exp(-100.)=-.43e34 on IRIS

[Glenn Randers-Pehrson, WMB]

The behaviour of the EXP() function for large negative numbers on the
IRIS is shameful.  I've tried the following program on a number of
machines (VAX f77, Gould f77, Alliant FORTRAN, Cray-2 cft77) and all
return 0.0 for out-of-range negative numbers.  The IRIS returns
not-a-number if the floating point board isn't used.  Worse, if the
floating point board is used it returns wrong numbers.  This on a 3130
running 3.5r2 and a 2500T running 3.6.


      program bugexp
      dimension trial(10)
      data trial/-1., -20., -50., -80., -85., -88.02968, -88.02970,
     &           -100., -1000., -10000/
      do 10 i=1,10
      x=trial(i)
      xx=exp(x)
      write(*,'('' exp('',f13.6,'')='',g15.8)')x,xx
   10 continue
      stop
      end

Results with f77 -Zf:

 exp(    -1.000000)=  .36787950    
 exp(   -20.000000)=  .20611540E-08
 exp(   -50.000000)=  .19287500E-21
 exp(   -80.000000)=  .18048510E-34
 exp(   -85.000000)=  .12160990E-36
 exp(   -88.029680)=  .15974800E-42
 exp(   -88.029700)=???????????????
 exp(  -100.000000)= -.43075530E+34
 exp( -1000.000000)=  .12234650E+29
 exp(-10000.000000)=  .41732680E-27

 Programmed STOP