historical stuff in math(3)

[Peter Jeremy]

On Wed, Jan 21, 2004 at 06:41:49AM -0500, Thomas David Rivers wrote:
>> Isn't all the vax-D format related stuff math(3) getting pretty old ?
>
> It's very similar to the IBM mainframe format.  So, a port
> of FreeBSD to the IBM mainframe could still use it.  (The VAX
> format was just a copy of the IBM one with an extra precision bit 
> thrown in every now-and-then.)

Not really.  The IBM S/360 uses base-16 whereas virtually everyone
else (including VAX) uses binary.  The S/360 double precision format
has a 14-digit (56-bit) fraction (no implicit digit), a fraction sign
and a 7-bit signed exponent.  The VAX-D documentation in math(3) is
totally irrelevant to the S/360.  Any serious math library would need
significant re-work to handle the increased range and reduced/variable
precision.

Someone else mentioned the Alpha - VAX-format FP is specified in
the architecture to simplify migration from the VAX.  The early chips
included it in hardware - do the recent chips still include it?

> But - even the mainframe has an available/alternate IEEE format now,
> and the mainframe version of gcc uses that...

I think this must be new in the S/390.  It's definitely not part of
the S/360 or S/370 families and I don't believe it existed on the 30xx
or 43xx families.

Peter