Register usage

The following list shows the register usage and linkage.

     GPR1    => a list of argument addresses, terminated with an address
                containing a 1 in its high order bit
     GPR2-12 => Preserved
     GPR13   => an 18-word save area
     GPR14   => the return point in the caller's routine
     GPR15   => the entry point in the called routine
     FPRs    => not preserved, value undefined
     VR24-31 => arguments (depending upon type)