--- a/plotlib/ps_subs.f
+++ b/plotlib/ps_subs.f
@@ -559,7 +559,7 @@
 c                   in the iout array (3,1,3,1,3,1,3,1)
 c     Note: the bit mask is shifted to always start counting on a '1' bit.
 c
-c---  Uses the library routines and() and rshift() for bit manipulation 
+c---  Uses the library routines and() and ishft() for bit manipulation 
 c     which are present in most fortran libraries as extensions to f77.
 c
       dimension iout(*)
@@ -570,7 +570,7 @@
         ibitold = and(1,imask)
         if(ibitold.NE.0) go to 5
           nshft = n
-          imask = rshift(imask,1)
+          imask = ishft(imask,-1)
       end do
 c
  5    nout  = 0
@@ -594,7 +594,7 @@
 c
         ibitold = ibit
         nbits = nbits + 1
-        imask = rshift(imask,1)
+        imask = ishft(imask,-1)
       end do
 c--- Add final bit(s) to end of 16 bits checked
 c--- Now append any zero bits shifted out originally