For me, the official opcodes of the 6502 are enough to make a machine interesting and worth investigating, whether it's a TTL remake, an FPGA implementation, or an emulator.

But one could add more fidelity:

• some C02 opcodes
• some undefined opcodes
• accurate cycle counts
• cycle accurate bus behaviour
• cycle accurate interrupt behaviour

and all of these could be of interest, especially for intricately optimised game code or software protection code. But every one of those is an extra constraint, which makes the project more difficult to make, more difficult to verify, more complex and so perhaps more costly, and is likely to reduce clock speed.

I think choosing an appropriate degree of compatibility boils down to what a 6502 is for: is it a means of performing a computation, or a means of running a large body of existing software very accurately.

It might be the case, if you want to sell something, or maximise your audience, you might want to appeal to people who think of a home computer as being a games console. But I'm not at all sure why that audience might want something which runs at a high clock speed... so, it seems to me that the two goals, of high performance and a great degree of compatibility, wouldn't often be seen together.

(Not wanting to derail any specific threads or projects!)

A pertinent insight, Ed -- thanks for sharing that.

Regarding fidelity, I've added another point to your list:

• some C02 opcodes
• some undefined opcodes
• accurate cycle counts
• cycle accurate bus behaviour
• cycle accurate interrupt behaviour
• hardware compatibility; such as drive strength. Also,TTL vs CMOS voltage levels

Regarding the TTL 6502, Drass mentions dealing with a drive strength issue here. "The solution was to install a 100 Ohm series resistor on the A14 pin at the CPU to “weaken” it a little."

cheers
Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

A very good point! Which brings up another: the clock-to-Q time of outputs and other nanosecond-level timing can be important, to interoperate with memory and peripherals, and may not come for free.

Then a 6502A running at 1 MHz is - to some extend - incompatible to a 6502 due to its slightly faster timing. Then you may also differentiate between different lots from the same manufacturer, not to speak from CPUs of different brands like MOS itself vs. Synertek vs. Rockwell etc.

And even then, “very accurately” is a likely to be a subjective compromise. As we discovered during the TTL CPU project, it would be a monumental effort to model the “unstable” opcodes accurately. Their almost complete absence in popular software might prompt some to ignore them, and yet others to be excited by the esoteric challenge. There is no right answer, of course, and much depends on personal aspirations and predilections.

As a corollary to the above, in many instances the motivation is simply to learn, or to have fun, in which case we’re dealing with an audience of one, and “whatever floats your boat” might be the operative design principle.

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C74-6502 Website: https://c74project.com

Well said!