Register usage and linkage

     GPR1-3  => arguments (depending upon type)
     GPR4    => the caller's stack frame in the downward-
                growing stack. This is biased and actually points to 2048
                bytes before the real start of the stack frame.
     GPR5    => the called routine's environment pointer
     GPR6    => the entry point in the called routine if
                the call was made by a BASR instruction
     GPR7    => the return point in the caller's routine. The return point
                also contains information to determine if the call
                was made via BASR or branch relative.
     GPR8-15 => preserved
     FPRs    => arguments (depending upon type)
     VR24-31 => arguments (depending upon type)