Name: CPUはどのようにしてマシンコードを読み取るのか？- 6502パート2 (How do CPUs read machine code? — 6502 part 2)
Uploaded: 2021-01-14T10:16:27.000Z
Duration: 49 min 14 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

I set out to write the classic hello world Program, but not like this.

分詞構句

Not building on a whole stack of software, but starting with just a microprocessor.

And this is the 65 02 microprocessor that started the home computer revolution in the 19 eighties.

We hook this up and it is actually executing code, but it's just not doing anything all that interesting just yet.

You've got the data bus pins hooked up here through these resistors.

Eso, no matter what the processor does, it always reads 11101010 which is E.

So whenever tries to fetch an instruction, it always gets E A.

And that happens to be the op code for the no up or no operation instruction, which does nothing.

So we saw in the last video the microprocessor initialize is and fetches and executes instructions, but right now we aren't able to feed it any instructions from anything other than literally hard wiring the snow up instruction.

So it just sits here successfully doing nothing forever.

So I guess it's running a program, but not a very interesting one.

But let's back up a romp chip that we can program with some different instructions that the process will be able to fetch and execute using different addresses.

So this is a 28 C 2 50 60 prom, and the 2 56 refers to the fact that it can store 256 K or 256,000 bits.

That's bit so if you want to think in terms of bites, it's gonna be 32 kilobytes now.

If we look at the pin out for the problem, you know, it's pretty straightforward.

Then there's the eight data lines, the Io lines, and basically, the way this works is if we settle these address lines to particular address, then we get out some data on the data lines based on whatever we've programmed in the pram for that address.

So we can use this program or two to program it ahead of time with whatever instructions or data we want the microprocessor to read.

First, let's walk through exactly how this is gonna work now.

Roughly speaking, it's pretty straightforward.

We'll hook up 15 address lines from the microprocessor over to the prom here, and then we'll hook up the eight data lines between the processor and the problem.

And then whenever the processor wants to fetch an instruction from the prom, it'll set those 15 address lines to some particular address and use the data lines to read whatever data the prom spits out at it.

One thing you might notice is that a promise got 15 address lines, right?

A zero through a 14 but the microprocessors got 16 address lines.

Well, it was 16 address lines on the microprocessor.

The address could be anything from all zeros, which is, you know, an address of zero through all ones, which is an address of, you know, 65,000 or FFFF in Hex.

So basically the microprocessor can access, you know, up to 65,000 unique addresses.

But the rahm only needs 15 address lines because it can only hold 32 K of data.

If we just took a zero through a 14 directly from the microprocessor to the rahm, just like we have here.

Then when the microprocessor Reed's address zero through seven f f f or you know 7 32,067 Then it's gonna read Address zero through seven f f f for 7 32,067 from the prom, which means it's gonna be able to read from anywhere in the room, which is which is great.

But then, if the processor tries to read from address 8000 through F f f f this is this a 15 line will be high.

This this first bit here that's gonna be high.

But the rest of the rest of the address lines are still gonna count for essentially from zero through seven FFF.

Essentially, the processor is going to see the contents of the Rahm sort of repeated twice.

And, you know, maybe that's okay, but it's kind of a waste because we could use these addresses here for something else.

So what we could Dio is we could connect that that a 15 line that we otherwise weren't using, we could connect that up to the chip enable signal on the problem, and that's Ah, that's an active low signal.

So what this will do is we'll say, Well, the prom's only gonna output anything so the chip is enabled.

It's only gonna help put anything when when a 15 is low, because it's an active low signal that way, the wrong sort of maps to the first half of the address space.

But if the processor tries to read from the other half the address based, then you know that top bid that a 15 bit is gonna be high.

And so the room's not gonna output anything.

And that means we could have other things that these addresses, like RAM or input and output devices and so on.

But you might remember from the last video that when the microprocessor starts up, the first thing it does is it reads from address F, f, F, C and F F F D, and does that to figure out where to start fetching instructions.

So we'd really like the F, F, C and F FT addresses to be somewhere in our rum so that we can program that start addressing the Rome.

We can just invert a 15 before we before we connected to the chip enable signal.

So this way, when the microprocessor is reading from this upper half of the address space that a 15 line is gonna be high, that first bit is a one.

So this will be high, which when we inverted it will be low.

And then the low signal here will activate the chip enable, which is an active low pin.

And so this inverter will essentially move the E prom from the lower half of the address base to the upper half of the address space.

Which means we can get that f f f c n f f f d value out of the prom and then the lower part of the address space from zero all the way to seven FFF.

That'll be unused, and we can use that in the future for something else.

And actually, instead of an inverter, I'm going to use a NAND gate configured like this, and this is just the same as an inverter because both pins are tied together like this.

So both inputs air high than the outputs gonna be low And if both inputs are lower than the outputs, gonna be high and it could just as easily use an inverter.

But you'll you'll see later why I'm using the damn gate.

So at the prom and start by connecting power and ground.

Then there are a couple of control signals I can hook up.

So we got the right enable signal here, which is active low on Pin 27.

So I'll tie that high, since that's just used for programming.

Yes, even though it's a programmable realm, it's it's designed to function as a romp, and you read only memory.

So once it's programmed and put into a circuit like this, we really only want to read from it.

Next, we'll type in 22 which is output enable I'll tie that low so the outputs always enabled.

But of course, that's still only if the chip is enabled.

So, really, by tying the output, enable pin to ground like this, were saying that the output of the problem will be enabled anytime the chip is enabled.

Remember, we're gonna hook that through the inverter back Thio a 15 that the top address line.

I'll add a 74 h c 00 which is which has four nan gates on it.

And then I'll connect power and ground for this chip.

Then I'll collect the A 15 line, which is this pin right here.

This is the address 15 object that around Thio pins 12 and 13 which are just two of the two inputs for one of the the Nan gates On here.

I'll connect those two pins together, which basically turns this into an inverter and then the outputs on pin 11.

So this is just the inverted copy now of of a 15 and I'll connect that around to the chip enable signal over here on the prom.

So now if the microprocessors reading from the upper half of its address space, this a 15 line will be high, and then we inverted over here, So now it'll be low.

But then the chip enable signal here is active low, so the prom will be active whenever the processors reading from the upper half of the outer space, which is what we want.

And now that's just this NAND gate here, connecting from address 15 through to the chip enable line of the the problem.

Now the rest of this is pretty straightforward.

Is connecting address line zero through 14 and the data lines that the data lines.

I'll hook up the other 15 address lines from the processor to the e problem and again, starting with a zero on the processor, connecting to a zero on the prom and going all the way through a 14 on the processor, connecting to a 14 on the problem.

You know, maybe you bought the kids for me with all the parts.

Definitely check out the data sheets to make sure you're connecting the right address lines to the right address lines.

Once you got the address lines hooked up, then we want to do the same thing with the data lines.

But first make it these resistors out of here, since we're no longer hard coating that no up instruction.

So then I'll hook up the eight data lines from microprocessor to GE prom, and I'll start with D seven on the microprocessor and hook that to D seven on the prom and work my way back to D zero on the microprocessor, hooking to D zero on the problem.

So now whenever the processor wants to fetch an instruction from the e problem, it can read from any address in the in the top half of the address space and will be able to use these data lines these blue wires to read whatever data the E problem spits out at it.

So now let's program the prom with some instructions, and maybe a good place to start is toe.

So kind of where we started with resistors, hard wiring, that no up instruction.

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CPUはどのようにしてマシンコードを読み取るのか？- 6502パート2 (How do CPUs read machine code? — 6502 part 2)

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