There are many software implementations of the 6502 which is now owned by Western Digital. Zilog still owns the Z80 and still makes it.
I know that it is illegal for video game emulators to reverse engineer proprietary hardware. Also, it is obviously illegal to distribute proprietary
ROM images such as BIOS etc.

The general consensus on the internet, is it is in fact legal to develop and distribute emulators, but I'm not sure what the legal basis is for this.
Is not a 6502 instruction set proprietary? Ironically, the 6502 was made to be binary compatible with the 6800, and the Z80 with the 8080 etc.

If someone develops a simulator for the 6502 or Z80, what protects them from being sued by Western Digital or Zilog? Keep in mind, I'm not
asking the likely hood of this happening nor the ethics of reverse engineering. I'm searching for the legal stance of the USA and other countries
on this subject.

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http://www.thestarrlab.com

I'd say the 6502 being produced by Western Design Center would protect people from being sued by Western Digital!

I'm pretty sure reverse-engineering is not illegal at all unless you exactly copy it and make a commercial product.
Compaq, Phoenix Technologies etc reverse-engineered the IBM PC BIOS and reimplemented it clean-room style which was perfectly legal https://en.wikipedia.org/wiki/Phoenix_T ... BM_PC_BIOS

The 6502 is in no way binary compatible with the 6800. Furthermore, its index registers and the stack pointer are strictly 8-bits in width, and the stack is limited to the range 0x100-0x1FF. Finally, the 6502 has numerous additional addressing modes not found in the 6800, or almost any other contemporary microprocessor.

You may be confusing the 6502 with the 6501, which to the best of my knowledge, was made to be pin-compatible with the 6800. That feature brought on a lawsuit by Motorola which was settled by discontinuing the 6501.

The Z80 was binary compatible, but only for a portion of its instruction set and architecture. It's extensions set it apart from the 8080. To get around the copyright issue on the mnemonics for the 8080 instructions by Intel, Zilog used a different set of mnemonics for its assembly language.

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Michael A.

Edit to add: if you're a hobbyist, don't let any doubts stop you from designing and building whatever 6502-related project takes your fancy. If you're a business, take legal advice.


There's a lot of information on the internet! What you're looking for is a legal opinion, but where you're asking is a forum on the internet. I've certainly got an opinion about this, and indeed I believe my opinion is well-founded, but why should you believe me rather than anything else anyone writes?

You need one of two things: a legal opinion from a professional who puts their reputation and professional status behind what they say; or an understanding of how IP law works and the various kinds of protections available to products, inventions, and names. To get that understanding you need to do some study, and you need to be able to distinguish trusted and untrusted sources of information, as well as distinguishing the different kinds of protections.

I could write about what I believe I know, and elsewhere I may have done, but it's not going to carry any special weight.

(But to summarise, I believe there's nothing protected about the behaviour of sufficiently old circuits, and nothing protected about the 6502 instruction set or the mnemonics. You need to understand and distinguish copyright, mask copyright, patents, design patents, trademarks. You need to distinguish hardware from software, and understand the status of microcode. You need to understand the difference between an emulator and the software you run on the emulator. You may well need to understand the history of copyright as it applies to masks, to software in both source form and binary form, and to firmware. Fortunately, there is no microcode in the 6502.)

If you were making a product to sell, you'd certainly get legal advice, or you'd take your own advice, at your own risk. You'd be forming a view about the state of the law and the stance of the intellectual property holders.

To offer my own untrustworthy amateur voice:

I don't believe the 6502 instruction set is owned or protected. The specification has been publicly available, license-free, since the late 70s. By "license-free" I mean that nobody had to sign anything to obtain a specification. It's not a trade secret. It's public information, and I believe you're free to implement an assembler, disassembler, emulator, logic implementation, FPGA or ASIC. That said...

Again, my own non-legal view is that the WDC likely own (besides their own proprietary logic designs) the TRADEMARK and the PATENTS.

The trademark means the name "6502" (and 65C02). You're free to build a product and call it a 5266, that happens to use the same opcodes as the 6502 and operates (almost) identically. But it's likely you can't use "6502" in any advertising or warranty of operation... if you do then you're using a name that someone else owns. Just like I can't make a cola that tastes like Pepsi then sell it as "Sark's Pepsi Cola!".

There at least used to be patents related to decimal mode (at least). I don't know if the patents are still active, or what they specifically covered. I think it might have covered the _operation_ of decimal mode on the 6502 - which would make it a violation to implement that functionality... or it covers the specific transistor-level design (which wouldn't apply to a software implementation or an FPGA implementation). Again, I don't know the details here but the general principle is the same: if it's patent protected then you shouldn't do it.

As Big Ed said, if you're going to build something to sell, then you should find qualified lawyer. There have been a number of flashback / retro type products, such as C64 and Atari game emulators... all of which emulated (either in hardware or software) both the CPU and custom logic of machines of the era. I don't know if the developers signed IP licenses or releases with whoever owns the rights to that historic hardware, but I would bet they all had lawyers...

The timeframe is such that patents can no longer apply to the NMOS part and to the earliest CMOS parts.

Both Intel and Peugeot found that numbers alone can't be trademarked.

Remake products which use names such as Commodore or Atari will surely have sought licenses from the rights holders: but this isn't usually a concern for hobby projects. Bundling any kind of software is certainly something where copyright may apply, and usually the selection of software with such products shows signs of care being taken.

It's notable, perhaps, that quite a few hobbyist projects include Microsoft Basic images, which is most likely a copyright violation, and the reason that works out OK is that Microsoft have taken no interest. Copyright in software is a civil matter: if the owner doesn't sue, nothing happens.

It's also notable that some hobbyist projects use open source software without taking sufficient care of the details of the license. Again, if the owner doesn't sue or raise a stink, there's no visible consequence. Only a loss of reputation.

Even if they could sue -- and IANAL -- I'm not sure if I see a rational business case for doing so. You want to get people involved with your CPU, and sending lawyers after your potential customers might not be the way to do that. The chilling effect on the 6502 community would be brutal. In the end, these companies are trying to make money.

Like so often in life, we can look to Star Wars for guidance: They've let fans do amazing stuff with their "IP", like the mind-blowing complete remake of the first film as the Beatles' Sgt. Pepper album (https://www.inverse.com/article/31038-b ... swap-ninja). This has helped turn the dysfunctional Skywalker family members into cultural icons -- basically, Star Wars has replaced The Iliad for cultural references in our civilization. More to the point, it's helped them make money hand over fist.

If we go back to CPUs and look beyond the 6502, there is the "free and open" RISC-V ISA (https://riscv.org/) coming up just at the time when we're reading all these things about hardware bugs and secret Minix implementations in Intel CPUs. It would be interesting to know if Intel has told its lawyers to hold back, especially as ARM just had a bad PR day regarding RISC-V (https://www.theregister.co.uk/2018/07/1 ... v_website/). Is this a time when it makes money-sense to sue somebody because of your ISA?

One of the things that's happened with open source is the realisation that you can make a successful business from packaging, supporting and distributing free software. Red Hat and SUSE come to mind, but there are surely many others.

It would be interesting to know what WDC think of the many HDL implementations of the '02 and the 'C02. Those are probably much more of a concern than any software re-implementation. What WDC offer of course is risk reduction and productivity: they have the tool chain and the history of licensing their cores successfully.

There isn't an HDL implementation of the '816 - whether that's an important fact, I don't know. (Edit: there is now a VHDL implementation with GPL licence, as part of the MiSTer project's SNES subproject.)

It will be the same with RISC-V - it's a free ISA, but there will be businesses built around implementations, and extensions, and support, and verification, and so on.

(On the other hand, the people who own rights to names such as Commodore, Atari, Amiga, and the people who own rights to 8-bit games, will care about licensing because that's the business they are in. The likes of Nintendo care about protecting their exclusive right to their brand, and can also afford to "send a message" by pursuing any and all infringements. Indeed, for trademark ownership, you need to pursue, or you may lose the ownership.)

IANAL, but:

No, not illegal at all. You just have to be clean about it (true reverse engineering, not copying). The 'illegal' aspect would be if you then ended up selling something that is a patended method, but in that case you wouldn't have needed to reverse engineer at all - patents are public.

In the general case, yes (unless the proprietary ROM comes with a license that allows distribution, which is not unheard of)

No
No, not at all, not even the 6501 (it only had pin compatibility with the 6800, not instruction set compatibility. The architecture is also different (registers etc.), as MichaelM said.)
The Z80 is a binary superset of the 8080 (mostly), which means that 8080 *opcodes* will execute on the Z80. But the 8080 *mnemonics* are copyrighted, so Zilog came up with a new set of of mnemomics for the same opcodes.

As for emulating or simulating a particular CPU - that has been done since computers were invented, there are emulators or simulators for nearly everything (although very few for some), including minicomputers, IBM mainframes, every popular microprosessor. I've never heard about a lawsuit about that. That doesn't mean that there haven't been any, but if there's been any 'successful' ones then *some* people must have heard about it.

And that's how it should be.. but apparently it's a criminal offence in Japan, and it looks like there's a lot of lobbying to make it criminal in the U.S. and maybe elsewhere too. But that's just my impression.

Are you sure about that? A quick look at Wikipedia suggests that Japan is following other countries in making circumvention of copyright protection a criminal matter - that would apply, probably, to games systems which have some form of protection mechanism. It wouldn't apply to any ordinary sort of 8-bit software, or mask copyright, or any of the other IP protections in the realm of re-implementing microprocessors.

When I enter Japan I have to sign a form where I declare that I'm not bringing any copyrighted material with me (that I don't have a right to, e.g. copies of copyrighted material). I am not 100% certain of course, but to me that implies that it's not just about the means to circumvent protection.

I wonder if that's more about audio and video works? But it might be that transporting goods could be criminal where producing them is merely civil.

I've updated my first comment to this thread: I feel strongly that the kind of discussion you get on these kinds of topics doesn't help anyone. If you're a hobbyist, go ahead and pursue your hobby. If you're a business, get legal advice. If you want to study law, go and study law.

It might be interesting to chew over, but the headline topic is always rather FUDish. Fear, uncertainty and doubt, with the effect of sapping enthusiasm.

I very much agree with that. Thanks Ed.

A 6502 program is data. In truth, it's not even that. It's a bunch of voltages on wires. The response to that bunch of wires changing voltage values over some interval is not copyrightable in the first place. It's not even patentable.

A schematic is not copyrightable. A rendition of a schematic is (you can't just make a perfect copy of a schematic from a book and post it everywhere), but you can make your own. A schematic is a list of parts and how they're connected. Similarly, recipes are not copyrightable. Same thing, can't photocopy the book, but the recipe is the recipe -- a list of ingredients.

The technique to build such a device may be patentable, but not the mapping of data values to voltages on wires.

They could be a trade secret, but, obviously, that secret is out.

If you can write a NAND gate simulator, you can write a CPU simulator.