The Motorola 68000 team published several papers that are still available through IEEE that describe the design process. They built a prototype from TTL chips which allowed them to rapidly iterate the microcode, nanocode, and programmed logic arrays without having to wait for samples to come back from manufacturing.
>> 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 (over 20 acres or 8 hectares) — an area about 940 ft (286 m) square.
It would take light about 956 nanoseconds to travel the 940 feet across that theoretical CPU. Assuming that was the limiting factor in your clock speed, that would cap it at 1.05 Mhz.
Obviously, it's going to be far more complex than that, but I'd be really surprised if they could get the clock any faster than that.
edit: elaborated on art trend
If you're ever in Cambridge (UK), pop along to the Computing History Museum and see "Mega Processor" (http://www.megaprocessor.com/).
Though that is all true and good, decent speeds are possible with discrete designs. A decade before the 6502, discrete component mainframes hit clock speed like 35 Mhz.
Those used BJT's; that's the main difference. Low impedance inputs exhibiting low parasitic capacitance.
I wonder if it could be done in a similar way but run full speed? From the FAQ:
> 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.
I think I'm imagining most of an entire motherboard of something similar to the C64 or Apple ][ implemented this way, wall mounted along with a display and controls, with a classic software library.
So if you look at the highest performance discrete computers of their time, things like the CRAY the CDC machines, etc. You'll see that they consumed larger and larger currents to achieve the switching rates they needed to achieve and increasingly exotic cooling systems (with the Cray's flourinert systems being pretty damn awesome).
I find it fascinating to walk from the ENIAC to the 704 to the B5500s, and PDP-8's and 11's, to microprocessors to systems on chips. Each step in integration let you make things smaller and run them faster. It wasn't until Moore's law broke down in 2004 and the world became multi-core that this really stopped happening.
Much more expensive than regular PCBs, of course.
That said, I think the objective was to duplicate the processor rather than the systems it ran in so their existing MOSFET design is true to that.
Speed of light is really starting to matter. That's about a nanosecond just for signal to propogate, and that's assuming a straight wire with no intermediate components.
(At least it's not GPS where the orbit speed plus lack of gravity means you have to factor relativity into the mix too!)
Approximately 0m/s over any practical distance. :)
There were some pretty serious limitations on parallel buses and backplane designs back in the day. Hence everything went serial (USB, PCIe, etc).
Eh. That's kinda cheating.
(I'm joking of course… this is impressive. Apparently they had to use the chips because discrete 4-terminal NMOS transistors are no longer manufactured!)
>"Is it truly a "discrete 6502?"
I read the answer but didn't understand the answer given. Can someone elaborate on this? Wasn't the original 6502 an "integrated" chip? When I think about a "discrete" CPU I think of something like an older main frame where the different functional units of the CPU might be spread over a few different circuit boards with busses connecting those boards. Is my understanding of a "discrete" CPU incorrect?
When used in that manner, "discrete transistors" generally means that each individual transistor is on/in one single package. The item pictured here: https://www.sparkfun.com/products/retired/12852 is a "discrete transistor" (the plastic package contains one single transistor, the three leads connect directly to the collector, emitter, and base).
Now, in the case of this design, they explain that they are unable to purchase actual discrete transistor packages because no one makes the appropriate MOSFET transistors as single transistor to a package items anymore. So they had to use four transistor packages instead of single transistor packages. So under a strict definition of "discrete" they did not use "discrete transistors". They then provide their rationale for why one could view this as still discrete in a less strict usage. It is then up to you to decide if you accept their rationale.
Does it suffer from Spectre attacks ?
seeedstudio.com does online quotes, but you'll need design files.
I'd totally pay $300~400 for one.
A Z80 or 68k would be way cool!
It will inspire kids / young people to learn and create things beyond what's possible today.
I love the LEDs illustrating what is going on internally.