OpenVMS I64 only.

    Associates a named or anonymous linkage with a routine name. When
    the compiler sees a CALLS, CALLG, JSB, BSBB, or BSBW instruction
    with the routine name as the target, it will use the associated
    linkage to decide which registers need to be saved and restored
    around the call.

    Format

      .CALL_LINKAGE  routine_name [,linkage_name] [,input] [,output]

                     [,scratch] [,preserve]