So, throughout this week, we talked about the computer architecture and how the various shapes are synthesized into a single overall architecture and in project five you actually have to build this architecture and test it using our project materials. So, the purpose of this unit is to give you all sorts of tips and advice on how to do it. Well, before we get started, I want to begin with some overall, broad overview of what we have done so far. Or what we are doing in the course in general. Well, we are building a computer, and the computer is implemented as a very high level chip. And this chip makes use of two main components, CPU and a memory unit. The memory, in turn, consists of several RAM units which are pieced together, and every one of these RAM units is basically a collection of many registers. The CPU also includes several registers, if you recall, the D register, the A register. It also includes a program counter, and, of course, the ALU that we built in project two. We can continue to resolve these chip parts into simpler or more basic chip parts. For example, the program counter makes use of a register and an adder, and the ALU also makes use of another instance of an adder chip. And, of course, all these chips at the end of the story, are based on elementary logic gates, those that you built in the first week of the course. And also, other chips, higher level chips, make direct use every once in a while, of these lower level gates. Now if we look at what I've just described, bottom up, then in project one, we built the elementary logic gates. In project two, we built the family of adders that led up to the ALU. In project three, we built a family of registers that ended up with RAM units. And finally, in this project, we are going to take the fruits of all this hard work, and put it together into a complete computer system. So, basically, we have to implement the CPU, the memory unit, and then we have to implement the syntheses of these two high level chips, into a complete working general purpose computer. So, that's what we'll do in project five. So, here is the abstraction of the Hack computer. And here is also, the implementation that we discussed in the previous units. And we see that the architecture is based on a CPU, a data memory unit, and an instruction memory unit, which we called, ROM. So, let us begin with the CPU. Once again, here is the CPU, in in abstract terms. We see we see here, the API, or the input and output units of the CPU. And if we delve into the implementation, get this diagram, that we discussed earlier in the course. And here are some tips on how to build this relatively complex chip. Well, first of all, if you look carefully, you will see that all the chip parts here are chips that we've built before in previous projects. So, it's just a matter of taking these chips off the shelf, so to speak, and piecing them together in some clever way. So how do you do it? Well, another inspection of this diagram, reveals that we have all these C control bits, that you see here. And, most of these control bits come from the current instruction. So here's, for example, a C instruction that consists, if you recall, of several fields. And I used color which is, obviously is, is meaningless for the computer but, it helps me communicate with you so, we see an instruction coming in. And your job as, the designer of this chip, is to unpack these control bits and send them, or route them, using HDL, to their correct destinations in the various chip parts that make the overall architecture. So, if you do this carefully, if you figure out which control bit has to go where, you will basically glue all these parts together in a logical way and and at the end of this synthesis, you will get an overall CPU architecture. Now actually I, I really glossed over quite a few details because it's not just a matter of taking these control date, bits and sending them to some destinations. You may have to add some logic that does some processing of some of these control bits, in order to achieve the desired effect. And also bear in mind, that there are also some control bits that come out of the ALU. If you look at the diagram, I actually forgot to circle them as well. These control bits also play an important role in in the overall architecture. And once again, you have to write some HDL logic that takes these control bits, combines them with some other information that comes from the instruction, and and piece them together in a way that achieves the desired effect. Now, we have intentionally, we haven't given you exact instructions on how to do it. And intentionally, we just use this catch-all C label, because we, we want you to think about it and figure out yourself how to put all these things together. Once you do it, you will achieve the overall architecture of the CPU. Here is also the stub file that we are giving you. The stub file contains, you know the documentation of the CPU, the names of the input and output units, and then obviously you have to provide the the missing implementation. All right, so, moving along, we have resolved the CPU, and the next thing that I want to discuss is the memory unit. The data memory unit which is the, the unit that contains the, the running program and the data that the program generates, the variables. And in a higher level language objects arrays, and so on and so forth. All right, so this is the memory abstraction. It's a single address space that has an in, an address input, and an out, output. And it also has this nice side effect of refreshing the screen and probing what the user is is doing with the keyboard. Now the overall address space of of the Hack memory consists of three logical segments. Now there's no physical separation, but there's a logical separation, if you will. So the first 16K of the memory are dedicated to the running program and, and the data that it generates. The next 8K of the memory are dedicated to a screen memory map, and the last register in the memory is represents the keyboard memory map. So, how do we build it? Well, we have to realize this abstraction using chips that we already built before. And here is some high level diagram of this, architecture. So we have three basic chips. We have the RAM chip that we built in project three. Then we have a built-in screen chip which essentially is just a regular memory chip, so it's not really terribly interesting to implement it, if you implemented the RAM before. But, it has the very nice side effect of refreshing the physical screen. And finally, we have a humble register called, keyboard which holds the currently pressed, the scan code of the currently pressed key. So, you have to write HDL code that takes an incoming address, whatever this address may, be and funnel this address to the right chip part within the the overall address space. So if the address is below 16k, well, there's nothing to do with it. You simply retrieve the right register from the RAM chip part. But if the address is between 16,384, and 24,575, then you have to do something to take this address and, and route it to the right address in the screen, memory chip. And finally, if the address is 24,576, then you want to route it to the to the keyboard memory map. So, that's basically what your HDL code has to do, and if you will do it, you will achieve this glue that holds together these chip parts and delivers the overall functionality of the data memory. All right, so we've resolved the CPU and the memory, and the only thing that remains to do is to figure out how to implement the ROM. Well the ROM is really a simpler version of the RAM. It's, it's just, it's, it's a RAM chip and yet, it's, it's a read-only device. So, it's actually easier to build than a RAM chip. And therefore, we decided not to bother with with asking you to build it, and we supply it as a built-in chip. We didn't do it only for that reason. We also, we, we, we supplied built-in version of the ROM because we also endode, endowed this built-in chip with the ability to load the program within the the hardware simulator. So it's very convenient that we have this built-in chip, and all you have to do is take it and plug it into the overall architecture. So this is probably the easiest thing to do in this project. So, that's it. We we built all the the three components that make up the architecture and the only thing that remains is to actually effect in HDL the very diagram that you see here in front of you. So, we have to write a few HDL statements that connect the CPU to the memory, and the ROM 32K, in the same way that you see here in the diagram. How do we do it? Well, we start with a, a stub file that gives you the API of the computer, and then you write a few lines of HDL code that basically implement this diagram in HDL. In order to do this project, as usual, you have to go to the website. Read the project page that gives you all sorts of the documentation about the files that you are going to need. All these files are already available on your computer, as part of the NAND to Tetris folder. So you're all set and you have everything you need in order to finally complete the computer architecture. There are a few more resources, as usual that you are, you're welcome to use. And, that's it. If you do that you will end up having a running computer. And and this will be the end of the hardware part of this course. The next unit as usual, is Perspectives.
