Re: Thoughts about IEEE floating point (-ish) on vax

[Anders Magnusson <ragge%tethuvudet.se@localhost>]

Hi Mouse!

Den 2026-06-13 kl. 16:15, skrev Mouse:
> > At first; here is a brief overview what differs between vax and ieee
> > floating point:
>
> > 	* +NaN/-NaN:  vax do have "illegal number" which traps.
> > 		Not really NaN.
>
> Are IEEE NaNs signed?
Yes, as well as zeroes.

> > 	* Vax exponent bias is (for F-float) 128, IEEE (for single) 127.
> > 	* Vax hidden bit is 0.5, IEEE is 1.
>
> I'm not entirely sure what you mean here.  By the value of the hidden
> bit, you mean that (say) if the exponent bits are eee and the mantissa
> bits are mmmm, the value is .1mmmm * 2^eee versus 1.mmmm * 2^eee?
Now I'm not getting what you mean :-)
I try to rephrase:

vax:  With an (unbiased) exponent of zero and only the hidden bit set 
the decimal value is 0.5.
ieee: With an (unbiased) exponent of zero and only the hidden bit set
the decimal value is 1.0.

Then the bias is different as well.

Note that I have tested a bunch of this myself, on pdp-11, but that 
should not matter :-)

> To match up the VAX with the IEEE, it needs byteswapping something like
> PDP-endian (swap words-within-long but not bytes-within-words) and then
> an exponent bias difference of 1.
>
> IEEE may allow for layout variations, but I suspect some test suites
> don't.  Yes, that would mean the test suites are broken, but then,
> except for things that really do require IEEE for some reason, they're
> broken anyway for demanding IEEE.
I don't know about test suites, I just read the standard :-)



> > We note that the difference in bias and mantissa is a factor of 4.0.
>
> Looks like a factor of 2 to me.  After bit rearrangement, 1 is biased
> exponent 7f for IEEE (printing as 3f8 because the sign bit shifts it
> over by one) and 80 for VAX (400, ditto).
Yep, sorry, of course it's a factor of 2.  My fault :-)


> > So; what happens if we just consider a vax float to be an IEEE float?
> > 	* For add and sub it works just as well; since there is no
> > 	  dependency in these ops what the bias is.
>
> Well, I'd have to check; I'm not sure - but I *think* IEEE reserves two
> biased-exponent field values for unusual purposes whereas the VAX
> reserves only one.  If so, this means that either overflow or
> underflow, possibly both, must happen differently.
IEEE has either all bits set (for NaN/Inf), or all bits zero (for 0 or 
subnormal).  See the checks in the small code snippet I wrote.

-- Ragge