O que você alcançará: Nesse curso baseado em projeto é que você construirá um moderno sistema computacional, desde os fundamentos. Nós dividiremos esta fascinate jornada em seis projetos "mão na massa" que levará você desde a construção de portass lógicas básicas até a criação de um computador de uso geral completamente funcional. No processo, você aprenderá - na forma mais direta e construtiva - como computadores trabalham, e como eles são projetados. O que você vai precisar de: Este é um curso de auto-contido: todo o conhecimento necessário para ter sucesso no curso e construir o sistema computadorizado será dado como parte da experiência de aprendizagem. Portanto, não assumimos nenhum conhecimento de ciência da computação ou engenharia prévio, e todos os alunos são bem-vindos a bordo. Você não vai precisar de nenhum material físico, uma vez que você vai construir o computador no seu próprio PC, utilizando um simulador de hardware baseada em software, assim como computadores reais são projetados por engenheiros de computação no campo. O simulador de hardware, bem como outras ferramentas de software, serão fornecidos gratuitamente depois de se inscrever no course. formato do curso: O curso é composto por seis módulos, cada um composto por uma série de conferências de vídeo, e um projeto. Você vai precisar de cerca de 2-3 horas para assistir a palestras de cada módulo, e cerca de 5-10 horas para completar cada um dos seis projectos. O curso pode ser concluído em seis semanas, mas você é bem-vindo para levá-lo ao seu próprio ritmo. Você pode assistir a um TED Talk sobre este curso "Googando" "nand2tetris TED talk" . A respeito de Cursos Baseados em Projetos: Cursos baseados em projetos são planejados para ajudar você a completar um projeto no mundo real que seja pessoalmente significativo, com seu instrutor e uma comunidade de aprendizes com objetivos similares, fornecendo orientação e sugestões ao longo do caminho. Aplicando ativamente novos conceitos na medida em que você aprende, você irá dominar o conteúdo do curso mais eficientemente; você obterá uma vantagem inicial em usar suas habilidades que você obtém para fazer mudanças positivas em sua vida e carreira. Quando você completar o curso, você terá um projeto concluído o que você estará orgulhoso de usar e compartilhar.
Unit 2.2: Binary Addition
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Do curso por Hebrew University of Jerusalem
Construa um Moderno Computador desde os Primeiros Principios: De Nand até Tetris (Curso baseado em Projeto)
1459 classificações
Hebrew University of Jerusalem
1459 classificações
Na lição
Boolean Arithmetic and the ALU
Using the chipset that we've built in the previous module, we will now proceed to build a family of adders -- chips designed to add numbers. We will then take a big step forward and build an Arithmetic Logic Unit. The ALU, which is designed to perform a whole set of arithmetic and logical operations, is the computer's calculating brain. Later in the course we will use this ALU as the centerpiece chip from which we will build the computer's Central Processing Unit, or CPU. Since all these chips operate on binary numbers (0's and 1's), we will start this module with a general overview of binary arithmetic, and only then delve into building the ALU.
Conheça os instrutores
Shimon SchockenProfessor
Computer Science
Noam NisanProfessor
Computer Science and Engineering
In the last unit, we saw how to represent numbers in computers using binary bits,
but now we can represent numbers, that's an important thing.
Now the whole point of representing something is if we want to manipulate it.
What kind of manipulations do we want to do with bi, with numbers?
Well we want to add them, subtract them, multiply them and so on.
This is what we're going to learn how to do in this unit.
Well, not all of them.
What we will really be talking about is how to add.
Once we do that we'll basically get the whole rest of the other operations
almost for free.
It turns out that once we understand how to represent negative numbers,
which we will do in next unit, we will be able to get subtraction for free and
to understand which of two numbers is greater for free.
Multiplication and divisions are more complicated, but nicely enough,
we can actually postpone them to software.
We will not build in hardware any multiplication or division eh, circuitry.
But rather, we will actually let software do it and
things are much easier to do in software because you just have to
write little programs rather than actually connect stupid little devices.
So we get two sequences of binary numbers.
How can we add them?
Well one way we already know how to do, and that's going to be very easy.
We convert them to decimal.
We add the decimal numbers.
That we know how to do from second grade.
We get a decimal number.
We convert it back to binary.
That we just learned how to do.
And we get the answer.
This is great and fine, but of course that's not what a computer does.
A computer doesn't know how to add decimal numbers
without first converting them to binary numbers.
So we need to figure out how to actually do the addition of binary numbers, and
directly, without converting them to decimal.
And how are we going to do that?
As usual, everything I need to know I learned in second grade.
So we need to go back to second grade.
We need to add 5,783 plus another number.
How do we do this kind of addition?
What did we learn?
Well, we start with the ones, with a right-most digit, and we add 3 plus 6,
we get the 9 and we are all very happy with that.
That's easy.
But now what happens when we next, do the next digit and we have 8 plus 5?
Well, 8 5, as 8 plus 5 is 13, and
we cannot write 13 underneath the tens position because 13 is greater than 10.
So we all learned this important and amazing trick of say, writing only 3 and
having 1 as a carry to the 100s place.
And then we know how to con, we can continue from there.
And then we can con, add the 1 to the 7 and so on.
And after we finish the left-most digit, we have the complete result.
So, exactly the same thing we're going to do in binary numbers,
only it's going to be much, much easier.
So we take 1 plus 0, the right-most two digits, and we need to add them.
And that's easy, 1 plus 0 is 1, we can write it down.
If we have 0 plus 0, that's also going to be very easy.
We can write them down.
Now the first time we get into a problem is we have 1 plus 1.
because 1 plus 1 is 2.
2 is more than what we can write in a single digit, bit, Boolean bit.
So we actually need to do the same kind of trick, write down 0 and
carry the 1 to the next position.
And now we can continue.
We need to add 1 plus 0 plus 1.
That's again, more than what we can write in a single bit because that's 2.
So we write 0, we carry 1.
Now we have 1 plus 1 plus 1.
The answer is 3.
So we write down 1, and we carry another 1.
And that's how we continue, until we get the final solution, the final answer.
And that's all there is to it.
In, the rest of the unit we'll actually learn how to do this thing exactly,
how to do this thing completely mechanically,
in the sense of really understanding how every operation works in true, ter,
terms of binary operations that we've learned so far.
But the basic principle is extremely simple,
just like we learned in second grade.
Before we continue, there is one thing I want to take out of the way, and
that is a question of overflow.
Suppose that we were somehow unlucky and the, and
the two left-most bits of our, the two numbers we were adding were 1.
So what is the problem with that?
The problem is that, that when we add them,
we have a carry that needs to go to the left of the word size.
And there is no place to carry,
to put that carry bit because we finished our word size.
So what would we do?
Will we raise some warning or anything like that?
Well, the answer is very simple.
What is usually done in computer systems is nothing.
We just ignore any carry bit that does not fit into the word.
What does that mean, really?
So if you try to look at it from a mathematical point of view, what it means
is that the addition that we're actually doing in our hardware is not real integer
addition, because we cannot go beyond the numbers that fit inside the word size.
Instead, what we have is really an addition module 2 to the width
of the word size if you look at it mathematically.
In others, in, in other, in other words, the answer is correct, but may, except for
the case that it may be off by exactly 2 to the n where n is the word size.
If the result was more than 2 to the n,
the hardware automatically decreases 2 to the n,
which is basically the carry that we just threw because there was an overflow.
So that's what they usually do, and
the rest of anyone using a computer, anyone using computer and software,
needs to remember that if he exceeds the word size, then the result that you get,
that you get is not the true integer result of the integer addition, but
rather the truncated result after the overflow was already disposed of.
Once we took good, got that out of the way, let's go back and
try to understand how will we, we actually going to build this kind of an adder.
How can we actually get, build hardware that gets two numbers as bits,
as input bits, and the output is another number
that actually represents the sum of the two input numbers?
We'll do that in three easy stages.
In the first stage we'll just learn how to add two bits.
Very simple.
The second stage we'll learn how to add three bits.
It may seem there is a long way to go if we are progressing so slowly, but
then in one big step we'll be able to add any two numbers of any number of digits.
So let's start with that.
So let's look at the typical operation we were,
when we were adding two bits in the, in the process we just looked at.
How do we took a 1 plus 1 and we added it and we got a 0 sum and a 1 carry.
How did we do that?
Well, the fir, most important thing is to notice that
if the rest of the bits of all these two numbers do not add,
do not matter at all when we're doing just this one bit slice of operation.
Whatever the other bits were, as long as what we're adding now is 1 plus 1,
and as long as one additional important thing, the carry so
far, the carry we had to this place was 0,
whatever the other bits are, we still are doing the operation exactly the same.
We're going to put, write 0 below these two 1s,
and we're going to have a carry of 1.
This tells us that now we have really a just a simple binary operation.
Taking two bits, a and b, and producing two output bits which depend
only on them, which is one of them we're going to call the sum,
the binary sum of these two things, and the other is a carry.
And this is really the first step that we need to do.
This would actually allow us to add two bits.
Now this operation really, which is the slice one oper, one, one
step of the process that we saw so far, is that's naturally abstracted by a chip.
[COUGH] And the chip gets two inputs, a and b, two outputs, and we know exactly,
for every combination of inputs, what the output is supposed to be.
This chip is called the half adder, and
implementing it is the first thing that you'll do in the exercise for this week.
In fact, we're going to give you the exact HDL that describes the interface
of this chip, and you're just going to have to do the implementation, which
actually does implement this operation that we'll now understand what it is.
And we've finished the first step of our journey adding,
of journey to add numbers adding two bits.
Now if you remember, the only real restrictions that we had when,
when we were doing this addition of two bits was the fact that the carry to this
point was 0.
But that's not the general case.
What happens more generally, is there may be a carry.
Suppose now that we get another bit of the input, called c,
which describes the carry from the previous step, which can be either 0 or 1.
Now how do we do this addition?
Well, we know we end, add the three numbers and
we still get the sum and the carry.
Again, now we get a Boolean gate, a chip,
if you wish, that we know exactly its functionality.
We have three inputs, a, b, and c, two outputs, sum and carry.
And there are eight possibilities of the inputs.
And for each of them, we can very well know what the outputs are.
So that's another chip.
And that chip is called a full adder.
And again, you can go and implement it.
And indeed, this is the second part of we, of our joining to addition.
And again, we're giving you the HDL of this chip, and
please go ahead and implement it.
And now we're ready for the final step for doing everything.
We get two full numbers and we want to add them.
How are we going to do that?
Well, we already know how to do every single step of the process, so
we just have to repeat doing the single step.
So let's look at what we did.
We first, let's color the bits, the right-most bit yellow,
the next one green, the next one blue.
This is just so we can talk about them in terms of colors.
But of course in the implementation there are no colors, just different bits.
So we start of course, by just adding the two yellow bits.
And adding the two yellow bits is just a half adder because we have no
carry so far.
And we get a yellow sum and a yellow carry.
Now the next step is, we need to add the yellow carry to the two green bits.
And from that, we get a green sum and a green carry.
Now we take the green carry, add it to the two blue bits.
Each one of these colored steps is simply a full adder now.
The thing that takes that yellow carry, the two green bits, and
outputs a green carry and a green sum, that's just a full adder.
And that already we've implemented.
So to implement the whole thing, we just need to basically connect 16 of
these full adders, or maybe 15 full adders and 1 half adder for the right-most bit,
connect them together in the right way, and you get exactly a, an adder.
And this is what you're supposed to do.
This is our 16-bit adder.
It accepts two numbers now, each one of them is a bus of 16 bits, and
outputs a single number that is supposed to be their sum as 16-bit integers.
And again, we're going to give you the HDL for this.
It just specifies that you're going to get two numbers, two 16-bit numbers,
as busses in input, and produce a single number,
16-bit bus as output that is their sum in terms of 2 representation of the number.
And you can go ahead and implement that.
So we've just learned how to add two numbers
in a very concrete sense of building the chips that actually do that.
The next unit we'll actually go back and actually look at how do we represent
negative numbers, something that we still owe you from last unit.
Once we do that it will turn out that we'll get subtraction for free.
After we get that under our belt, then we'll go to the capstone of this week's
this week's lecture which is building a complete arithmetical logic unit.
And it turns out that most of the cleverness is already done.
The most clever thing that we have in a ALU unit is just adding two numbers.
But, of course, we need a lot of logic around it, and
that's what we will do in the fourth unit.
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