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Unit 3.2: Flip Flops

Having built the computer's ALU, this module we turn to building the computer's main memory unit, also known as Random Access Memory, or RAM. This will be done gradually, going bottom-up from elementary flip-flop gates to one-bit registers to n-bit registers to a family of RAM chips. Unlike the computer's processing chips, which are based on combinational logic, the computer's memory logic requires a clock-based sequential logic. We will start with an overview of this theoretical background, and then move on to build our memory chipset.

依据基本原理构建现代计算机：从与非门到俄罗斯方块（基于项目的课程）

希伯来大学

What you’ll achieve: In this project-centered course* you will build a modern computer system, from the ground up. We’ll divide this fascinating journey into six hands-on projects that will take you from constructing elementary logic gates all the way through creating a fully functioning general purpose computer. In the process, you will learn - in the most direct and constructive way - how computers work, and how they are designed. What you’ll need: This is a self-contained course: all the knowledge necessary to succeed in the course and build the computer system will be given as part of the learning experience. Therefore, we assume no previous computer science or engineering knowledge, and all learners are welcome aboard. You will need no physical materials, since you will build the computer on your own PC, using a software-based hardware simulator, just like real computers are designed by computer engineers in the field. The hardware simulator, as well as other software tools, will be supplied freely after you enroll in the course. Course format: The course consists of six modules, each comprising a series of video lectures, and a project. You will need about 2-3 hours to watch each module's lectures, and about 5-10 hours to complete each one of the six projects. The course can be completed in six weeks, but you are welcome to take it at your own pace. You can watch a TED talk about this course by Googling "nand2tetris TED talk". *About Project-Centered Courses: Project-centered courses are designed to help you complete a personally meaningful real-world project, with your instructor and a community of learners with similar goals providing guidance and suggestions along the way. By actively applying new concepts as you learn, you’ll master the course content more efficiently; you’ll also get a head start on using the skills you gain to make positive changes in your life and career. When you complete the course, you’ll have a finished project that you’ll be proud to use and share.

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您将学习的技能

Logic Gate, Computer Architecture, Assembly Languages, Hardware Description Language (HDL)

教学方

Shimon Schocken
Professor
Noam Nisan
Professor

脚本

In a previous unit, we discussed how we're dealing with time and

sequential logic in computing systems.

What we'll do in this unit is actually talk about the actual elements,

the chips that will allow us to do, provide this kind of functionality.

So, let us say, let us recall how we started the last unit

where we started the way we deal with time, we add integer time units.

At every time unit t, we want something that can depend with value to compute some

kind of function and the output.

Some kind of function and the value at the previous time unit, the time t minus 1.

So what kind of element do we need in order to provide such a functionality?

Well, the missing element, we need something as a very basic point,

is we need something to actually remember one bit of information,

move one bit of information from time t minus 1 to time t.

This is what we have missing.

We have lots of combinatorial log, logic so

far that can do any kind of manipulations that we want within a single time unit.

But actually moving information from time t minus 1 to time t is

something that is still missing.

Now, if we look about that kind of thing in the way we're,

we're in this abstract way that we're thinking about these discrete time units,

at that exact time when we switch between time unit t minus 1 to time unit t.

This new element must remember the bit.

At that point it has to remember what happened previously and

carry it on to the next stage.

Without that we cannot have this kind of functionality.

Now, this means that at this transition point between two consecutive time units

it must have state.

It must remember whether it's now remembering 0 or it's now remembering 1.

That means it has to be in two different physical states in, in its implementation.

These two physical states, it must be able to move

between them according to the logic of the previous time unit.

Which means, which means it needs to be able to flip between these two

kind of different, two, two different physical states.

An element that can do that thing, that can flip change situation between

two different kind of such states are called Flip-Flops.

They flip to 0 and then can flop back to 1, and

the point is that this flipping and flopping is something they remember.

It's not just a function of the current input, but

its something internal to them they remember between time units.

So let us view now the basic flip flops that we will you,

be using in this course which is called the Clocked Data Flip Flop.

This flip flop has a single input and a single output and it basically

remembers the input from last time unit and outputs it in the next time unit.

So, if we again look at the diagram of our time units and

assume that our input looks like this.

If it starts with a value of 1, its time unit 1 goes down to 0 for

time units 2 and 3, and then goes up to 1 and down to 0 again.

Then when we look at what the D flip flop will do, at any time unit it

will actually return the value that was in the input in the previous time unit.

So at time 1, we don't exactly know what our output will be because we haven't

specified what happened in the previous time unit.

So for us, it's gray, we just don't know what the output is.

But once we get to time unit 2, then we know exactly what the output needs to be.

It needs to be exactly what the input was at time 1.

Similarly at time unit 3, we know it has to be 0 because that was what

was the input was at time 2, and so on.

At every time point we are having simply the previous signal that was fed to

the input, shifted one time unit to the right.

And this is what the Clocked Data Flip Flop does.

And this is going to be the only element that will provide

all the sequential logic that we need in this course.

And from this we'll need all the kinds of the sequential circuitry that we need,

memories, counters, and so on.

One thing that I'd like to mention at this point is the meaning of the little

triangles that we see at the bottom, at the bottom of the D flip flop diagram.

The meaning of this is that we have a sequential chip,

a chip that depends on time.

As opposed to all previous chips are all combinatorial chips that we had so

far, whose output only depended on their own inputs at any given point in time.

Here we have a chip whose output also depends on what happened previously and

state that's kept inside this chip.

This is the only point that we care for this logical dependency of time.

Although it's probably also interesting to note that in any physical

implementation of a D flip flop.

This means that the implementation will also need some kind of

access to this clock that we mentioned in the last unit,

that actually breaks down the physical continuous time into discrete time units.

Again we won't care about this because we're living in our abstraction of

discrete time, but

from our point of view this is just a way to remember that this chip has state.

And can remember and ha, and, and,

depends out and its output also depends on what happens in previous time units.

Now at this point, let us say,

just say one word about the implementation of the D Flip Flop.

In this course, we are going to deal with it as a completely primitive operation.

This is something given to you that cannot be manufactured from anything else.

But just, you are going to use it, just like you viewed the Nand gate and

from D Flip Flop, and

the Nand gate you will provide, you will actually build everything in the course.

In many other courses,

you actually eh, follow what happens in most real life hardware.

You can actually construct the flip flop from Nand gates eh, when you actually

take the Nand gate and put them into some kind of a loop eh, between each other.

This kind of loop basically has them amplifying the same signal again and

it allows them to get stuck either in a zero or one state.

And with some additional logic you will also be able to manipulate this

date from the outside.

Eh, this kind of logic usually has two steps.

The first step is one, is what I've just described having this kind of cycle that

allows it to somehow remember information.

And the second type step is to actually have some kind of logic that actually

provide isolation between subsequent time units.

We will not describe in this course how this is done even though it's extremely

elegant and beautiful because we basically think it's confusing.

We do not think that is the right way to think about a logical circuitry.

We think that it's cont, completely worthwhile to keep separate in your head.

The combinatorial logic which happens instantaneously and

the sequential logic which we don't want to, to thinking about how it,

how it is constructed from lower levels combinatorial stuff.

But rather think about it as its own primitive block.

So now that we can remember one bit from this, we can build everything else.

In fact, there is a generic paradigm of how we're going to build

all our logic in the computer, and it's going to be a combination of

remembering information via this basic D flip flops.

And then manipulating them using combinatorial logics that we built in

the first two lectures.

So in particular the usual way we do things is we have an array of D flip flops

which basically compromise all of our memory in the system.

Their output is going to be fed into some combinatorial logic

together with the new input that you get in this time unit.

And all of this is going to change the state that we have in the D flip flop for

the next time unit.

This is the general way we're going to build everything whether it's a memory or

a counter.

For example, if it's a counter we're going to remember a number in all these flip

flops and the combinatorial logic will basically add one to the counter.

In this way, every time unit,

the new value that we have will be 1 more than the old unit.

Let us say, actually now look at the first bit that we're actually going to the first

device, the first chip that we're going to actually be, build from a D flip flop.

And this is a device that actually builds, remembers a bit forever.

So the D flip flop provided the basic functionality of remembering a bit for

one time unit.

If we want to build a memory we want something that can remember a bit forever.

We store it, we tell now start remembering this bit and

then it keeps on remembering the bit.

Such a device, such as chip eh, is defined by the following API,

by the following chip that we have that we call the bit chip.

It has two inputs, an input bit and a load bit.

Once we take the load bit and

put the 1 into it we want to remember the input bit at that time.

When the load bit goes down to zero we want to the, the chip to

keep remembering the last time that the, the last input that was loaded into it for

infinity until a new load operation is performed.

So this is basically the logic that we have.

If load at time t minus 1 is 1, then we want to the, the next out,

the out at time t to be exactly the input at time t minus 1.

Otherwise, we want to output the time t to just keep on being what it is now.

Basically the old value that we remember the same as out t minus 1.

How can we build such a chip from our humble D flip flop?

Before we look at that, let us see what this bit chip needs to do.

So again, let's look at two possible signals for load and for in.

So for example, we ask for loading in time units 1 and time unit 4, while input is,

let's say it's 1 in the first time unit and then it goes down, down to 0.

So let's see what do we expect the output of this bit gate to be.

Well on the first time unit we don't know the history so

we have no idea what the output is, let's put it in gray.

Now in time unit 2, it's supposed to be, since we asked for

a load at time unit 1, we, what we want in time unit 2 is for

the output is to be exactly like the input at time unit 1.

Because we have a load at time 1, the value of any time 1 is 1,

the output at time 2 should be 1.

Now after that, load goes down to 0 for the next 2 time units.

So basically what we want is we want the same output to, the same output to remain,

just because load is going to 0 even if input changes.

So we're going to keep on maintaining the same value for the next 2 time units

until load goes up to 1 and then we'll have to see what we're going to do.

Now notice at for this time units, whatever input was,

whether input went up or down, it does not matter.

In any case, we will not change the output of our chip.

Now let's see what happened at the fourth time unit.

At thefourth time unit, load goes up to 1.

Load goes up to 1, so now we're asking the chip to load the value again,

and the value that we load is the value of in right now,

which is the value at time four which is zero.

Notice that, that we have to load at time four, but this will only affect the output

at time five, because the output at time five is basically

the change to the input at time four, only if load at time four asked it to do it.

So that means that at time 5, our output will go down to 0 as planned.

So this is the intended functionality of the 1-Bit Register.

How can we do that?

How can we pipe and pipe the value that we remember so

it will keep on being remembered until we ask, we ask to change it?

Well here is the first naive approach.

We will take the output, what we currently remember and

let's plug it back into the input of the D flip flop.

And this way, the D flip flop, unless we tell it to load the new value,

we just keep on having the same value and looping inside it if you wish.

This is a very basic the ba, the correct idea in general but

of course this is not the real chip.

How do we connect?

How do we put the real input in,

if we want to also put the output of the D flip flop back into itself?

How does this connection done?

So this doesn't quite work, really the way to combine these two possible sources

into the D flip-flop, one source which is an output from the previous stage, and

another source which is a new input.

And which one of them we want to actually plug into the D flip-flop depends of

course on whether the load bit is set or not.

But we already know exactly how to combine two sources into one output and

this is exactly the multiplexer.

So, if we actually take the input and fit it into one input of the multiplexer.

Take the previous output and fit it into another input of the multiplexer and

then load, choose between them, this is exactly the correct functionality.

We can actually see how that works by following step by step.

So, let's try to follow this implementation and

see how it works eh, real time.

Again, let us take two possible example inputs,

load them in that we get from some other source that we don't care about.

And see what our new implementation does when it's fed these two signals and

inputs, and what it produces as output.

Let us start with time step 1, at time step 1 the ma, there is 1 point,

piece of information that we don't know.

We don't know what the previous state of the D flip flop was,

what the previous out was because it's not specified in this example.

So for us it's going to be a question mark.

All the other bits we can complete.

We know that in is one because that's what we see in our input.

We know that load is 1 and so on.

Now notice that the multiplexer here, because load is 1 and

because in is 1 even though we don't know what is the input to its other input.

To its eh, down to its lower input, we still can tell very well

what the output is, what the output of the multiplexer is and that's going to be 1.

Now eh, that we know what the input to the D flip flop at time unit 1 is, so

even though we don't know the output at this point time unit

we know everything about the input.

Now we can see,

now this implies exactly what is going to happen in the next time step.

In the next time step, the one that was the input

of the D flip flop is going to be passed and is going to be the output of the D

flip flop in the next time unit exactly when we switch between time units.

Now we can fill up the rest of the information about what

happens in the second time unit.

In the second time unit, we know what in is, that was our input.

We know what load is, we have the previous out so

we can calculate exactly what the multiplexer does.

And the multiplexer basically, basically picks the previous output

from the ori, from the old value of the DFF.

And then we exactly, we again, we know what the input to the, to the D, D,

D flip flop is going to be.

It is going to be again a 1, and this we can keep on doing step after step.

At each point in time we know the previous values.

We know what load is, we know what in is, we can compute what the next situation is,

and at every given point in time, we can write down the values and

all the different wires just like we did, did in combinatorial logic.

And as we see, this implementation actually provides the required

functionality, the functionality of whenever load is being pulled to high.

We load the input from in and keep it until the next time that a load is asked.

So at this point, we've looked, we've, we've looked at

what is the basic unit that allows us to do sequential logic on a computer.

And we've built the first interesting element from it, our one bit memory.

Once we have a one bit memory, in the next unit we'll actually build a whole

huge memory of lots and lots of bytes and words.

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