The central processing unit, also called the CPU, is the centerpiece of every computer hardware architecture. It is the hub of computation, if you will, because this is where all the calculations of the machine take place. And it is also the seat of control. This is where decisions are made about which instruction should be fetched and executed next. In this unit, we're going to talk about the Hack CPU, the specification of the, of the CPU, and how we actually are going to build it. Now, as usual, it always pays off to start with the abstraction. So we can think about the Hack CPU as some sort of a black box magic that performs two truly remarkable things. First of all, well, it's a 16-bit processor, like, the entire computer. And the first thing that it can do is, given a certain instruction, written in the Hack language, it will execute this instruction. Now, this is something which in my opinion is truly remarkable and dramatic. Because up until this point, when you wrote programs, the programs existed on a piece of paper, they were completely static, they were simply a bunch of symbols that described some fantasy of the programmer who wants to actually make something happen. But now, we can actually take these programs and put it into a machine that turns them into reality. The programs become music that you can listen to or video clips that you can watch. Or some airline reservation system or whatever. So the CPU is this very unique and special agent that takes this program specification and turns it into a reality. And if this were not enough in its own right, the CPU can also do something not less remarkable. In the process of executing the current instruction, it also figures out somehow which instruction should be executed next in the program which is currently running. And, of course, since we talk about the Hack computer, we assume all these programs are written in the Hack machine language that we studied or looked at in, in week four. So given that this is the CPU's abstraction, the next thing that I'd like to discuss is the more detailed specification of this apparatus. Well, the first thing that we have to remember is that the CPU does not work in isolation. It is connected to other devices Inside the computer. In particular, in this particular Hack architecture, the CPU is connected both to the instruction memory and to the data memory. With that in mind, here is the specific input and output connections of the CPU. And let us begin to review them from left to right and from top to bottom. So, first of all, we have three inputs coming from three completely different sources. First of all, there is the 16-bit data value, which is the value of the currently selected data memory register. This is the value that the CPU is going to operate on. Now, what exactly do we want the CPU to do? Well, the next input, which we decided to call instruction, which is a very sensible name, describes a 16 bit input which is the value of the selected instruction memory register. And remember that at any given point of time, there is always a selected register in the instruction memory, and there is always a selected memory register in the data memory. So always something comes into the CPU. The third input, which we decided to call the reset, is a one bit input which I will discuss later in this unit. On the right hand side, we see the outputs of the ALU. So, first of all, if the ALU wants to write something to the data memory, it has to specify three different things. Number one, what is it that we want to write. And this is stored in or admitted through an output called outM. Number two, where do we want to write it. We have to provide an address, and this is the the job of the next data output. And, the third output is a load bit that enables the data memory for write operations. So these are the three data outputs, if you will, of the CPU. In addition, there's one extremely important output, which we decided to call PC for reasons that will become clear in a few minutes. And this output holds, by some sort of magic, the address of the next instruction that has to be fetched and executed in the next cycle of this computer. So how do we implement this magic? How do we make all these good things actually happen? Well, here is one way of doing it. This is our proposed implementation of the functionality that we just described. It is not the only way to build a CPU, but this is a, a pretty good one, because it's very elegant and simple and relatively easy to do. So before we get into the actual details of this architecture, let us look at the interface. If we look at the interface of this hardware diagram, we see that it is 100% identical to what we saw before. We have three inputs coming in and we have four outputs coming out. And, given that we made this observation, we can now delve into the details of the actual architecture. Another important thing that we have to look at before we begin to understand before we begin to understand what is going on here, is that within this architecture, we have many labels, labeled by the catch all symbol C, which I use to represent the notion of control bits. So in order to make all these chips work together, the designer of the CPU has to make sure the different bits, different control bits, arrive to the right locations, and these multiple messages together will cause the CPU to do what it's what it's supposed to be doing. Now, if you look at this architecture, you know, after the first minute or two of bewilderment, you will realize that everything here is extremely standard. You see the LU, which you built in in project two. We see three registers, which we call A, D and [COUGH] and PC, and these registers are identicals to chips that you built in in project three. And we see two muxes, which we built in in week one of the course, in project one. And therefore, putting together this architecture is just a matter of assembling chip parts that you already built in previous weeks in this course. Another thing that you see in this diagram is all these C labels, which indicate control bits that go into various locations in various chips. Now, we have intentionally left those the specification of these control bits somewhat opaque, because we want you to figure out how to actually piece all these things together. And we'll have much more to say about it in the unit that discusses how we actually build this architecture. Now, before we build it, we have to describe what this architecture actually does. And in doing so I find it kind of, interesting to think about this architecture as an orchestra. An orchestra that works together, many different instruments in order to produce some great symphony and the symphony is the execution of the current program. You know, just like a regular orchestra, it can, it can play many different pieces of music. And the current program determines what it actually executes. So I would like in the remainder of this unit to focus on different sections in this architecture. And let us begin with this section that deals with the current instruction. So zooming in on this section, we see that it consists, mainly of, a register called A. And an instruction input that connects to this register through a multiplexer. And here is, an A instruction arriving to the, machine in the current cycle or some cycle. And here is also the symbolic mnemonic of the same instruction which is easier to read but obviously the computer doesn't care about it all this is just for communications between me and you. So we see that this instruction seeks to load the value 3,001 into a register. And of course, to do everything in binary. So in order to carry out this instruction, the CPU has to do several things. First of all, it has to decode the instruction. It has to take the 16-bit value and separate it into an op-code and a 15-bit address or a 15-bit value. By the way, how do we know that it's an A instruction? Well, we can look at the op-code. The op-code is zero, and according to the heck machine language specification it means that it's an A-instruction. So the CPU says, aha, I know what to do with it. I have to take the next 15-bit, value and put it inside the A register. And notice that this is exactly what the, the gate diagram does. The the 16 bit values go through the register and, and into the register. Another thing that the CPU does and we can not see it in this diagram in order to avoid clutter. But this is something very simple. It takes the output of the A-register and admits it outside the [COUGH] the CPU via the output which we call I think address M or M address. You can look it up in the overall architecture of the computer. So this is how the CPU handles an A instruction. Now the next thing that we are going to look at is how the CPU handles a C instruction because after all in this, hack machine language we have two generic instruction types. An A instruction and a C instruction. So we have to be able to entertain and handle any one of these instructions. Well the C-instruction, like the A-instruction, consists of different fields. And the first field is the op-code which happens to be one. This is how the CPU can tell that this is a C-instruction. But in addition to this, we have several other fields. And we can see them, we can see them more, more clearly if we look at the symbolic manifestation of the same instruction. Now notice that I'm using the color coding to associate the bits with the symbolic mnemonics but these colors obviously are completely meaningless for the computer. They are just a way for us to, to communicate more easily in the course. So how does the CPU handle with such an instruction? Well, you may have guessed the first thing that needs to be done is to decode the instruction.So the ALU takes this, 16 bit value and it decodes it into four different fields. And these fields will later on be used, in order to, power up and manipulate different elements inside the computer architecture. What we discussed so far was how, the CPU handles the instruction, but if you look at the gate diagram, you'll see that the same A register can be also fed, not necessarily from the instruction input, but rather from the ALU output. So we have to decide in some cases we want the A register to be fed from the instruction and in other cases we want the area to be fed from the ALU output. No you may have guessed how we do it. We know that in different situations we deal with two different constructions, in some cases its an A instruction with a op-code of 0, and in this case we want the input to come from the instruction. In other cases its C instruction, with an op-code of 1, and in this case we want to, to route the input of the A, A register in such a way that the input will come from the ALU. So it is the job of the CPU, designer to inspect the op-code of the incoming instruction and decide from which source the A register will take it's it's next contents so to speak. Alright, so this is roughly how the CPU handles incoming instructions and let's go back to the overall architecture and focus on the next section that I would like to discuss which is the operation of the ALU, which is by far the most, complex, part of this architecture. All right. Now, we see here, this, segment, that deals with the ALU, and we see a C instruction coming in, instead of the instruction that we saw before. And let us remember that the C instruction consists of different fields of bits, each one of which means something else. And another thing that we have to remember is that the ALU is a combinational chip. It always computes something, there's always some inputs coming in and there's always some output coming out. And when you look at the, diagram you see that the inputs of the ALU come from two different sources. One of them is the D register, the current value of the D register and the other one is either the value of the A register or the value of the selected memory, memory register. And there is a multiplexer that takes care of this decision of where to take the value from. And the control bit of this multiplexer is one of the bits in the instruction. So, once again, the CPU designer has to take care to take this bit that determines from which we want to read the input of the ALU. And then the ALU will then get the correct inputs, or the inputs that the programmer intended it to receive. So we have the inputs lodged in the input spans of the ALU, and, the ALU goes to work, and how does it know what to do? Well, the ALU also has, six control bits that, basically, taken together, tell or specify the ALU which operation we want to carry out. Now, these bits are taken from the instruction, and I'm using some color coding here to, to help distinguish which bits go where, but of course these colors are completely meaningless as far as the computer is concerned And I'm using them only for didactic purposes. So the CPU designer has to take these six green bits or more accurately copies of these bits and root them all the way to the control bits of the ALU. So now that we have all this information, we have, the input data. We have the, operation that we have to carry out. We can actually, you can actually compute something. And out comes the ALU output. Now, inspecting the diagram, we see that the ALU output is simultaneously fed Into three different destinations. The first destination is the D-register. The second destination is the A-register and it goes through a multiplexer. And the final destination is outside the interface or the chip, so to speak. So the same ALU is fanned out into three different places and so we have this situation that we have the same ALU output knocking on three different doors. But the first the fact that it knocks on these doors does not necessarily mean that the doors are going to open. We have to decide or the programmer has to decide, which door has to be opened. This decision is made by the next field of bits that I want to focus on and these are called in our jargon, we call them destination bits. We have three destination bits and these bits determine whether or not to open the D-register, the A-register and the data memory to accept the ALU output. So, if for example, the green bytes would have been 000, then it's very nice the ALU computes something, but this value would be lost. No container in this computer will will be open for business, so to speak. And if we set these green bits to 1111, then all of the containers will receive the same output simultaneously. So we can play with these bits as, as we've done in week four when we wrote programs although we did it in assembly and then symbolic way, we can do it with machine language. And once again we have a way to program this architecture so that the ALU output, output would be selectively absorbed by different locations in this architecture. All right. Moving along, another thing that we have to remember is that the ALU also computes and outputs to control outputs, which I think we call the ZR and, and NG. And these two control bits are document whether or not the ALU output is negative or zero. And these two outputs play a key role in what will be described next, which is the control logic of the CPU. But before we talk about this logic, I want to as promised, I want to say a few words about the reset input that we see coming, coming in to the diagram from the left hand side at the bottom of the slide. And of course, when you build and shape all these sides, up, down, left, right, mean absolutely nothing, but it's, it's useful h, to communicate about it when we talk about gate diagrams. So let us say a few words about this recent bit. And in order to do it, we have to try to envision how the heck computer will look like if we actually set out to build it. Well, it will be a black box and as you see, this black box is equipped with a single push down button. And this push down button is called reset in our in our jargon. And the idea is the following. This black box, the hack, the head computer is already loaded with some program written in Hack machine language and up until now, it does nothing. The program is inside, but nothing is happening. Well, if you take your finger and push this reset button in and let, let go, what will happen is that the computer will start running the current program. Well, you look at whatever comes out of this computer and then in a few minutes you will see that we can attach a screen to it and a keyboard. You look at the program's execution and at some point, you decide to reset the computer or rerun the same program again. You take your finger, you push it inside the the reset button. You let go and by doing this, the computer will restart running the same program again. Okay? So this is the kind of behavior that we want to implement in the in the control logic of the CPU, among other things. All right. So now that we understand how the reset button works from the outside of the computer, so to speak. Let's go back to the CPU logic and explore the control logic of the CPU. Well it doesn't make sense to talk about control if we don't have some instruction that that gives meaning to to the control operation. So here is an example of a instruction and it's a schematic instruction, it's not zeros and ones. But they used some mnemonics to and also color coding, which is completely meaningless as far as the computer goes. I use these mnemonics and colors in order to emphasize that we have here, here different fields of bits each field is designed to do something else in the overall computer architecture. And when we talk about control, I want you to focus on these 3 bits, which we call the junk bits. If you recall, if the three jump bits are zero, it represents a no jump situation. If the three jump bits are one, it represents unconditional go to. And any other combination of zeros and ones in the jump bits represents a conditional go to. What I've just said is part of the Hack machine language specification. And now, it's the job of the CPU to turn this specification into reality, to realize this obstruction. So how do we do it? Well, the hero of our story is a humble register called program counter, it's actually a counter and the program counter is also called PC in our jargon. And in what funnels, we want to focus on what happens inside the PC. So here is the desired operation of a typical program counter. First of all, and, an for most the one thing we want the program counter to do is to always emit the address of the next instruction that has to be executed. So let's keep this in mind. Now, if you want to start or restart the computer. Well, in this case, we have to set PC to zero, because we want to execute the first instruction in the program. So PC equals zero makes a lot of sense. Otherwise, if we have a no jump situation, if the current instruction is such that the three j bits are zero, then we want the program counter to increment by one step. So that the next instruction that we will execute will be instruction number one, and then instruction number two and three and four and so on as long as we don't have a jump. However, if the current instruction is such that all the j bits are one, then we have a conditional, I'm sorry. We have an unconditional go to. And in the case of an unconditional go to, we want to set the PC to A. Why? Because if the programmer, if the programmer knows what he is doing, then the programmer has all, already taken care to enter the address you want to jump to, to the A register. So if we do PC equals A, PC, the program counter will emit the address of the next instruction that has to be executed. And finally, if only some of the j bits are, are, are one. One or two of the j bits are one then we have a condition of go to. If we have a conditional go to, we have to look at the ALU output and decide if this go to should actually materialize. That's the whole idea of conditional. We have to check if the condition is true or false. If the condition is true, Then, we want to make a jump. Otherwise, we want to ignore the go to and just do PC++. So, I hope that you are convinced that this abstraction makes sense, that's exactly how a program counter has to work in every computer, and, what remains to be seen is how do we actually make it happen? Well, the answer is already, on the screen here in front of you, the, logic architecture is such that the PC will do exactly what we want it to do. And let us convince ourself that this is indeed the case. Well, let's look at the, reset input, and remember that in order to start the computer, in order to cause the computer to start, executing the kind, the kind of program, we push the reset button in, and we let go. So if PC, I'm sorry, if the register is one, we want to set PC to zero. Otherwise, we have to look at the current instruction. And the current instruction may or may not have, jump bits. If the jump bits are. Well, given any combination of jump bits, you know, all the way from zero zero zero zero to one one one and all the possible eight combinations of three, of three, jump bits. Given every one of these combinations, we have to look at the jump bits and simultaneously look at the ALU output. And decide if we really want to carry out the jump. So, in the slide I describe this, decision as a function. This f function which I made up. Completely meaningless in the computer, architecture. But, I'm using this, symbol f to to try to say that we look at these two different pieces of data together. We combine somehow, this information, and we make a decision. Do we want to jump or not? I store the result of this decision in the symbol load. And if you look at the diagram, you can see that this load is actually the load bit of the program counter. So if load ends up being one, yes, we want to jump. So we PC equals A. PC equals. The current, value on the register which contains the address to which we want to jump. Otherwise we do PC pass pass. At the end of all this operation and all the dust clears what we'll get is that the PC always emits the address of the next instruction that has to be fetched and executed. This, pretty much raps up the, the description of the CPU implementation, and the only thing that remains to do is to actually build it. But before we set out to build it, we have to remember that all this complexity is just one element in the overall computer architecture. That said, it is by far the most complex and interesting element in the architecture. And, we're going to build it. Not now, but, after the next unit. Because in the next unit, we'll describe the overall, architecture of the Hack computer, and then we'll get our hands dirty and actually set out to build both the CPU and the entire Hack computer.
