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Re: Default value of x87 registers' reserved bits

Stathis Kamperis <> wrote:

> So all of the reserved bits seem to be set to 1. Is it fine to assume
> that this is true in all cases ? Could someone please confirm it with
> an AMD processor ?

I think you should be _extremely_ cautious about relying on
non-specified behaviour. Although I'm sure it's attractive
from a performance standpoint, you have no guarantee that all
future CPUs will remain consistent with current behaviour.

I'd even be wary of a runtime test because only some
instructions or sequence of instructions might use the
"reserved" bits. OTOH, it's always possible they're not used
at all and never will be.

Summary is (IMHO) that if the CPU architects have reserved the
bits then there is at least some chance they'll use them some
day even if they're not using them already, and I suspect that
FP code in general and all the flavours and extensions of FP
instructions on the x86 family of CPUs are considered esoteric
enough by many people that identifying such a problem would be


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