Hi, this is Wayne again with a topic “The Monster 6502 is a gargantuan version of a famous microchip”.
Hi, my name is eric and i brought a 6502 microprocessor here at maker faire. The 6502 is a very influential microprocessor. It was essentially the first low-cost 8-bit computer brain that became available in the late 1970s and it got used in computers, including the apple one, and the apple ii, the commodore 64 and even the nintendo entertainment system uses the 6502. So you might be thinking that i brought just a small computer chip. The problem is that a computer chip is really hard to see and you can’t see inside it to see how it works, and so what i’ve done is i’ve blown it up into a large circuit board with leds on it, and so you can actually see the Data move around from register to register and get processed by the alu all the mathematical operations. So i started this project early last year. It’S been actually a little bit over a year since i’ve been working on it, the actual hard work, the schematic capture and the layout took six months of continuous work.
So there are several big problems, i would say the biggest one is figuring out the feasibility. Would this even work because there are so many transistors? There are over 3200 transistors in the design, and so i did a lot of experiments to try and prove to myself that it would work, and it did so. I made a small program counter with a subset of the circuit elements that i use in the design and it worked perfectly and so at that point i knew i had to sit down and put down the rest of the transistors. So, let’s look at how a single instruction flows through the 6502 microprocessor.
It begins at the data bus, which is this section right here on the side of the chip and the data comes in and it gets latched in a special register called the pre-decode register. The 6502 uses that to get a preview of this instruction before it starts executing it, so it can find out how many bytes are in the instruction and what basic path it needs to take through the decode logic from there. It gets latched into the instruction register up here. This is what the 6502 uses to remember the instruction that it’s currently executing as data flows into and out of the chip. So the instruction once it’s latched in this register goes into a very special decode rom that contains a special code to essentially implement the instruction set. So that’s how the 6502 knows what to do with each instruction that results in 131 control lines that enter the section in the middle of the chip and that’s called the random control logic, and so there’s a lot of different logical operations that happen here.
That turn those 131 control lines into a set of what are called 40 data path controls. Those are these guys right down here with the colorful leds. Each datapack control line tells the registers what to do. For example, it might tell the accumulator register to put its data onto the data bus, or it might tell the x register to latch its information. That is, pull information off of the data bus and store it in memory. If you want to learn more go to monster6502.com, you .