MOnSter 6502 (prototype PCB)

The MOnSter 6502

A new dis-integrated circuit project to make a complete, working transistor-scale replica of the classic MOS 6502 microprocessor. We brought our work in progress to show off at the 2016 Bay Area Maker Faire!


The MOnSter 6502 is a work in progress, designed by Eric Schlaepfer, in collaboration with Evil Mad Scientist Laboratories. Together, we're the folks that brought you the "Three Fives" discrete 555 timer and the XL741 discrete 741 op-amp.

Special thanks to Ken Shirriff,, Chuck Peddle, and Bunnie Huang.



The 6502 is the famous processor found at the core of such influential computer systems as the Apple ][, the Commodore PET, the Atari 400 and 800 home video game consoles, the BBC Micro, and the Tamagotchi digital pet. Slight variations of it were found in the Commodore 64, the Atari 2600, and the original Nintendo Entertainment System.

What's with the name and capitalization?

MOnSter 6502 is a play on the original manufacturer and device name (MOS 6502) as well as acknowledging its large size.

How big is it?

It's a four layer circuit board, 12 × 15 inches, 0.1 inches thick, with surface mount components on both sides.

How many components are there on the board?

In total, there are 4304 components on the board. There are 3218 transistors and 1019 resistors that comprise the "functional" part of the 6502. In addition to these, there are also LEDs sprinkled throughout that indicate the values of various control lines, registers, and status bits, as well as additional transistors and resistors (not counted in those "functional" totals) that are necessary to drive those LEDs.

As of the current design, the statistics are as follows:

Are you nuts?


What is the current status of the project?

MOnSter 6502 (assembled prototype, on the work bench)

We have just fabricated the first full-scale prototype, publicly demonstrated it (in progress) at the the 2016 Bay Area Maker Faire, and since then brought it up to the stage of successfully running BASIC code.

The photo above shows the assembled board, still in very early stages of testing. The 40 square pads around the edge of the circuit board correspond to the 40 pins of the original 6502 integrated circuit, and can be connected up with alligator clips. They're also designed to look like the wire-bond contact pads on an IC die. Some of the indicator LEDs are lit up as well. (You can see a bunch of green LEDs lit up less brightly if you look closely; we need to tweak the brightness values of the LEDs in the next revision.)

Since that photo was taken, we've replaced the dimmer green LEDs with brighter ones, found and fixed a few soldering errors, added some needed capacitance to the bus lines and have (finally) made it to the stage of actually running programs on the board. (Yes, there are a few issues still remaining, but so far it looks like we'll be able to take care of everything in the next revision of the board.)

Does it run at the full speed of an original 6502 chip?

No. The MOnSter 6502 is relatively slow compared to the original, thanks to the much larger capacitance of the design. The maximum reliable clock rate is not yet determined, but we expect it to be in the tens to (low) hundreds of kHz. The clock speed is limited (amongst other things) by the gate capacitance of the MOSFETs that we are using.

Can you hook it up inside an Apple ][ and run Oregon Trail?

No, not directly. It's neat to think of plugging the MOnSter 6502's in-circuit emulator (ICE) in-circuit replica (ICR) cable directly into a socket inside an Apple ][, but that wouldn't actually work. The Apple ][ design relies on a number of clever tricks that derive timing for video generation and peripheral control from the main clock signal — all of which will fail if you need to run at a slower speed.

There are some ways to get around limitations like these. For example, the Replica I computer (an Apple I clone) uses a Parallax Propeller chip to emulate a system clock and some of the timing-dependent external processing.

So what will it be able to do?

It can act as an in-circuit emulator for a 6502 integrated circuit, in any circuit that can run at a lower clock rate. We're working on bringing it up on a custom 6502 development board that is currently running BASIC, much as you would find on an Apple ][. We are definitely interested in finding other applications where it could be substituted for an original 6502.

How long did it take?

This has been a year-long project, thus far. The primary design work was done over six months, from July 3 to December 1, 2015.

Is it truly a "discrete 6502?"

Not in the strictest sense. It depends upon how picky you would like to be.

The MOnSter 6502 uses the original dynamic NMOS logic design, implemented at the individual transistor level.

Dynamic NMOS requires a large number of "transmission gate" transistors that are used to switch currents. For various technical reasons, only a 4-terminal MOSFET can make an effective NMOS transmission gate. Unfortunately, individually packaged 4-terminal MOSFETs are no longer commercially available. However, they do still make arrays of 2 or 4 MOSFETs on a single chip with a separate substrate pin. We used the 4-pack version — These are the quad transistor array chips that we mentioned earlier.

Because these transistors do share a pin, there are (strictly speaking) integrated circuits in the MOnSter 6502. However, one might still argue that it is a discrete transistor design since there are not (for example) any logic gate chips in the circuit.

How big would the MOnSter 6502 be if it were made with through-hole parts instead of surface mount parts?

About 19 square feet (1.7 square meters).

How big would a 68000 microprocessor be at the scale of the MOnSter 6502?

Also about 19 square feet (1.7 square meters).

How big would a modern CPU be at this scale?

The Apple A8X, found in the iPad Air 2, contains about 3 billion transistors. (This is comparable to the number of transistors in modern desktop computer CPUs as well.) At the scale of the MOnSter 6502, that would take about 885,000 square feet (82000 square meters) — an area about 940 ft (286 m) square.

How big is the MOnSter 6502 compared to the original 6502 die?

The original device was 153 × 168 mils (3.9 × 4.3 mm) or an area of 16.6 square mm. Given that ours is 12 × 15 inches, that makes it about 7000 times actual size.

Are you going to make one out of vacuum tubes next?


Can I buy one?

No. (Not yet, at least.) This is very much a hobby project at the moment. However, we are considering various avenues for the future of the project. Sign up for our mailing list if you would like to receive any announcements that we may eventually make with regards to that kind of thing.

Is it expensive?

It is definitely not cheap to make one of these. If we had to ballpark what one of these would sell for — assembled and working — it would certainly be larger than $1k and smaller than $5k. While the circuit board itself is large and a little bit expensive, the cost is actually dominated by the component and assembly costs of an extremely large number of tiny components, each of which is individually quite inexpensive. Add to that the setup and test costs of building complex things like these in small batches, and you'll immediately see how it adds up.

Is there going to be a soldering kit version of this?

No. (But on the other hand, "Anything is a soldering kit if you're brave enough!")

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