Hacker News Reader

> literally nobody else could or would implement the same Apple-proprietary ISA extensions.

That is maybe a little excessive, technically they could probably pay an other architecture license holder to do it for them.

View on HN

No, because the architecture license doesn't allow it. Apple got special treatment here.

Oh, the architecture license doesn't allow bespoke ISA extension?

Nope, you need to be compatible with the architecture specification, and it lays out exactly what can be implementation defined and what can't. E.g. you're allowed some freedom in what features to implement (Apple doesn't implement EL3 and this is fine), and you can add implementation defined system registers (Apple has a huge number of them, e.g. to implement TSO for Rosetta). But you can't decide not to support mandatory features (Apple forces on VHE mode, which is not legal - VHE is optional, non-VHE mode isn't), nor can you add extensions to the core ISA. Apple added AMX, memory compression/decompression, a variant of the AT instructions that outputs to a GPR, and the whole Guarded Execution feature (two new parallel exception levels and machinery to call to/from them and lock down things to them), and possibly more, all of them in reserved instruction encoding space.

Reply on news.ycombinator.com