function r9pak (y, n)
  c december 1979 edition. w. fullerton, c3, los alamos scientific lab.
  c
  c pack a base 2 exponent into floating point number x.  this routine
  c is almost the inverse of r9upak.  it is not exactly the inverse,
  c because abs(x) need not be between 0.5 and 1.0.  if both r9pak and
  c 2.0**n were known to be in range, we could compute
  c             r9pak = x * 2.0**n .
  c
        external i1mach, r1mach
        data nmin, nmax / 2*0 /
        data aln210 / 3.321928094 887362 e0 /
  c
        if (nmin.ne.0) go to 10
        aln2b = 1.0
        if (i1mach(10).ne.2) aln2b = r1mach(5)*aln210
        nmin = aln2b*float(i1mach(12))
        nmax = aln2b*float(i1mach(13))
  c
   10   call r9upak (y, r9pak, ny)
  c
        nsum = n + ny
        if (nsum.lt.nmin) go to 40
        if (nsum.gt.nmax) call seteru (
       1  31hr9pak   packed number overflows, 31, 2, 2)
  c
        if (nsum.eq.0) return
        if (nsum.gt.0) go to 30
  c
   20   r9pak = 0.5*r9pak
        nsum = nsum + 1
        if (nsum.ne.0) go to 20
        return
  c
   30   r9pak = 2.0*r9pak
        nsum = nsum - 1
        if (nsum.ne.0) go to 30
        return
  c
   40   call seteru (32hr9pak   packed number underflows, 32, 1, 0)
        r9pak = 0.0
        return
  c
        end