historical stuff in math(3)

RMH rmhlldr at yahoo.co.uk
Thu Jan 22 03:41:53 PST 2004

Peter Jeremy wrote:
> 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?
All 21264 CPUs (EV6, EV67, EV68, EV68AF, EV68CB, and EV68DC) support
32-bit and 64-bit VAX F, G, and D floating-point formats, in addition
to IEEE formats. You probably mean VAX-F format; it's supported fully.
VAX-D has a limited support, but still available.

Besides, all Alpha floating-point instructions are separated as IEEE,
VAX, and independent. Thus, it's not a good idea to remove any of
VAX floating-point formats from math(3).


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