
00:34:25,426 --> 00:34:29,966
Standard IO bits, or the CS50
library bits, in other words,

719
00:34:29,966 --> 00:34:33,986
its dynamic in that you need,
in order to run this program,

720
00:34:33,986 --> 00:34:37,626
you need to have those other
files installed on the computer.

721
00:34:37,936 --> 00:34:40,086
So, this is good and bad.

722
00:34:40,086 --> 00:34:42,836
What is a upside, based
on that definition

723
00:34:42,836 --> 00:34:44,476
of statically linking
your programs?

724
00:34:44,646 --> 00:34:46,776
What's one good thing about
it that comes to mind.

725
00:34:47,166 --> 00:34:47,346
Yes?

726
00:34:47,496 --> 00:34:49,886
>>It takes up less space because
you don't have repetition

727
00:34:49,886 --> 00:34:51,446
of the Standard IO in every.

728
00:34:51,596 --> 00:34:52,636
>>David: If you statically link.

729
00:34:53,386 --> 00:34:53,856
>>Right.

730
00:34:53,856 --> 00:34:55,176
>>David: If you, that's dynamic.

731
00:34:56,546 --> 00:34:58,646
Okay. So, an advantage
of dynamic linking is

732
00:34:58,646 --> 00:35:00,546
that you don't have
this redundancy, right?

733
00:35:00,546 --> 00:35:03,156
Because if you're dynamically
linking, you just have one copy

734
00:35:03,156 --> 00:35:08,206
of Standard IO dot O or CS50 dot
O, and you dynamically linked

735
00:35:08,206 --> 00:35:10,296
against them so that you
don't make copies of all

736
00:35:10,296 --> 00:35:12,116
of the bits comprised
in those libraries again

737
00:35:12,116 --> 00:35:12,756
and again and again.

738
00:35:13,176 --> 00:35:14,666
All right, so that's one
advantage, and in fact,

739
00:35:14,666 --> 00:35:17,026
that's why Microsoft does it
with so many of their products,

740
00:35:17,246 --> 00:35:19,036
because you can have
these shared libraries,

741
00:35:19,036 --> 00:35:20,426
these dynamic link libraries,

742
00:35:20,596 --> 00:35:23,036
and just not waste disc
space unnecessarily.

743
00:35:23,296 --> 00:35:24,356
So, what's now an upside

744
00:35:24,356 --> 00:35:26,376
of statically linking
your programs together?

745
00:35:27,516 --> 00:35:28,446
Anyone at all?

746
00:35:29,066 --> 00:35:29,226
Okay.

747
00:35:29,226 --> 00:35:32,336
>>You don't have the
problem that I always get

748
00:35:32,336 --> 00:35:35,856
if you're missing a DLL file
and nothing [inaudible].

749
00:35:35,976 --> 00:35:36,076
>>David: Good.

750
00:35:36,186 --> 00:35:39,066
So you don't have the problem
of double-clicking that program,

751
00:35:39,066 --> 00:35:41,466
or running it at the command
line and being told, cannot run,

752
00:35:41,466 --> 00:35:44,696
cannot find foo dot DLL or
some other program like that.

753
00:35:45,016 --> 00:35:47,106
It's actually self-contained
program,

754
00:35:47,316 --> 00:35:49,506
a program that's completely
portable, you can copy it,

755
00:35:49,506 --> 00:35:52,546
move it anywhere, and it
simply works anywhere else.

756
00:35:52,776 --> 00:35:54,876
So, now you, so we bring
this up for two reasons.

757
00:35:54,876 --> 00:35:57,326
So, one, most everything
you've learned in this course,

758
00:35:57,326 --> 00:35:59,486
you can certainly apply
to Windows programming,

759
00:35:59,486 --> 00:36:02,406
so Microsoft and its
compilers generally add a bunch

760
00:36:02,406 --> 00:36:05,666
of new syntax and conventions
that you don't see necessarily

761
00:36:05,666 --> 00:36:08,166
in the Lennox world, for
instance, there's different ways

762
00:36:08,166 --> 00:36:11,216
of writing a main function when
you code on a Windows platform,

763
00:36:11,406 --> 00:36:13,276
and that's just because
it's a Microsoft convention.

764
00:36:13,276 --> 00:36:15,356
But, all of the basics
are still actually there,

765
00:36:15,356 --> 00:36:17,436
and MAC OS is much more
similar say to Lennox,

766
00:36:17,766 --> 00:36:19,746
which is what you guys have
been dabbling in thus far.

767
00:36:19,996 --> 00:36:21,196
But, what tends to happen,

768
00:36:21,196 --> 00:36:23,256
especially at final project
time, or when you're just

769
00:36:23,256 --> 00:36:25,746
out in the real world, or just
trying to do something for fun

770
00:36:25,746 --> 00:36:27,946
on your computer, you'll
run into these kinds

771
00:36:27,946 --> 00:36:31,006
of stupid headaches, like why
can't I run this program someone

772
00:36:31,006 --> 00:36:31,446
sent me?

773
00:36:31,636 --> 00:36:33,256
Well, if they dynamically
linked it,

774
00:36:33,256 --> 00:36:35,136
it means they didn't
send you all of the files

775
00:36:35,136 --> 00:36:35,986
that are prerequisite.

776
00:36:36,226 --> 00:36:38,026
If you are actually
running, writing a program

777
00:36:38,026 --> 00:36:40,746
that you want other people to
be able to use, you might need

778
00:36:40,746 --> 00:36:42,976
to statically link things,
all be it at the expense

779
00:36:43,246 --> 00:36:45,726
of making things much
bigger than it needs to be.

780
00:36:46,306 --> 00:36:49,226
All right, so if we now have
programs that are being compiled

781
00:36:49,336 --> 00:36:52,076
down to zeros and ones, we
have these things called CPU's,

782
00:36:52,076 --> 00:36:53,906
which are the brains of your
computer, which know how

783
00:36:53,906 --> 00:36:55,556
to understand those
zeros and ones,

784
00:36:55,906 --> 00:36:59,836
how do we actually start storing
these things in the real world?

785
00:36:59,916 --> 00:37:02,106
Up until now, we've
completely taken for granted

786
00:37:02,366 --> 00:37:04,856
that we have hard drives
or solid state drives,

787
00:37:04,856 --> 00:37:08,546
or flash drives, or floppy's or
CD's, there's just some means

788
00:37:08,546 --> 00:37:10,716
of encoding data on computers.

789
00:37:10,716 --> 00:37:11,526
So how is that done?

790
00:37:11,526 --> 00:37:13,816
If you have a hard drive,
the mechanical type,

791
00:37:13,816 --> 00:37:15,936
which is in most of your
laptops still these days,

792
00:37:16,156 --> 00:37:17,856
how is data stored
inside that thing?

793
00:37:18,636 --> 00:37:19,176
Anyone know?

794
00:37:19,386 --> 00:37:20,746
What's inside of a hard drive?

795
00:37:20,746 --> 00:37:21,286
>>[Inaudible Speaker Comment]

796
00:37:21,286 --> 00:37:23,386
>>David: what's that?

797
00:37:23,386 --> 00:37:23,836
>>[Inaudible Speaker Comment]

798
00:37:23,836 --> 00:37:25,716
>>David: Plates, okay,
so these metal plates,

799
00:37:25,716 --> 00:37:26,816
otherwise called platters,

800
00:37:26,856 --> 00:37:28,946
they're these little
circular things, and in fact,

801
00:37:28,946 --> 00:37:31,506
if you put your ear to a
desktop computer or a lap top,

802
00:37:31,506 --> 00:37:32,996
you'll often hear
something spinning,

803
00:37:33,216 --> 00:37:34,626
and that is these platters.

804
00:37:34,626 --> 00:37:37,516
So, most of you might know,
at least, what a record is,

805
00:37:37,516 --> 00:37:40,886
or a phonograph player, right,
it's actually very similar idea.

806
00:37:41,236 --> 00:37:44,986
You have data encoded in
circles, on that disc,

807
00:37:44,986 --> 00:37:47,196
and the thing spins so
that the computer can read

808
00:37:47,196 --> 00:37:51,696
in those bits at, from
start to finish, so,

809
00:37:51,956 --> 00:37:54,066
I think a little
picture, or better

810
00:37:54,066 --> 00:37:57,026
yet a little video can do this
more justice than my own words,

811
00:37:57,026 --> 00:37:58,186
so let me go ahead and open

812
00:37:58,186 --> 00:38:02,796
up this little demo
here, for hard drives.

813
00:38:03,406 --> 00:38:04,956
And let's see if this
doesn't elucidate.

814
00:38:06,256 --> 00:38:08,776
>>The hard drive is
where your PC stores most

815
00:38:08,776 --> 00:38:10,166
of its permanent data.

816
00:38:10,476 --> 00:38:12,476
To do that, the data travels

817
00:38:12,546 --> 00:38:14,696
from RAM along the
software signals

818
00:38:14,696 --> 00:38:17,106
that tell the hard drive
how to store that data.

819
00:38:17,906 --> 00:38:20,316
The hard drive circuits
translate those signals

820
00:38:20,486 --> 00:38:22,016
into voltage fluctuations.

821
00:38:22,736 --> 00:38:25,476
These, in turn, control the
hard drives moving parts,

822
00:38:26,026 --> 00:38:28,976
some of the few moving parts
left in the modern computer.

823
00:38:29,706 --> 00:38:31,356
Some of the signals
control a motor,

824
00:38:31,466 --> 00:38:33,206
which spins metal-coated
platters.

825
00:38:33,946 --> 00:38:36,626
Your data is actually
stored on these platters.

826
00:38:37,266 --> 00:38:40,476
Other signals move the
read-write head to read

827
00:38:40,476 --> 00:38:41,796
or write data on the platters.

828
00:38:42,536 --> 00:38:45,156
This machinery is so precise,

829
00:38:45,276 --> 00:38:47,486
that human hair couldn't
even pass between the heads

830
00:38:47,486 --> 00:38:48,396
and spinning platters.

831
00:38:49,016 --> 00:38:51,546
Yet, it all works
at terrific speeds.

832
00:38:52,426 --> 00:38:53,976
>>David: In fact, when
it says terrific speeds,

833
00:38:53,976 --> 00:38:55,736
how fast does this
particular hard drive spin?

834
00:38:55,736 --> 00:38:57,276
If you've ever noticed
on your own computer

835
00:38:57,276 --> 00:38:58,516
or at Best Buy when buying one?

836
00:38:58,516 --> 00:39:00,076
>>[Inaudible Speaker Comment]

837
00:39:00,076 --> 00:39:05,326
>>David: Its 50, so 54 hundred
RPM's, so rotations per minute,

838
00:39:05,326 --> 00:39:08,216
or 72 hundred RPM or if you
buy really expensive discs,

839
00:39:08,256 --> 00:39:11,986
10 thousand RPM's, that means
its spinning several thousand

840
00:39:11,986 --> 00:39:15,016
times per minute and on each
revolution can some data

841
00:39:15,016 --> 00:39:16,666
actually be read
off, and the reason

842
00:39:16,666 --> 00:39:19,386
for the multiple platters there
is, as you may have guessed,

843
00:39:19,386 --> 00:39:21,586
just for efficiency, like
why just have one platter,

844
00:39:21,586 --> 00:39:22,386
if you can have four.

845
00:39:22,386 --> 00:39:25,246
That means you can store four
times as much data on there.

846
00:39:25,436 --> 00:39:27,916
Well, let's go into slightly
more technical detail,

847
00:39:27,916 --> 00:39:30,676
still quite accessible,
with this follow up version.

848
00:39:31,356 --> 00:39:34,226
>>Let's look at what we
just saw in slow motion.

849
00:39:35,036 --> 00:39:40,456
When a brief pulse of
electricity is sent to the read-

850
00:39:40,776 --> 00:39:45,706
write head, it flips on
a tiny electro magnet

851
00:39:45,826 --> 00:39:46,786
for a fraction of a section.

852
00:39:46,786 --> 00:39:49,256
The magnet creates a field,
which changes the polarity

853
00:39:49,256 --> 00:39:52,726
of a tiny tiny portion
of the metal particles,

854
00:39:52,726 --> 00:39:53,436
which coat each platter surface.

855
00:39:53,466 --> 00:39:54,726
A pattern series of these tiny
[inaudible] varies on the disc,

856
00:39:54,756 --> 00:39:55,686
represents a single bit of data

857
00:39:55,716 --> 00:39:56,976
in the binary number
system used by computers.

858
00:39:57,046 --> 00:39:59,776
Now if the current
is sent one way

859
00:39:59,776 --> 00:40:00,286
through the read-write
[inaudible],

860
00:40:00,706 --> 00:40:02,606
the areas polarized
in one direction.

861
00:40:02,606 --> 00:40:05,246
If the current is sent in
the opposite direction,

862
00:40:05,246 --> 00:40:08,766
the polarization is reversed.

863
00:40:08,766 --> 00:40:13,496
How do you get data
off the hard disc?

864
00:40:13,496 --> 00:40:14,416
Just reverse the process.

865
00:40:14,766 --> 00:40:16,366
So, if the particles on the
disc that get the current

866
00:40:16,396 --> 00:40:17,086
in the read-write head moving.

867
00:40:17,116 --> 00:40:18,436
Put together millions of
these magnetized segments,

868
00:40:18,466 --> 00:40:18,976
and you've got a file.

869
00:40:19,526 --> 00:40:22,806
Now, the pieces of a single
file may be scattered all

870
00:40:22,806 --> 00:40:25,136
over a drive's platters,
kind of like the mess

871
00:40:25,276 --> 00:40:26,696
of papers on your desk.

872
00:40:26,696 --> 00:40:30,716
So, a special extra file keeps
track of where everything is.

873
00:40:30,896 --> 00:40:32,786
Don't you wish you had
something like that?

874
00:40:33,386 --> 00:40:35,346
>>David: Sorry, they
like to add that.

875
00:40:35,806 --> 00:40:37,606
So, but, one of the things
I've actually always thought is

876
00:40:37,606 --> 00:40:40,016
really neat about Computer
Science or about Engineering is

877
00:40:40,016 --> 00:40:43,216
that unlike some of the physical
sciences like Biology and such,

878
00:40:43,446 --> 00:40:46,866
like we, humanity, actually
understand everything that goes

879
00:40:46,906 --> 00:40:50,526
on inside of this computer, and
frankly, even though I, frankly,

880
00:40:50,526 --> 00:40:52,946
would not know how to go
about making a hard drive,

881
00:40:53,006 --> 00:40:55,066
like what metal to use, how
to wire all of this together,

882
00:40:55,066 --> 00:40:57,706
like that's certainly not my
forte, what's kind of neat,

883
00:40:57,706 --> 00:40:59,436
I think is that at
the end of the day,

884
00:40:59,616 --> 00:41:03,206
I mean all of the comprehension
of how this stuff works

885
00:41:03,206 --> 00:41:06,736
in your pocket and on your desk
is all quite accessible, right?

886
00:41:06,736 --> 00:41:08,216
Even if some of that
was over your head,

887
00:41:08,216 --> 00:41:10,716
hopefully you realize, oh,
well if I just have some way

888
00:41:10,716 --> 00:41:13,016
of laying out particles,
magnetic particles like this,

889
00:41:13,336 --> 00:41:15,246
let's call that a one,
if its instead like this,

890
00:41:15,366 --> 00:41:16,476
let's call that a zero.

891
00:41:16,476 --> 00:41:18,846
Well, as soon as you have
means of encoding two pieces

892
00:41:18,846 --> 00:41:21,806
of information, we've seen
from week one and two,

893
00:41:21,956 --> 00:41:24,086
how you can actually start
to represent numbers,

894
00:41:24,086 --> 00:41:25,566
how you can start
to represent letters

895
00:41:25,566 --> 00:41:28,186
and then ultimately
actual computer programs.

896
00:41:28,186 --> 00:41:32,206
So, let's go ahead and
take a look at this one.

897
00:41:32,206 --> 00:41:32,856
Oh, all right.

898
00:41:32,856 --> 00:41:36,116
So I always offer this to, just
so, if that was a little so much

899
00:41:36,116 --> 00:41:38,956
like this, we also have
something for those

900
00:41:38,956 --> 00:41:41,346
who still worry that they're
among those less comfortable,

901
00:41:41,386 --> 00:41:42,876
aw damn, won't work.

902
00:41:43,266 --> 00:41:45,106
All right, little fail
there, never mind.

903
00:41:45,276 --> 00:41:47,986
It's going to be a,
oh I won't tell you.

904
00:41:48,366 --> 00:41:51,386
I'll tell you, it was
going, no I won't tell you.

905
00:41:51,386 --> 00:41:53,816
Okay, we'll leave it at that.

906
00:41:53,816 --> 00:41:54,616
Okay, well, how about this.

907
00:41:54,716 --> 00:41:55,586
So this was just for fun.

908
00:41:55,586 --> 00:41:57,656
I'm not going to try to
justify this with any academics,

909
00:41:57,656 --> 00:42:00,346
it's just kind of interesting
I think, because CD's work

910
00:42:00,346 --> 00:42:02,956
in very, in a very similar
way, but clearly all

911
00:42:02,956 --> 00:42:04,936
of us have the means of
burning CD's, or DVD's

912
00:42:04,996 --> 00:42:09,796
and sometimes you can do this
multiple times, if its CD-RW's

913
00:42:09,796 --> 00:42:12,896
or DVD-RW's, so this
ones, again,

914
00:42:12,896 --> 00:42:14,676
just to help you understand
what you've been doing

915
00:42:14,676 --> 00:42:16,966
when you actually burn
a disc or music or data.

916
00:42:17,046 --> 00:42:19,136
>>How can a CD-ROM
disc hold so much more

917
00:42:19,136 --> 00:42:20,356
than a conventional disc?

918
00:42:21,046 --> 00:42:24,466
The CD drive reads data with
a beam of light so narrow

919
00:42:24,466 --> 00:42:27,296
that the information can be
squeezed together much tighter.

920
00:42:27,346 --> 00:42:30,456
You see, a laser [inaudible]
creates this concentrated beam

921
00:42:30,956 --> 00:42:31,906
of light.

922
00:42:32,566 --> 00:42:35,956
The light travels through a
prism, then through a lens

923
00:42:35,956 --> 00:42:38,736
and magnetic coil that
focus the beam even more.

924
00:42:39,386 --> 00:42:42,986
On the underside of the contact
disc its self, are millions

925
00:42:42,986 --> 00:42:45,076
of tiny bumps called PITS.

926
00:42:45,466 --> 00:42:46,926
That's right, the
bumps are called PITS.

927
00:42:47,166 --> 00:42:51,086
The same surface has
smooth areas called LANDS.

928
00:42:51,976 --> 00:42:54,396
These PITS and LANDS
are translated

929
00:42:54,396 --> 00:42:58,776
into the binary language of bits
and bytes used by the computer.

930
00:42:59,426 --> 00:43:02,366
>>David: And, part two of
that same sequence is this.

931
00:43:04,696 --> 00:43:06,556
>>Patterns of PITS
and LANDS are laid

932
00:43:06,556 --> 00:43:08,186
out along a continuous spiral.

933
00:43:08,796 --> 00:43:11,886
As the disc turns, a precise
motor keeps the laser beam

934
00:43:11,886 --> 00:43:12,756
in place on the path.

935
00:43:13,276 --> 00:43:15,866
Where the laser beam hits a
PIT, the light is scattered.

936
00:43:16,466 --> 00:43:17,636
But where it hits a LAND,

937
00:43:17,636 --> 00:43:22,556
the beam is reflected straight
back along its original path.

938
00:43:23,076 --> 00:43:24,546
The light enters
the prism again,

939
00:43:24,576 --> 00:43:26,016
but this time it's
reflected at a 90 degree angle

940
00:43:26,046 --> 00:43:26,976
and strikes the device
called a diode.

941
00:43:27,076 --> 00:43:30,776
The diode creates an electric
pulse each time the light

942
00:43:30,896 --> 00:43:31,206
hit it.

943
00:43:31,726 --> 00:43:34,756
So, when the laser hit a
PIT, no light bounces back.

944
00:43:35,226 --> 00:43:36,726
When it strikes a LAND,

945
00:43:36,726 --> 00:43:38,906
the diode sees the
reflection and sends a pulse.

946
00:43:39,636 --> 00:43:42,546
These blanks and pulses
are sent to the computer,

947
00:43:42,546 --> 00:43:44,656
which interprets them as a
pattern of zeros and ones.

948
00:43:45,276 --> 00:43:46,976
In other words, into
binary code.

949
00:43:47,646 --> 00:43:49,426
>>David: So that's how
real CD's are actually made

950
00:43:49,426 --> 00:43:51,706
when you buy them from store,
with a game on it, or you buy it

951
00:43:51,706 --> 00:43:53,696
from a music store
with music on it.

952
00:43:53,956 --> 00:43:55,926
When you actually
burn discs yourself,

953
00:43:55,926 --> 00:43:58,096
what those things really are,
they're really dirt cheap,

954
00:43:58,096 --> 00:44:00,146
they're just pieces of
plastic, and you can see this

955
00:44:00,146 --> 00:44:02,076
if you just run your
fingernails or something over it

956
00:44:02,076 --> 00:44:04,186
and start scrap, do this on
something you didn't buy,

957
00:44:04,726 --> 00:44:07,836
you scrape the label off of it,
generally there's just plastic,

958
00:44:08,076 --> 00:44:09,786
there's a label for
marketing purposes,

959
00:44:09,786 --> 00:44:11,986
and then there's some kind
of dye on the other side,

960
00:44:11,986 --> 00:44:15,866
and that dye, which usually
looks green or gold or purple,

961
00:44:16,066 --> 00:44:18,426
what happens is, when
you do burn a CD,

962
00:44:18,496 --> 00:44:20,166
some laser light
insider of your computer,

963
00:44:20,166 --> 00:44:23,186
essentially just distorts
portions of that dye,

964
00:44:23,186 --> 00:44:24,416
here and there and there,

965
00:44:24,646 --> 00:44:27,156
so that the next time a
laser light actually shines

966
00:44:27,156 --> 00:44:29,226
on that CD, if it
bounces right back,

967
00:44:29,396 --> 00:44:31,576
that means it's a zero
or a one, whichever.

968
00:44:31,826 --> 00:44:34,336
But if it actually hits the
part where you distorted the dye

969
00:44:34,336 --> 00:44:35,856
and kind of veers
off somewhere else,

970
00:44:35,996 --> 00:44:38,846
well that representing the
opposite, the one or the zero,

971
00:44:38,936 --> 00:44:41,346
so its still, again,
fairly, rudimentary.

972
00:44:41,346 --> 00:44:43,206
Again, not sure how I
even myself would start

973
00:44:43,206 --> 00:44:46,146
to build something like this,
but the comprehensions probably,

974
00:44:46,216 --> 00:44:47,276
probably quite accessible.

975
00:44:47,276 --> 00:44:49,256
In fact, one of your
classmates, on the survey's,

976
00:44:49,256 --> 00:44:52,926
said a comment along the line
of, when I come into lecture,

977
00:44:52,926 --> 00:44:55,726
I always exit realizing how
much I don't actually know,

978
00:44:55,726 --> 00:44:58,756
and I think they actually said
that somewhat out of frustration

979
00:44:58,756 --> 00:45:01,366
or just being a bit overwhelmed,
which is totally fine,

980
00:45:01,366 --> 00:45:03,916
and understandable,
but I mean, frankly,

981
00:45:03,916 --> 00:45:05,626
that's kind of what
makes stuff like this,

982
00:45:05,716 --> 00:45:06,876
I think, a little exciting.

983
00:45:06,876 --> 00:45:09,186
I mean we could go off in
so many different directions

984
00:45:09,186 --> 00:45:11,336
at this point in the
semester, we're not going to,

985
00:45:11,626 --> 00:45:14,216
but even today, it's meant to
just give you a teaser that,

986
00:45:14,216 --> 00:45:16,786
once you start to understand
what's underneath the hood,

987
00:45:17,056 --> 00:45:18,756
just a lot more stuff
starts to makes sense.

988
00:45:18,756 --> 00:45:20,866
Google is going to be your
friend, and Wikipedia is going

989
00:45:20,866 --> 00:45:22,386
to be your friend,
and even those of you

990
00:45:22,386 --> 00:45:24,686
who don't consider yourself
Engineers, and are going to go

991
00:45:24,686 --> 00:45:27,846
through OCS and get consulting
jobs or iBanking or whatever,

992
00:45:28,196 --> 00:45:30,986
what's really kind of cool about
having some technical knowhow,

993
00:45:30,986 --> 00:45:33,636
whether you are or not a
computer science major,

994
00:45:33,946 --> 00:45:36,526
is that you can start to
tell, much more effectively,

995
00:45:36,526 --> 00:45:39,246
when people are BS'ing
in the real world.

996
00:45:39,246 --> 00:45:41,346
All right, there's a lot of
people who kind of talk, talk,

997
00:45:41,346 --> 00:45:44,306
talk about technology and
this and that, and hopefully,

998
00:45:44,506 --> 00:45:47,066
and after a couple of more
weeks of this class, you know,

999
00:45:47,066 --> 00:45:50,126
you'll start to be able to
see who's quite just talking

1000
00:45:50,126 --> 00:45:53,026
and not really understanding
what it is that they're saying.

1001
00:45:53,026 --> 00:45:54,516
You'd be surprised how
often this happens.

1002
00:45:54,516 --> 00:45:58,066
Frankly, go into Best Buy or a
technology store, and already,

1003
00:45:58,066 --> 00:45:59,686
even just after seeing
videos like that,

1004
00:45:59,686 --> 00:46:01,826
might you understand what's
being sold a bit better

1005
00:46:02,076 --> 00:46:04,006
than the person selling
it to you sometimes.

1006
00:46:04,256 --> 00:46:06,986
All right, so you have one of
these things called floppy discs

1007
00:46:06,986 --> 00:46:09,896
in your hand, might not have
used, or seen these things

1008
00:46:09,896 --> 00:46:12,086
in some time, they do
actually still sell them,

1009
00:46:12,336 --> 00:46:13,516
they only store how much data?

1010
00:46:13,516 --> 00:46:15,326
>>[Inaudible Speaker Comment]

1011
00:46:15,326 --> 00:46:16,746
>>David: Yeah, 1.44 megabytes,

1012
00:46:16,746 --> 00:46:19,496
so a million bytes
or 8 million bits.

1013
00:46:19,496 --> 00:46:20,936
Well, let's take a
look at what's inside,

1014
00:46:20,936 --> 00:46:23,386
and you'll find it quite
similar to what's inside

1015
00:46:23,386 --> 00:46:26,176
of a hard drive, but much
cheaper and much slower.

1016
00:46:27,076 --> 00:46:28,916
>>The floppy drive you
see here works with three

1017
00:46:28,916 --> 00:46:30,306
and half inch floppy discs.

1018
00:46:31,106 --> 00:46:35,826
They're the type most often
used to carry new programs,

1019
00:46:35,826 --> 00:46:40,696
save data, or to move files
from one PC to another.

1020
00:46:40,856 --> 00:46:46,056
The part of the disc we see
is actually just the hard

1021
00:46:46,256 --> 00:46:46,776
plastic shell.

1022
00:46:46,876 --> 00:46:51,876
The working disc, which
is inside, is protected

1023
00:46:52,226 --> 00:46:52,886
by a sliding metal shutter.

1024
00:46:52,916 --> 00:46:54,176
This thin inner disc, called
the cookie, is coated not

1025
00:46:54,206 --> 00:46:55,346
with chocolate chips, but
with a very thin layer

1026
00:46:55,376 --> 00:46:55,976
of magnetic material.

1027
00:46:56,186 --> 00:46:59,206
When you slip the disc
into your floppy drive,

1028
00:46:59,546 --> 00:47:02,396
a system of levers pushes
back the metal shutter,

1029
00:47:03,066 --> 00:47:06,166
the levers also pinch two
read-write head closer

1030
00:47:06,166 --> 00:47:09,626
so they almost touch the cookie.

1031
00:47:09,816 --> 00:47:11,656
A motor at the base of the
drive spins the cookie based

1032
00:47:11,656 --> 00:47:13,406
on commands from your PC.

1033
00:47:13,686 --> 00:47:15,696
The PC also signals
another motor

1034
00:47:15,696 --> 00:47:18,956
to move the read-write heads
back and forth over the surface

1035
00:47:19,116 --> 00:47:20,976
of the disc, so they
can read or write data.

1036
00:47:22,366 --> 00:47:24,106
Before your PC writes data,

1037
00:47:24,666 --> 00:47:26,636
your drive first checks
the right protect tab

1038
00:47:26,926 --> 00:47:28,736
in the corner of
the floppy disc.

1039
00:47:29,456 --> 00:47:33,386
If it is open, light from
a tiny diode shines through

1040
00:47:33,386 --> 00:47:35,086
and strikes a diode
on the other side.

1041
00:47:36,206 --> 00:47:40,666
This diode then says to your
PC, "Don't write on this disc."

1042
00:47:40,916 --> 00:47:43,926
But if the tab is closed,
no light gets through,

1043
00:47:44,116 --> 00:47:46,796
and the PC knows it's
okay to write data.

1044
00:47:47,676 --> 00:47:49,386
>>David: So, be careful
because there is, in fact,

1045
00:47:49,386 --> 00:47:51,536
little bits on this things,
but you can probably see,

1046
00:47:51,536 --> 00:47:53,126
and the clicking
suggests, that you know,

1047
00:47:53,236 --> 00:47:55,086
you just move this cheap
little metal shutter,

1048
00:47:55,086 --> 00:47:57,856
and inside of there is, in
fact, this cookie, in fact,

1049
00:47:57,856 --> 00:48:00,076
it's alright if you ruin a few
bits, go ahead and just push

1050
00:48:00,076 --> 00:48:02,616
down on that, and the reason
these things are called floppy

1051
00:48:02,616 --> 00:48:04,986
is not because they're actually
pretty hard, because they're

1052
00:48:04,986 --> 00:48:07,296
in this plastic shell, but
because that thing inside

1053
00:48:07,296 --> 00:48:10,276
of there is, in fact,
itself very floppy.

1054
00:48:10,656 --> 00:48:13,036
Let's see if we can't dive
a little deeper, but again,

1055
00:48:13,036 --> 00:48:14,546
to this point of
things being sort

1056
00:48:14,546 --> 00:48:16,456
of simpler underneath the
hood than you might think,

1057
00:48:16,536 --> 00:48:17,946
it's such a stupid thing.

1058
00:48:17,946 --> 00:48:20,396
Well, let's make a whole
in the piece of plastic,

1059
00:48:20,396 --> 00:48:22,326
and if light shines
through it, it's locked.

1060
00:48:22,326 --> 00:48:25,646
If light doesn't shine through
it, it's not locked, so again,

1061
00:48:25,756 --> 00:48:27,196
very basic things under here.

1062
00:48:27,196 --> 00:48:28,806
Let's look at this
final clip here.

1063
00:48:30,956 --> 00:48:33,216
>>To write data onto a floppy
disc, your PC tells the drive

1064
00:48:33,216 --> 00:48:38,466
to send tiny pulses of
electricity through the heads.

1065
00:48:38,626 --> 00:48:41,356
The pulses make the head
act little electro magnets.

1066
00:48:42,116 --> 00:48:43,726
Each head creates
a magnetic field

1067
00:48:43,796 --> 00:48:45,666
that reaches the
surface of the disc.

1068
00:48:46,026 --> 00:48:48,126
Remember the magnetic
coating on the cookie?

1069
00:48:48,756 --> 00:48:52,406
The magnetic field alters the
tiny particles in this coating.

1070
00:48:52,406 --> 00:48:54,236
If current runs through
the heads one way,

1071
00:48:54,376 --> 00:48:55,916
the particles are
arranged with their North

1072
00:48:55,976 --> 00:48:57,496
and South Poles in
one direction.

1073
00:48:58,246 --> 00:49:00,466
But if current flows through
the head the opposite way,

1074
00:49:00,856 --> 00:49:01,916
the polarity reverses.

1075
00:49:02,556 --> 00:49:04,776
To read data from a disc,
the read-write heads move

1076
00:49:05,356 --> 00:49:07,436
into the same position
over the cookie.

1077
00:49:07,836 --> 00:49:09,776
But this time, the
process is reversed.

1078
00:49:10,456 --> 00:49:13,026
The cookie particles create
a magnetic field in the coils

1079
00:49:13,026 --> 00:49:16,906
of wire, and this creates a
current in the read-write heads.

1080
00:49:16,906 --> 00:49:19,306
The disc drive detects
this flow of electricity

1081
00:49:19,306 --> 00:49:20,976
and passes it on to the PC.

1082
00:49:21,046 --> 00:49:24,386
The PC translates the back
and forth current changes

1083
00:49:24,386 --> 00:49:26,476
into a series of ones and zeros,

1084
00:49:27,346 --> 00:49:29,976
the binary language
of computer data.

1085
00:49:30,456 --> 00:49:32,786
>>David: So, we're well aware
that many of you would really

1086
00:49:32,786 --> 00:49:35,346
like to just break something
when you're working on CS50,

1087
00:49:35,346 --> 00:49:36,636
so we thought we'd give you

1088
00:49:36,636 --> 00:49:38,716
that opportunity here toward
the end of the semester.

1089
00:49:39,186 --> 00:49:42,276
Beware the little spring, that's
likely to hit your seat mates

1090
00:49:42,276 --> 00:49:45,666
in the eye, but go ahead and
just remove this plastic,

1091
00:49:45,666 --> 00:49:46,756
or this metal shutter,

1092
00:49:46,796 --> 00:49:49,936
and you'll actually now
expose the inner cookie,

1093
00:49:49,936 --> 00:49:52,886
and this is the most gratifying
part of all, insert your fingers

1094
00:49:52,886 --> 00:49:57,776
at the bottom of the plastic
and wa-la, a little souvenir

1095
00:49:58,596 --> 00:50:01,326
and the entire magic
that made for years,

1096
00:50:01,326 --> 00:50:03,956
software distribution possible,
and the storage of millions

1097
00:50:03,956 --> 00:50:06,046
of bits, this little
thing in there.

1098
00:50:06,116 --> 00:50:08,146
So whether you were among
those less comfortable,

1099
00:50:08,146 --> 00:50:10,366
somewhere in between,
or more comfortable,

1100
00:50:10,366 --> 00:50:13,466
a lot of the stuff we've been
talking about, really boils

1101
00:50:13,466 --> 00:50:15,056
down to some fairly basics.

1102
00:50:15,486 --> 00:50:19,766
Why don't we embrace the
fun that you've just had,

1103
00:50:19,766 --> 00:50:22,066
actually it doesn't sound
like you're having much fun,

1104
00:50:22,206 --> 00:50:24,396
but consider this your souvenir,
please take it with you

1105
00:50:24,396 --> 00:50:26,196
when you go, that's
it for today.

1106
00:50:26,196 --> 00:50:27,036
No class Wednesday.

1107
00:50:27,036 --> 00:50:29,976
We will see you on Monday.

1108
00:50:30,516 --> 00:50:36,040
[Background talking]