If "casx" isn't an intel instruction, then forget about using it on that processor!

All modern SMP & multi-core processors have 'atomic compare and swap' instructions. (IBM CPUs invented them back in the early 1970s.)

On Intel & AMD x64 processors thay are called variously: CMPXCHG8B & CMPXCHG16B (amongst other variations).

The question was purely about the particular semantics of the Solaris version as that was used by the researchers of the paper I was reading. It was important for me to understand those semantics so that I could work out whether the x64 equivalents were compatible with their algorithms. They are (kinda).

None of this seems relevant to perl shared memory though - where does perl come into this???

It is. Or rather it will be soon.