Re: [fpu] Architecture

["Vladimir" <vladimir@fotocomp.fi>]

> Do we really need 80 and 128 bit precision? I guess at least for a start
> single and double precision would be enough. In fact I would put in our
> first silicon test run (hopefully this will happen at the end of summer)
> only single precision FPU since I think it is important to have small die
> (at least for the first silicon). Idea is to be able to "sell" this first
> implementation chips for a price of a hamburger. ;-)

The IEEE 754 does not insist on implementing double precision but strongly
recommends that an implementation should support the "extended precision"
format corresponding to the widest basic. In case of 64-bit double format
the extended has _at least_ 79 significant bits. For 32-bit single it should
have _at least_ 43. This policy is needed to support a sequence of
continuous operations with a little bit higher precision before rounding
them to the basic format. That is why x87s and later Intel FPUs keep the
internal representation 80-bit wide. It is easy as long as you use a
separate register space.

As far as I could understand you want to use double precision because it
fits well into 64-bit register file. Strictly speaking you may pretend that
your implementation conforms to IEEE 754 implementing only the single basic
precision and using double as its extension. You cannot claim to support the
double precision without implementing the extended format for it.

Vladimir Ushakov