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The MOnSter 6502

A 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. This year we're bringing it back to show off the fully working version!

If you'd like to see it in person and try it out, come meet us at the 2017 Bay Area Maker Faire, May 19-21 in San Mateo CA.

Credits

The MOnSter 6502 is a continuing 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, visual6502.org, Chuck Peddle, and Bunnie Huang.

FAQ

Sixty-Five-Oh-What?

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:

• Components that correspond 1:1 with transistors in the original 6502:
  • Total active transistors: 4237
  • 3218 enhancement mode n-channel MOSFETs
    • 2599 discrete
    • 619 located on 161 quad transistor array chips (25 of these 644 transistors were not used)
  • 1019 depletion mode MOSFETs (the MOnSter 6502 uses resistors in place of these MOSFETs)
• 525 Additional parts present only in the MOnSter 6502:
  • 167 LEDs
  • 123 extra MOSFETs for driving the LEDs
  • 20 filter capacitors
  • 2 zero-ohm jumpers for net tie reasons
  • 8 current limit resistors
  • 167 resistors for the LEDs
  • 36 diodes (for ESD protection)
  • 2 connectors (5 V power, 40-pin "ICR" ribbon cable)
• Total parts: 4304

Are you nuts?

Probably.

What is the current status of the project?

We have fabricated the first full-scale prototype and publicly demonstrated it (in progress) at the the 2016 Bay Area Maker Faire. Since then we have brought it up to the stage of successfully running BASIC code. Our current work in progress is building up the capability around this processor — keyboard, monitor, programming interfaces and so forth — so that you can actually use it.

The photos above show our prototype, hooked up via a ribbon cable to a custom single board computer that uses the MOnSter 6502 as its CPU. (The 6502 is just a microprocessor; it needs additional logic board elements including external memory to do anything.) 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; the 167 LEDs that we added show the register values, data flow control bits, and so forth.

Getting to this point was not easy. Since first fabricating the board, 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, in both BASIC and FORTH

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 around 60 kHz. The primary limit to the clock speed is the gate capacitance of the MOSFETs that we are using, which is much larger than the capacitance of the MOSFETs on an original 6502 die.

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 currently running it 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 ~2 year project, thus far. The primary design work was done over six months, from July 3 to December 1, 2015. There have been several stages of reviews and revisions since then.

Is it truly a "discrete 6502?"

Not in the strictest sense. However, it really 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 credibly 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 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?

No.

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

Also about 19 square feet (1.7 square meters).

Are you going to make a 68000 next?

No.

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.

Are you going to make a dis-integrated modern CPU next?

No.

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!")

Stay up to date!

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