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This expression means nothing to process. Since "Null" expression does nothing, the next statement is proessed. In general, it is used to with "case" statement, when user wants the program do nothing for specified conditions.

NOTE: Null is the default type for allocator operations and variables in access type. Null means that, no object is pointed


case CODE is
       when "001" => Data <= A and B;
       when "010" => Data <= A or B;
       when "100" =>Data <= not A;
       when others => null; -- CODE is blocked except for the three processes written above.
end case;







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