input=<>

    Register set that indicates those registers from which the
    routine receives input values.

    This register set informs the compiler that the registers
    specified have meaningful values at routine entry and are
    unavailable for use as temporary registers even before the first
    compiler-detected use of the registers. Specifying registers in
    this register set affects compiler temporary register usage in
    this case:

    o  If you are explicitly using any of the Alpha or Itanium
       registers (R13 and above).

    In either of these cases, if you do not specify a register that
    is being used as input in the input argument, the compiler may
    use the register as a temporary register, corrupting the input
    value.

    This register set has no effect on the compiler's default
    register preservation behavior. If you are not using the VAXREGS
    optimization switch or any of the Alpha registers, the input mask
    is used only to document your routine.

 output=<>

    Register set that indicates those registers to which the routine
    assigns values that are returned to the routine's caller.
    Registers included in this register set are not saved and
    restored by the compiler, even if they are modified by the
    routine.

    This register set also informs the compiler that the registers
    specified have meaningful values at routine exit and are
    unavailable for use as temporary registers even after the last
    compiler-detected use of the registers. Specifying registers in
    this register set affects compiler temporary register usage in
    this case:

    o  If you are explicitly using any of the Alpha or Itanium
       registers (R13 and above).

    In either of these cases, if you do not specify a register that
    is being used as output in the output argument, the compiler may
    use the register as a temporary register, corrupting the output
    value.

 scratch=<>

    Register set that indicates registers that are used within the
    routine but which should not be saved and restored at routine
    entry and exit. The caller of the routine does not expect to
    receive output values nor does it expect the registers to be
    preserved. Registers included in this register set are not saved
    and restored by the compiler, even if they are modified by the
    routine.

    On OpenVMS I64 systems, the compiler will not use these registers
    as temporary registers.

 preserve=<>

    Register set that indicates those registers that should be
    preserved over the routine call. This should include only those
    registers that are modified and whose full 64-bit contents should
    be saved and restored.

    This register set causes registers to be preserved whether or
    not they would have been preserved automatically by the compiler.
    Note that because R0 and R1 are scratch registers, by calling
    standard definition, the compiler never saves and restores them
    unless you specify them in this register set.

    This register set overrides the output and scratch register sets.
    If you specify a register both in the preserve register set and
    in the output or scratch register sets, the compiler will report
    the following warning:

    %AMAC-W-REGDECCON, register declaration conflict in routine A