> IIRC this was not ALWAYS the case, on x86 not too long ago the CPU might choose to put your operation in an 80-bit fp register, and if due to multitasking the CPU state got evicted, it would only be able to store it in a 32-bit slot while it's waiting to be scheduled back in?

I don't think the CPU was ever allowed to do that, but with your average compiler you were playing with fire.

Did any actual OS mess up state like that? They could and should save the full registers. There's even a bultin instruction for this, FSAVE.