AMD and Intel license IA-32/AMD64 and extensions from eachother, and they have agreements which make this mutual licensing apply to basically any ISA extension. The original x86 patents are long since expired, but every year they add extensions and cross-license them to eachother.
Base x86 is partially open partially owned by Intel. X86-64 has components owned by AMD. AMD and Intel cross-license. You can’t start a new x86 chip company without paying amd/intel (if you want to use modern isa)
AFAICT, ISA's are heavily patent protected. The patents are usually on implementation details, but it's difficult to implement without stepping on the patents, so...
Intel & AMD have a cross license, x86-64 was actually created by AMD. And before then, (ie, in the 70s & 80s) Intel was forced to license to third parties to qualify for defense & government contracts.
I assume the "forced to license to third parties" part applies in the case of the Zilog Z80?
But re: patents, in the case of Cyrix:
> Focused on removing potential competitors, Intel spent many years in legal battles with Cyrix, consuming Cyrix financial resources, claiming that the Cyrix 486 violated Intel's patents, when in reality the design was proven independent.
[And this is despite Cyrix having white-box reverse-engineered Intel's chips to figure out how to be software- and socket-compatible with them! That seemingly didn't matter, as long as in the end the design they actually put in their own chip wasn't encumbered by Intel's patents.]
> Intel lost the Cyrix case, which included multiple lawsuits in both federal and state courts in Texas. Some of the matters were settled out-of-court and some of the matters were settled by the court. In the end after all appeals, the courts ruled that Cyrix had the right to produce their own x86 designs in any foundry that held an Intel license. Cyrix was found to never have infringed any patent held by Intel.
With that case-law in place, it sounds like anyone demanding that third-parties license their ISA these days is effectively bluffing.
Relatively few companies have an ISA license; I doubt it's a significant income source for ARM versus licensing their core designs: only Apple and Qualcomm are left producing parts based on their own implementations in any real volume.
What's the IP that Apple would otherwise be infringing upon if they didn't license? The "ARM" trademark?