o  See the OpenVMS Calling Standard for a full description of
       procedure descriptors.

    o  You must specify .PROCEDURE_DESCRIPTOR before the code of the
       routine it describes.

    o  See the descriptions for the $PROCEDURE_DESCRIPTOR and
       $ROUTINE library macros. These macros define the procedure
       identifier and define the storage for the procedure
       descriptor.

    o  You can only use this directive within data or mixed psects
       (psects that have either the NOEXE or MIX attributes).

    o  If automatic data alignment is enabled, this directive aligns
       the current location counter to a quadword (64-bit) boundary
       before defining the procedure identifier.