of this course is a fish way either 15 213 from computer science or 18 213 from

electrical and computer engineering but a large number of people enrolled in

this course are in one called 513 which is for graduate students and to be

totally honest you're not supposed to be here because you supposed to watch the

videos the lectures by video later but we're not checking any cards so i don't

really know who you are

Oh anyways this course is a as you can see a very popular course on campus we

have over 700 700 600 shouldn't exaggerate students on campus and rolled

in one of those three versions of the course which if you think about a

university of the size of CMU it's like about five percent of the total student

population so we're very glad to have you here in this game is too high think

we overdid the game try that

that's better so my name is Randy Bryant and my co-instructor is Dave O'Halloran

and those names might be familiar if you have been to the bookstore

because the book was written by us and we actually wrote this book of we

started to 13 is a course in 1998 are and I know that your children them and

all that stuff but up out of response for our new type of course that you'll

see today that sort of gives people are in depth understanding of systems but

more from a high level or programmers perspective and we'll talk some about

the philosophy of the course later

so this book as you see are just came out its third edition last March and

that's the required text for the course and there is nothing you can do to avoid

buying that book and it's not because we're greedy actually we take the money

we get from royalty for students in this course or any CMU course and we donate

it to CMU so we're not actually making any money out of you buying the book but

it's a simple reality that the new version is different than the old we

expect you to be using the new version there are no electronic copies there are

no pirated version so you basically have to buy the book but and I don't really

apologize for that because i know you or your parents or somebody's paying a lot

of money for you to be here and so the price of the book is really a relatively

small amount relative to that and up this course by the way is is not some

courses there's oh yeah this book you might want to look at it once in awhile

but actually the course has nothing to do with it

this course the book and the course are one they're very tied together there'll

be parts of the book that we don't go into all the details that are in the

book sometimes but we expect you to be able to figure it out and so really the

course in the book are tied together and they very much the the progress the

course the topics covered how it's covered everything about it is

consistent with the book and we wrote the book because of the course think of

them as course notes so and the reason why we are teaching this term by the way

we haven't talked together in several years but because this is the first roll

out of this book and not only is it being used here but it's actually used

by about 250 schools around the world and many of those people are also going

through the same of activities of teaching for from this new edition for

the first time and so we wanted to make sure all the material for the course was

sort of put in order and thanks

because other universities and other colleges use this material of the

supporting material we've developed so that's the course and today is the first

election we're sort of doing a tag team here I'll talk the high-level what the

course is about what the main themes are and today we'll talk more about the

logistics so we refer that the saying we have one of the very first time I type

the number of this course into a text editor i realized i've typed those five

digits many times in my career because it's the zip code of CMU 1521 three so

that's where we come up with the same so arm there is a few things about this

course for the most part you know and your normal undergraduate curriculum

when you learn programming you've been very much separated from the realities

of the Machine you you just think about code just you put some text into some

little box somehow and outcomes of behavior that it hopefully is what you

intended the program to do the purpose of this course is to give you enough

understanding of what that box is doing when it executes your code and through

that to help you

I'll become better at what you're trying to do so some of the outcomes is there's

really two types of outcomes from this course one is that if this is the only

systems course you ever taken your whole life you will get useful material from

it you will learn tools tricks methods that will help you if you ever are

involved in software development a large-scale software engineering

projects systems hardware design any aspect of of computer technology

this will help you be better at what you do you'll understand what programs do

how they work

what the machines that support them do why sometimes they work really well and

why sometimes they don't work so well

oh it also is intended as a sort of stepping stone into a whole number of

other courses at CMU that will give you more in-depth understanding of computer

technology but are sort of specialized by topical area whether it's computer

networking or operating systems or embedded systems where you'll take the

sort of ideas from this course and be able to then learn it as somewhat

narrower but deeper sense what's really going on and so it really is trying to

serve those two goals of making you more effective giving you useful ideas and

tools right away but also preparing you for later courses so one way we talk

about this is sort of what why what kind of stuff will you learn from this course

is to go through a series of what we call great realities places where sort

of computers meetup real-life computers get meet up against your expectations

and maybe they're not quite the same

so one of them is in the first part of the course is going to take a fairly

detailed look at how numbers are represented in computers and you'll

learn some things that are on one hand surprising and another

your hand will start to make sense when you understand it better

so are a simple case is for numbers i don't know when it was but it was

probably in about eight grade algebra that I've learned that if you square a

number it will be at least 0 if it's not an imaginary number

so a either an integer or a real number you'd expect to square and it to be a

positive value or perhaps 0 right arm and that's actually generally true with

floats a representation of floating-point numbers but with integers

are hints you know the the computer representation of integers it's not so

clear so for example a few square 40,000 on most computers then you'll get

$PERCENT of whatever that should be as you'd expect but what if you square

50,000 so you could do this in your head but actually built into this laptop is a

computer so i'm going to go ahead and use it just sighs

and use the to all which on unix systems is called know what's going on before

we are on our linux systems is called GDB but on a macintosh OS X it's called

lld but they're pretty much the same program you will get to know this

program really really well this term so like i said if you square 40,000 you'll

get what you'd expect but let's change this to 50,000 and you get a very

peculiar number that doesn't look anything like you'd expect 50,000

squared to be and in fact it's negative and so that might just seem like well

there must be a mistake or something know that that's just the way it is

because of this computer is expecting numbers to be represented as 32-bit

values and the bit pattern that you get when you do this multiplication happens

to be the representation of a negative number so that's a an example of where

your normal expectations about integer arithmetic may or may not hold up on the

other hand there are some places like if i try to do the same thing multiplying

300 by forward by 500 x 600 then all of a sudden i also get a number that

clearly is not what you'd expect because the same thing has happened

I've gotten an overflow it happens that it's overflowed to value that's positive

not negative but it still is obviously not the integer product of those four

numbers on the other hand one thing you'll find is even though this

arithmetic is not a normal sort of integer arithmetic it actually has some

well-behaved properties so for example if you look at it

what i've done is i just moved the 300 around from the beginning of the product

the end of the product and so now if you think about how associate ility and

community works basically multiplying these four numbers in a different order

but what you see is you get the same funny-looking result

uh no matter how you do it so what you can say from this is integer arithmetic

is commutative and associative sobes some of the conventional mathematical

properties it just isn't what you'd expect it to be it's not a up

normal sort of mathematical integer arithmetic so the cartoon here shows an

example that similar possibility we get to it

oh I guess it comes at the end up so that the next question is is edition

associative right can you order that the numbers and then you probably might have

figured both it in the integer arithmetic even though as this potential

for overflow it is associative and it's a community of two but four floats it's

not really quite the same because the range of values you can get in floating

point or so extreme that some numbers kind of disappear on you so the example

I'll show without having to use a computer to do it is if you take a big

number and subtract it from itself you'll get zero so if you add that to

3.14 you'll get 3.14 but if you take those two numbers and you reorder the

how you combine them so that that 3.14 compared to minus ten to the twentieth

is so insignificant that result gets turned into a minus ten to the twentieth

and you add that to one ten to the twentieth and you end up with zero so

it's not associative and so what you see is the both these numbers systems have

some peculiarities and it all comes down to the fact that they use finite

representations of things that are potentially infinite and their expands

and so there's some compromises in how those work and what the compromises you

can overflow an integer and and run out of room and in floating point numbers

you have roundoff problems where you should drop the digits that are

significant you can also over floating point but the more common problem is

round of so this is shown in this cartoon this idea of overflow of

somebody counting sheep which in the u.s. is are in english language at least

a way to fall asleep and when he or she goes from 30 2767 and then increment set

by one gets minus-30 2700

68 and we'll see exactly why that happens but basically the numbers are

going up to the largest value can represent and then when it goes one more

becomes a negative number so these are really important understood thanks to

understand I mean ninety percent of the time maybe you can just get by writing

programs and not worrying about whether your numbers are going to exceed their

possible range but they sometimes when this could be really important if you're

like you know controlling a rocket you really don't want the positive thrust to

become negative or something like that so you can see you there

this could be an important consideration similar way there is well known

instances of security vulnerabilities where somebody wrote code that sort of

expected a positive number in someplace and clever person figured out if I

supply a negative number i can fool the system and get it to do then thanks

so these are the kind of corner cases that you need to understand better if

you're either working in programs where it's really really important that it

worked correctly or you're really worried about security vulnerabilities

anytime you have possible for corner cases you have to understand these

nuances better assembly for floating point if you're going to use floating

point for serious computation whether its scientific research or for designing

bridges or nuclear power plants or something

you better understand what the characteristics are so that's our first

one and we'll spend a couple weeks talking about numbers and number

representations in their properties here

the second is we're going to spend a lot of time in this course learning about

machine level programming meaning what

the instructions are that actually get executed by the computer and that can be

diff described in assembly language which is a text version of it or an

object code which is the actual bit-level binary encoding of

instructions and we'll learn

a fair bit about that and spend a fair amount of time seeing how code that you

write in C I'll gets turned into machine code and how that gets executed on the

machine

one thing I'll say is it historically courses like this would teach assembly

code by having you write programs and assembly that I do various things not