[LegionMammal978]

Google v. Oracle was not about interoperability: as the decision presents it, Google copied the APIs for Android specifically because they didn't want false interoperability with desktop Java. Instead, the court found that the use was transformative (bringing Java to a new, distinct platform), insubstantial (the declarations being non-central to the overall implementation), and with little market effect (Sun/Oracle having failed to make its own mobile Java viable).

In contrast, reimplementing a copyrighted ISA would quite clearly be substantial and could easily have large market effects. So you'd have to lean heavily into the idea of interoperability to overwhelm the lack of these other factors, or argue that the ISA is not copyrightable per se.

Also, even if an ISA isn't copyrightable, you have to watch out for patent rights the owner might have over any part of the ISA.

[wk_end]

Just to prod at this a little bit:

The vast majority of ARM licensees aren't doing anything transformative, that's for sure. But would something like the M1 - bringing ARM to the desktop when all past attempts have failed - have been considered transformative?

The question of what constitutes a "large market effect" is a big one...if I make a chip to serve as a controller in my brand of hard drives, is that a large market effect? ARM loses out on a few pennies per drive or something.

And I'd dispute the characterization of micro-architecture design as "insubstantial". Not accusing you of this, but this really has been the most frustrating thing about RISC-V - software engineers who don't know anything about CPU design comparing it to Linux when really it's more like POSIX or, yes, the Java class library. The insides of microcomputer CPUs haven't really resembled their interfaces very much since the early 80s - even the 68000, a consumer-grade CPU that's 45 years old (!) was designed so that the microcode engine that drove it could be used to implement an array of instruction sets. Any useful implementation of a CPU has an enormous number of internal details that aren't implied by the ISA.

[LegionMammal978]

> Any useful implementation of a CPU has an enormous number of internal details that aren't implied by the ISA.

If anything, I'd say that it's precisely because of this independence that an ISA can be seen as a separate work from any of its existing implementations. So what I'm trying to get at, is that if an ISA is copyrightable at all, in whole or in part, then any complete re-implementation will be copying most of the ISA's own substance, regardless of differing internal details. (Where by 'ISA' I specifically mean the external interface that software can access.)