If you came here from 6502.org, welcome! And even if you didn't, welcome anyway! (But if you didn't, you should check it out; It's simply the web's best portal site for all things 6502.)
After nearly 20 years, Google decided to disable my Google webspace (which I was using to host the images here) without notifying me, so unfortunately the pictures which used to be here are gone. You can see the YouTube playlist I made showing more or less this same process here.
To this day, people debate the relative merits of the 6502 and the Z80 CPUs. They had different features and different architectures, and this stands as one of the classic "holy wars" of the computer world. But whatever their technical merits might have been, nobody can argue that the 6502 definitely had one thing that the Z80 never quite attained: Popularity. The Z80 was only used in one well-known computer system, the TRS-80, so widely disliked that it was nicknamed the "Trash-80". (Some people may dispute this, since the Z80 was used in several other computers (most notably the Osborne line of systems), but few of those computers endure in the memories of modern-day microcomputer enthusiasts.) The 6502, on the other hand, was used in almost all of the classic microcomputer platforms, including the Apple II line of computers (except the Apple IIgs), the Commodore PET (the Commodore 64, probably the most classic microcomputer ever, used the 6510, a close cousin of the 6502), the Atari 400 and 800, and even the Nintendo Entertainment System (NES) video game console. Clearly, the 6502 was the chip to use in the early-to-mid 1980s, and today it remains an important chip among vintage electronics hobbyists.
Personally, although the 6502 is a fine CPU, I prefer the Z80 after working with both of them. The Z80 is simpler to hack with (largely because it does not use dynamic registers, so you can slow its clock speed way down), it is better-documented by Zilog (the company that first manufactured it), and as a bonus, it is more readily-available (and therefore considerably cheaper) than the 6502. While a 6502 chip from an electronics supply store might cost you about $7 today, the Z80 costs less than $2. Indeed, it seems there is an upcoming shortage of 6502s driving their prices higher, while the Z80 is still easier to get your hands on.
This project pretty much duplicates the function of my First Great Z80 Project: It uses a 6502 with a ROM chip and a 6522 VIA to control some LEDs.
Obviously, building something like this requires a few parts. However, the parts that you need to build your own computer from scratch are actually remarkably few, and also amazingly cheap for the most part; About the only really expensive item on the list below is the breadboard, which may run you almost $50 if you get a large one (which is recommended, since setting up all these chips and their interconnecting wires requires a lot of room).
As you put this project together, you will need some way to hold the parts together and connect them, while keeping everything (including the wires) in place so they don't flop around. That's what the breadboard is for. Get the biggest breadboard you can find; Trust me, when you start building projects like this, you'll be glad for the extra space eventually. For this project, I recommend you put the CPU in the center, and the ROM and the VIA on either side of the CPU. This is because the ROM does not have any connections to the VIA, but the CPU has plenty of connections to both of them; This way, you will reduce how much you have to extend your wiring.
The industry-standard pin-numbering scheme for DIPs is to place pin 1 in the upper-left corner, and begin moving down, until you reach the half-way pin in the lower-left corner. From there, move to the right side and move back up, ending up with the highest-numbered pin in the upper-right corner. For example, on a 40-pin chip, pin 1 is in the upper-left corner, pin 20 is in the lower-left corner, pin 21 is in the lower-right corner, and pin 40 is in the upper-right corner. Note that this requires knowing which side is "up" on the chip; Most chips are manufactured with a small orienting notch carved into one side. This notch is meant to be at the top in all pin diagrams.
Let's start with the centerpiece of the computer, the CPU. As has been mentioned, for this project we're using the classic 6502 CPU. In terms of non-Intel CPU platforms, this CPU probably stands next to the Motorola 6800-series as the most-used for microcomputer platforms. The pinout for this CPU is as follows:
1: Vss (Ground) 2: RDY 3: CLK1 (Out) 4: /IRQ 5: NC (Not Connected) 6: /NMI 7: SYNC 8: Vcc (+5V) 9: A0 10: A1 11: A2 12: A3 13: A4 14: A5 15: A6 16: A7 17: A8 18: A9 19: A10 20: A11 21: Vss (Ground) 22: A12 23: A13 24: A14 25: A15 26: D7 27: D6 28: D5 29: D4 30: D3 31: D2 32: D1 33: D0 34: R/W 35: NC (Not Connected) 36: NC (Not Connected) 37: CLK0 (In) 38: SO 39: CLK2 (Out) 40: /RES
The 6502 has, weirdly enough, three power pins; One positive voltage-in and two ground pins. Connect all three (1 and 21 to ground, and 8 to +5 V DC) to give your CPU power.
Make sure that RDY (Ready, which is pin 2) is tied high, or else you might have problems with the CPU halting later. Also tie /IRQ and /NMI high through 3K-ohm resistors. (The official documentation for the 6502 specifies 3K pull-up resistors for these pins.) There is some controversy attached to pin 38, the SO (Set Overflow) pin; It simply sets the overflow register inside the CPU. It doesn't seem to make much difference regarding operation of the CPU, and I used to recommend leaving it unconnected. This is what I did when I made my projects, and they worked fine, but in digital design, it's never a good idea to leave a digital input unconnected, because then the pin will float and could cause unpredictable behavior inside the chip. The SO pin is active-low, so go ahead and connect pin 38 to the high side of the power supply so it stays inactive. (In the photos on this page, however, you'll see that pin 38 stays unconnected, since that's how it was when I took these photos.) (Thanks to Roy J. Tellason for pointing out to me the folly of leaving a digital input unconnected.)
Note that in this picture, you can tell the resistors really are 3,000 ohms by their color bands: The color bands are orange, black, and red. The first two digits, then, are 30 (orange = 3, black = 0), and the multiplier is 100 (red = X100), so 30 times 100 is indeed 3,000.
The clock pins of the 6502 are interesting beasts. However, the only one that you really need to concern yourself with right now is pin 37, which is the only clock input on the 6502. Every CPU chip has a clock pin, and this pin is simply a trigger for the CPU: Every time the clock input pulses, the CPU performs a cycle (which basically means it runs an instruction). That's right, the speed of the CPU is externally-controlled. You can run the CPU as fast or as slow as you want to, simply by changing how fast the CLK pin pulses up and down, although if you make it too high, the CPU will stop being able to keep up and may overheat from trying too hard. This is why every CPU in the world has a hertz rating, which is how fast it can handle clock pulses before it stops working reliably. You can go under this speed by simply lowering your clock pulse, but you can't go much over it without risking damage to the CPU.
The clock input can be a little difficult to generate if you're not familiar with analog electronics. Because the timing of it is so precise, you cannot simply manually cycle the clock by flipping it from low to high with a switch. Doing this will make a split-second instability during each transition, because whenever you press or release a hand-operated switch, the circuit state does not go instantly from low to high, or high to low; Rather, it fluctuates between them for just a fraction of a second. Although it doesn't seem like a long time to you, the clock input of a CPU is so sensitive that trying to pulse it by hand will make the CPU crash instantly. (This behaviour of switches is called "switch bounce", and is an inescapable fact of life when working with switches in electronics.)
So what must you do with this clock input? You need to supply it with a good clean square wave, which is an electric signal which rises and falls instantly (or pretty close to instantly). This creates a waveform which looks like a series of three-sided squares or rectangles, hence its name:
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The creation of square-wave clock signals is a whole world of a topic unto itself. As such, it is out of the scope of this file; For now, a very quick cook-book recipe for making a square wave suitable for use with a CPU like the 6502 is as follows:
Take a 7404 chip, which is a typical TTL logic inverter. (You shold be able to use most variants, including 74F04, 74LS04, etc.) Power it up by connecting its pin 14 to +5 volts and pin 7 to ground. Connect pin 1 to pin 2 through a 1,000-ohm resistor. Also connect pin 1 to pin 4 through a 2 MHz two-pin crystal. Connect pin 2 to pin 3 through a 0.05 microfarad capacitor. Connect pin 3 to pin 4 through another 1,000-ohm resistor. Finally, connect pin 4 directly to pin 5. Pin 6 is now your clock output, and you can connect it directly to pin 37 on the 6502 CPU.
You may need to play around with this clock circuit a bit; However, it is the one specified by Harry The Bastard, who used to have a good website about the 6502 with this picture of the circuit I just described. His site was formerly at www.harry-the-b.demon.co.uk, then he got it moved to a dedicated domain name at www.harrythebastard.org. Unfortunately, harrythebastard.org seems to have been neglected, and the domain got snatched up by a company calling itself Ultimate Search, which seems to have nothing better to do than steal abandoned domain names to spread the virus of their ugly web directory. (Same problem that famously happened to Underdogs a while back.) You may wish to see if you have good results with a 1 MHz crystal, a different-rated capacitor, etc. However, this setup worked fine for me.
And now we come to the RESET pin. Like any CPU, the 6502 must be reset when it is first powered on. This is because as the power is first coming on, it goes through a transition state where the power is not quite fully on yet. This kind of marginal voltage confuses digital electronics, and so all CPUs must be immediately reset after they're turned on to clear them out and re-start them with the now-fully-established power. Desktop computers have built-in circuits which automatically reset the CPU when the power comes on, but for this project, we can stand to keep things simple by manually resetting the CPU with a button. Some would argue that resetting the CPU with a button creates the same variation problems that crop up when you try to clock-pulse the CPU with a button; However, this worked for me. Resetting the CPU with a button seems to work well enough. For this, I recommend one of those tiny push-button switches, the kind which are meant to be mounted on a printed circuit board. You can get them to fit on a breadboard too; They take up very little space and they work quite well. Use a momentary, normally-open push-button switch. (The "momentary" part means that it only takes effect when you push it, as opposed to a toggle switch, which flips back and forth each time you push it. The "normally-open" part means that it is not connected when it's not pushed.)
Begin by connecting the button to the 6502's RESET pin, and connecting the other side of the button to ground. This way, when you push the button, the RESET pin will go to ground, and since it is active-low, pushing the button will reset the CPU. But this leaves us with another interesting electronic problem: When we let go of the button, the RESET pin needs to go high. How can we make it do this? We could certainly just connect it directly to the high end of the power supply, but then when we pushed the button... Well, you can hopefully guess what would happen. We'd be connecting the two sides of the power supply directly together, which would result in a short-circuit. To deal with this problem, we'll use what is technically called a "pull-up resistor", a resistor used to keep something high when it will sometimes be connected directly to ground. Run a 3,000-ohm resistor from the 6502's RESET pin to the positive end of the power supply. Now, normally the resistor will keep the pin high, so the CPU does not reset. But when we push the button, the button provides a direct path to ground, and that direct path is stronger than the path to the high voltage (because there's a 3K-ohm resistor in the way), so we can reset the CPU without fear of it getting confused or short-circuiting.
That pretty much leaves us with the A and D pins of the CPU, which constitute the address bus and the data bus, respectively. The R/W and CLK2 pins also remain; We'll get to all of these in the appropriate sections of the parts decribing the other chips in this project.
As an afterthought, you may want to take a moment to add a capacitor to your CPU. This should simply be a capacitor connected directly between the positive and negative sides of the power supply, connected as close to the CPU as possible; This adds some stability to digital ICs, and generally you should have one on any CPU you use. (This is called a "decoupling capacitor".) The capacitor should be fast (i.e. have a low farad rating, somewhere from 0.1 to 4.7 microfarads), and it should prefereably be ceramic, although I used a mylar one.
For that added touch to your computer, you can add a reset IC, which is an ultra-simple IC which resets your CPU for you automatically. Essentially, a reset IC is a chip with three pins, two of which are simply connected to the power supply, and the third of which is a reset output. If for any reason the power wavers or is otherwise not appropriate for powering digital electronics (such as when the power supply is first powering up when you turn it on), the reset IC sends a reset signal out; When the power stabilizes, it releases the reset signal. It really is that simple. These reset ICs are very cheap and they eliminate the one step that you need to do to get your computer working (besides plugging it in). If you don't use such a device then you just need to reset the system manually with a switch every time you turn it on. I must admit that I thought this would be a problem with the 6502, because the reset button suffers from the same problems of switch bounce as a manual clock button would, and much has been said about "dirty" reset signals and how they tend to prevent CPUs from working properly. This was not such a problem with the Z80 projects, because they were clocked so low that the button's input rose and fell before a single clock cycle completed, but the 6502 must be clocked much faster, which could create a problem. In my experiment, however, I found the 6502 worked just fine with a manual push-button reset. If you're really concerned about reliability, a reset IC is not an expensive addition to your system.
You are now finished wiring your 6502 CPU chip.
(For my earlier quick-start guide to the 6502, click here.)
For this project, we're using a 2865 EEPROM chip, which is an 8-bit, 8-kilobyte, 28-pin ROM chip that can be easily programmed and re-programmed without special chip-erasing-and-burning hardware. It's also fairly inexpensive (mine was $5), making it a good choice for a small hobby project. The pinout for the 2865 is as follows:
1: READY/BUSY 2: A12 3: A7 4: A8 5: A5 6: A4 7: A3 8: A2 9: A1 10: A0 11: D1 12: D2 13: D3 14: Vss (Ground) 15: D4 16: D5 17: D6 18: D7 19: D8 20: /CE (Chip Enable) 21: A10 22: /OE (Output Enable) 23: A11 24: A9 25: A6 26: N.C. (Not Connected) 27: /WE (Write Enable) 28: Vcc (+5 Volts)
The very first thing you need to do with the ROM chip, before you can actually incorporate it into your circuit, is program it. For this project, we'll use a rather short program, written in pure 6502 machine language. Program your 2865 with the following byte values:
0000000000000: 10101001 (A9) ;LDA #$FF 0000000000001: 11111111 (FF) 0000000000010: 10001101 (8D) ;STA $8002 0000000000011: 00000010 (02) 0000000000100: 10000000 (80) 0000000000101: 10001101 (8D) ;STA $8003 0000000000110: 00000011 (03) 0000000000111: 10000000 (80) 0000000001000: 10101001 (A9) ;LDA #$AA 0000000001001: 10101010 (AA) 0000000001010: 10001101 (8D) ;STA $8000 0000000001011: 00000000 (00) 0000000001100: 10000000 (80) 0000000001101: 10001101 (8D) ;STA $8001 0000000001110: 00000001 (01) 0000000001111: 10000000 (80) 0000000010000: 01001100 (4C) ;JMP $E010 0000000010001: 00010000 (10) 0000000010010: 11100000 (E0)
(The first number is the byte address on the chip, the second number is the actual data value that should be in that memory cell. The third number, in parentheses, is the hexadecimal value of the data byte, and the text after some of those bytes is just the assembly-language instructions, placed there for easy reference.) Remember that in this kind of binary work, bit numbers are read from right to left, so in the first byte (which is 10101001), pin D3 on the ROM chip should be 0, NOT 1, which is how it would be if the pins were numbered from left to right.) It is beyond the scope of this file to detail how to program an EEPROM chip; If you have one and don't know how, read my separate electronics writeup, How to program an EEPROM.
There are two more bytes on the ROM that need to be coded for this project to work properly. They are located at addresses 1111111111100 (1FFCh) and 1111111111101 (1FFDh). Set address 1FFCh to 00h, and set address 1FFDh to E0h (which is 11100000 in binary).
After the chip is programmed, you can drop it onto your circuit board. Begin by powering it up (connecting it appropriately to your power supply via pins 28 and 14). Keep pin 27 (Write Enable) high, since that pin is only used to write to the ROM, and you do not want to overwrite this program that you worked so hard to get into it in the first place.
Connect the ROM's data bus to the CPU's data bus. This is basically a matter of simple name-matching with wires, EXCEPT for one small anomaly: The ROM's data bus pins are numbered beginning with 1, while the 6502 CPU's data bus is numbered beginning with 0, resulting in an off-by-one difference for each pin mating. Below is a table indicating where each pin on the CPU should meet each pin on the ROM (the CPU's pin is on the left, the ROM's pin is on the right):
D0 --- D1 D1 --- D2 D2 --- D3 D3 --- D4 D4 --- D5 D5 --- D6 D6 --- D7 D7 --- D8
Connect the ROM's address bus pins to the CPU in exactly the same way. However, because the ROM we're using only has 13 address bus pins and the CPU has 16, you can just leave the top three address bus pins of the CPU unconnected to the ROM. (Inexplicably, while the 2865 numbers its D pins beginning with 1, the A pins begin with 0 just as they do on the 6502, so you can match the names directly.)
Note that because the connections from the CPU's data and address buses to the ROM chip have so many connections, I will not actually be connecting them in the pictures yet; They would create a huge ugly mess of wires and would obscure the other connections, so I will leave them for now and get back to them when we have made the other connections on the project. It's not crucial that they be done now, just as long as you remember to do them when you've finished the other parts of the project.
Now all that's left on the 2865 is the CE (Chip Enable) pin and the OE (Output Enable) pin, which, when both activated, will make the ROM spit out whatever is in the selected memory address. These pins will be covered below, in the section on the 74LS138 3-to-8 decoder.
You are now finished wiring your 2865 ROM chip.
If you really want to cut down on the number of obscure parts you're using for this project, you can certainly omit the I/O chip and create your own I/O circuitry. All it really is is a buffer of the data bus, so you can set up some kind of glue logic that latches the data bus and maintains it on some other wires, but the I/O chip we're using is a fairly simple beast, and it will make things a lot easier on you later when you want multiple peripherals. Trust me, it's worth getting an I/O chip and learning to use it, it will make your life easier.
I'm a fan of the 6522 VIA (Versatile Interface Adapter) chip. It's simply and logically designed, yet quite powerful. It's fairly readily available and very cheap. And it was designed to work with the 6502 anyway, so it's a match made in heaven. (Or at least in the factory.)
The 6522's pinout is as follows:
1: Vss (Ground) 2: PA0 3: PA1 4: PA2 5: PA3 6: PA4 7: PA5 8: PA6 9: PA7 10: PB0 11: PB1 12: PB2 13: PB3 14: PB4 15: PB5 16: PB6 17: PB7 18: CB1 (Control for Port B) 19: CB2 (Control for Port B) 20: Vcc (+5V) 21: IRQ (Active-low; Output to the CPU, not an input to the VIA) 22: R/W (High = read, low = write) 23: CS2 (Chip Select 1; Active-low) 24: CS1 (Chip Select 2; Active-high) 25: CLK2 (Enable) 26: D7 27: D6 28: D5 29: D4 30: D3 31: D2 32: D1 33: D0 34: RES (Reset; active-low) 35: RS3 (Register Select 3) 36: RS2 (Register Select 2) 37: RS1 (Register Select 1) 38: RS0 (Register Select 0) 39: CA2 (Control for Port A) 40: CA1 (Control for Port A)
Ignore the CA and CB pins for this project; We won't be using them at all.
Begin by powering up the VIA by connecting pins 20 and 1 to your power supply as appropriate.
The RS pins on the VIA are normally connected to a CPU's address bus, and that's exactly what we're going to do here. Connect RS0 on the VIA to A0 on the 6502, RS1 on the VIA to A1 on the 6502, RS2 on the VIA to A2 on the 6502, and RS3 on the VIA to A3 on the 6502. Similarly, connect all the D pins to each other; This is just a matter of pin-name-matching, once again.
For the 6522 to work, both CS (Chip Select) pins must be active. The trick here is that CS1 is active-high and CS2 is active-low; So tie CS1 to the high side of your power supply, and we'll worry about CS2 when we get to the 74LS138 chip. Also tie R/W directly to the R/W pin on the CPU. Similarly, the CLK2 pin can be connected directly to the 6502's CLK2 output; See how easy things are when chips are made for each other?
Almost done! The 6522 has a reset pin, just like the CPU. Add a reset button to it, just like the CPU; Connect the pin to ground through the button, and add a 3,000-ohm pull-up resistor so the pin normally stays high.
Now all that's left is those PA and PB pins. Those pins are the actual I/O buses, the two I/O buses that are built into the 6522. For this project, all the pins will change, but you don't have to connect anything to them; You can just check their status with a voltmeter after the computer is turned on. If you'd like a really nifty-looking LED setup though, you can connect LEDs to each of the I/O bus pins (bear in mind that they are positive outputs, so connect the positive end of the LED to the I/O bus pin and the negative side to ground).
Note that in the picture below, I didn't quite connect all the I/O bus pins to LEDs, because I didn't have quite enough space to comfortably do so; This should still be enough to give you an idea of what's going on.
You are now finished wiring your 6522 I/O chip.
(For my earlier introduction to the 6522, click here.)
For the ultra-simple Z80 projects which had no RAM (only ROM), it was not necessary to use a memory selection chip. In a "real" computer, however, it is necessary to have a mechanism to divide the RAM from the ROM. Although this computer does not have any RAM either, it suffers from an irritating aspect of the 6502: The 6502 requires "memory-mapped I/O", which means that it must have the I/O addresses actually mapped into the system memory space, which creates a "hole" in your memory which cannot be used for data storage because it's being used by your peripheral controllers(s). Since the 6502 treats your I/O chip as just another memory chip, we end up with two memory-mapped chips on this computer, and thus we need a way for the CPU to choose which is which.
That's where the memory decoder chip comes in. It is a rather simple, but useful device which has a number of inputs which are connected to the top of the CPU's address bus, and a number of outputs, one of which turns on depending on the state of the inputs. The number of outputs on such a chip is 2 to the power of X, where X is how many inputs there are. We're using a 74LS138 chip, which is a 3-to-8 decoder, meaning it has 3 inputs and 8 outputs. The pinout for the 74LS138 is as follows:
1: A0 2: A1 3: A2 4: /E1 (Active-low) 5: /E2 (Active-low) 6: E3 (Active-high) 7: /Y7 8: Ground 9: /Y6 10: /Y5 11: /Y4 12: /Y3 13: /Y2 14: /Y1 15: /Y0 16: Vcc (+5 V)
Of course, I like to power chips up before anything else, so connect pins 8 and 16 to your power supply as appropriate. Now we can start doing some digital memory logic conversion!
Begin by connecting the 74LS138 to the top 3 pins of the CPU's address bus: Connect the 6502's A13 to the 74LS138's A0, connect the 6502's A14 to the 74LS138's A1, and connect the 6502's A15 to the 74LS138's A2.
Keep all three of the 74LS138's enable pins (the ones labeled E) enabled: Tie pins 4 and 5 to ground, and tie pin 6 high.
We're placing the ROM at memory range E000h to FFFFh. This corresponds to the /Y7 pin (pin number 7) on the 74LS138, so connect pin 7 on the 74LS138 directly to pins 20 and 22 (Chip Enable and Output Enable) on the ROM. This way, when the CPU is accessing any memory address in the range from E000h to FFFFh, the ROM will be enabled and it will output the contents of the memory location selected on the CPU's lower 13 address bus pins.
We're putting the VIA at memory location 8000h to 9FFFh, which corresponds to pin /Y4 on the 74LS138. So tie pin 11 on the 74LS138 to /CS2 (pin 23) on the VIA. This way, when the CPU is accessing any memory address in the range rom 8000h to 9FFFh, the VIA will be enabled and it will access the register selected on the CPU's lower 4 address bus pins.
You are now finished wiring your 74LS138 memory decoder chip.
We've gone through all the chips in the project. At this point, your project board should look like this:
We're almost done putting things together, except for one little thing: Remember those wires we left off the ROM chip? Well, it's time to go back and hook up the address and data buses of the ROM chip. When this is done, your computer should look something like this:
Congratulations! You are also finished assembling your single-board computer.
Begin by powering up your power supply. Hold down both reset buttons for a few seconds, then release them. (The 6522 VIA resets almost instantly, but the CPU's reset line must be held low for a few clock cycles in order for it to reset properly, so hold down its button for perhaps 1 or 2 full seconds. In any case, make sure you release the 6522's reset button BEFORE the one for the 6502, or the 6502 will start sending signals to the 6522, but the 6522 won't do anything with them because it's still resetting.) After this, all that's left is to simply check the VIA's I/O bus. They should be in an on/off pattern: One pin should be on, the next should be off, and so on. You can check this with a voltmeter, or if you elected to install LEDs on the VIA so you could see its state at a glance, then the LEDs should reflect this. If this happens, congratulations! You have successfully built, programmed, and operated a scratch-made computer!
The green LEDs I originally used in the pictures above are pretty good LEDs, but they're not quite bright enough to show up well in a photograph. Thus, ironically, the only part of this computer that really shows it's working properly--the LEDs--is the one part that doesn't work right in the pictures above: They're too dim. I took them all out and did what I should have done in the first place: Replaced them with "super brite", mini-LEDs. These LEDs have two advantages: First, as the "super brite" part suggests, they are indeed quite bright for such small things, and they do show in a photo. Secondly, as mini-LEDs (3 millimetres in width), they're much smaller than the regular LEDs pictured above, so you can fit them more easily into tight circuit board spacings like this, although there is still a little crowding. The resultant LED setup, when the computer is turned on and operated, should look like this:
You can clearly see that the eight red LEDs connected to PA and the eight green LEDs connected to PB are in an alternating on/off pattern.
As soon as the 6502 CPU is reset, the first thing it does is check a very specific memory address; Actually, two very specific memory addresses. These are FFFCh and FFFDh. Together, these two bytes form the 6502's reset vector, which is the memory address it should start running instructions from when it is reset.
Because the top address bus pins of the CPU all turn on while it looks for the reset vector (because FFFCh and FFFDh both begin with 111 in binary), the ROM chip is enabled. This is because 111 in binary corresponds to 7, and so when the 74LS138 chip gets all of its address inputs turned on, it activates output number 7, which we connected to the ROM; When this happens, the ROM chip is enabled and it outputs whatever is at the selected memory address.
We programmed the ROM at addresses 1FFC and 1FFD. This is because the ROM has only 13 address pins (the top three CPU address bus lines only go to the memory decoder chip), and so when the CPU is looking at address FFFC, the ROM only gets 1FFC. (The ROM could not tell the difference whether the CPU were looking at address FFFC or 1FFC, because the lower 13 bits of those addresses are identical.) Thus, we can see that the top three address bus lines are used solely for selecting a chip, and only the lower 13 are actually used for memory addressing.
Recall that we programmed the ROM with a value of 00 at 1FFC, and E0h (which is 11100000 in binary) at 1FFD. So now when the CPU checks its reset vector, it will get a vector of E000h (because the two bytes are stored backwards, for some odd reason). In binary, this is 1110000000000000. Can you see where we're going with this? This means that the CPU will start running program instructions at that address; However, all the address bus lines are off except for the first three, and the first three are only for chip selection. In this way, we can begin placing our program instructions at the beginning of the ROM chip. The CPU will consider itself to be at address E000, but as far as the ROM can tell, it's at address 0. This is perfectly fine and does not constitute any problems, as long as you can remember that the CPU and the ROM are not in agreement about the currently-chosen address. Because the CPU looks for its reset vector at almost the very top of the memory space, there is no way around this; That is, there is no way to place the ROM at the bottom of the CPU's address space unless you use a second ROM chip, and we don't want to go to that kind of trouble right now.
And so the CPU goes merrily on its way, executing instructions from the beginning of the ROM. What instructions does it run? The program we put in the ROM is repeated below in assembly language for easy reference:
LDA #$FF STA $8002 STA $8003 LDA #$AA STA $8000 STA $8001 JMP $E010
LDA is short for "Load into Accumulator"; It loads a value into the accumulator, which is the general-purpose register inside the 6502. The first instruction loads it with the value of FFh. (In 6502 assembly language, a number sign (#) indicates an immediate (constant) value, and a dollar sign ($) indicates a hexadecimal number.) The second and third commands are STA (Store Accumulator) commands; They store the contents of the accumulator in memory addresses 8002h and 8003h. 8002h is 1000000000000010 in binary; Again, perhaps you can notice the effect of this. That's right: The first three bits of the CPU's address bus become 100, which is the number 4. And since our VIA is connected to output 4 of the memory decoder chip, this has the effect of activating the VIA and then selecting its register number 2. (Because the bottom 4 bits of the address bus are connected to the VIA's register select pins.) So the value FF is actually sent to the VIA's ports 2 and 3, which are the two DDRs (data direction registers). This has the effect of making all the VIA's I/O bus pins be outputs, which is what we want, because we're outputting through them, not inputting from them.
The next three commands are similar in form to the first three. The value of AAh (10101010 in binary) is loaded into the accumulator, and then placed on registers 0 and 1 of the VIA, which are the two output registers. This sets the status of all the VIA's I/O pins; They will form an alternating on/off pattern. One pin will be on, the next will be off, the next will be on again, and so on.
The final instruction is simply a JMP instruction, which jumps to a specific memory address. It is jumping to address E010h, which happens to be the very address of this instruction itself. This makes the CPU keep on repeating that same instruction endlessly, entering an infinite loop. This is done to effectively lock up the CPU so it does not go haywire, running through the memory and executing whatever random instructions it might find. When all of this is done, the CPU is pretty much idle (it just keeps jumping to the same instruction infinitely), but the state of the VIA's interface stays the same, and you can see the effect of this in LEDs if you hook them up.
