In this unit, we're going to talk about the very basic thing that a computer does, which is execute one instruction after the other. We're going to look more specifically at how exactly is that done. Well, really, it's very simple. You first fetch the instruction that you need to execute, and then you just execute it. And then you repeat. And that's the only thing that a CPU does. Fetch an instruction, execute it, fetch another instruction, execute it, and so on and so forth. So let us look specifically at each of these two stages, what it exactly happens there and how does that translate into what happens in hardware. So let's first talk about fetching. So, how do you get the neck, next instruction that you need to execute. Well, where is the next instruction located at? It's in the program memory, it's in the memory. Where in the memory? In the address that's specified by our program counter, probably. So, we need to basically put the location of the next instruction, of the instruction that you want to execute next, into the address of the program memory. And then we need to read the contents of that memory and then we will get the basic instructions that we need to execute, the code of the instructions that we need to execute. The next instruction will usually just be the one after the one that we just did in the previous. That is if we now finished instruction number eight, usually the next instruction will be instruction number nine. Sometimes we already know that we jump and we're going to need, want to execute the different instruction. So how do we actually go about putting the address of the next instruction into the address input of the program memory? Well, we're going to have to put it somewhere and that's going to be into some counter, into some register and that is usually called the program counter. So basically the hard words of exec, executes the fetch, if you wish, part of the cycle is the following, there is a program counter, when we need to jump into a new location or we need to just increase and go to the next instruction, we need to manipulate the program counter so it will have the address of the next instruction. The output of the program counter feeds into the address specification of our program memory. And then, out of the program memories output comes the actual code of instructions that we need to execute. So that is basically what happens in the fetch part of the fetch and execute cycle. So now, once we have the code of the instruction, now we need to execute it. So the instruction code itself has all the specifics of what we're going to do now. Which a, calculations that we're supposed to do. Are we touching this register or that register and so on. And whether we want to, need to jump afterwards and so on. So usually all that information is coded somehow inside the instructions that we already have because that we got it in the instruction part in the fetch part of the cycle. And usually the way it's coded is in a simple way. So that is basically, different parts of the instruction, bits, actually control the different parts that we need to do right now. Sometimes it's more complicated, sometimes it's more simple but that's a basic idea. So the basic execution executing the current instruction basically mean taking the bits from the instruction code that specifies what to do, and actually doing what needs to be done. Looking at it in the hardware phase, from the hardware point of view, the instruction we already got from our fetch, that is now going to feed into the control bus of our CPU, of our computer. And that control bus, basically controls everything. It tell the ALU what instruction to compute, to add numbers, subtract them, or so on. It also tells where do the, data pieces come from. Do they come from which which register or from the data memory or so on. So we basically have the instruction memory that we just fetched, basically tells us exactly each part of the system what to do right now in the execute cycle. And that really controls whether we read from the data memory, whether we read from the registers, what do we compute, where to write and so on? Of course there are many details. How is that exactly done? How do we, how does the bytes from the instructions specifically tell the ALU what to do which registers to choose and so on? That is best a, viewed through the lens of a particular computer. Specifically we will actually get into these details when we talk about our computer, about the HACK computer. And that is what we are going to do, be doing the next few units where you're actually going to see how we actually control each and every part of the system from the instruction bits. So, we're not going to delve deeper into the specifics of this part, which is really the most, more, most of the work that is done and by the computer. But rather, I want to take a step backwards and look at a certain problem that I've sort of glossed over so far, and that is the fact that we really have a clash between the fetch cycle and the execute cycle. You see, both the program and the data reside in memory. In the fetch cycle, basically we need to get from the program memory, the next instruction. So we need to put into the address of the memory, the address of the next instruction and get the instructions output. On the other hand, in the execute cycle, we need to access data that also resides in memory. So we need to put into the address of the memory, the address of the data piece that we want to operate on, which has nothing to do with the program piece that gave us the instruction. And because we have a single memory, that is a clash, because what are we going to put into the address, are we going to put there, the pro, the address of the instruction, or is address of the data piece? We need to do both and that's a problem. So how do we solve that? Well, basically we are going to do one after the other, that is the usual way. How do we do one after the other? There's going to be a multiplexer that feeds into the address of the memory. In the first part of the cycle in the fetch cycle, we're going to actually set the multiplexer to plug into the address input of the memory, the program counter that is the location of the next instruction. While in the execute cycle, the multiplexer will actually set the memory to actually point into the data address that we need to access. And of course, the output of the memory will go into both places. Both into the, the bit part of this system that needs instruction and into the part of the system that are used inside when executing to actually put information there. A, you may wonder, okay, so how do we do both of them together? Well, when you look at it you really need to remember the instruction from the fetch cycle to used for the execution cycle. So that is what we're going to do. When we actually a, are in the fetch cycle, we put the address of the next instruction, we get the next instruction, and then we need to remember it inside an instruction register. And that instruction register is exactly what is remains holding the value of the instruction that we're now executing in the execute cycle. And then of course, in the execute cycle now we have the instruction already stored in this register. And we can work with all the information that we need a, for the execute cycle using the data memory that is being addressed in the second part of the cycle. So, this is the usual way it's done. Now there is a shortcut and that is simply keep the two different parts of the memory separate. That is, have one unit holding the data memory and another unit holding the program memory. Each one of these units has its own address. And then we don't need to worry about, a, be, between switching between them, a, as we've just shown. Eh, this shortcut makes everything simple. It is sometimes called the Harvard architecture, but we just view it as a variant of the von Neumann Architecture and keep using it due to its simplicity. So, now we've basically finished talking about the general a, architecture of computers. How in general do you actually a, construct a computer? What we're going to switch in the next unit is talking specifically about our HACK computer. And how it is built a, exactly.
