Variable declarations

[ shared ] variable identifier_list : subtype_indication [ := expression ] ;

• entity_declarative_part
• architecture_declarative_part
• package
• package_body
• block_declarative_part
• function_declarative_part
• procedure_declarative_part
• process_declarative_part

identifier {, identifier }

[ resolution_function_ name ] type_mark [ constraint ]

• relation { and relation }
• relation { or relation }
• relation { xor relation }
• relation [ nand relation ]
• relation [ nor relation ]
• relation { xnor relation }

Shared variables declarations are allowed only in the declarative part of entities, architectures, packages, package bodies and blocks. Shared variables declarations must not be used in processes and subprograms.

More than one process can access shared variables. If several processes access a given shared variable in the same simulation cycle, the sequence of accesses is not prescribed. It is neither the actual value of the variable after the simulation cycle nor the different values of the variable read within the simulation cycle are deterministic. See example below.

The variable count of the type positive is declared.

VARIABLE count : positive ;

The variables index and memory are declared as subtypes of the types integer and bit_matrix .

VARIABLE index : integer
      RANGE 0 TO 99 := 0 ;
VARIABLE memory : bit_matrix
      ( 0 TO 7, 0 TO 1023 ) ;

• address_bus is declared as a variable of the type bit8 and initialized with the value “00110110”.
• integer_address is declared as a variable of the type integer and initialized with (2**nb_bit-1).
• The variable bittab is declared as a subtype of the type bit_vector and initialized with “000111Z1Z”.
• The variable tab is declared as a subtype of the type table_type and initialized with the values 2, 3, 4, -2 and 0.

VARIABLE address_bus : bit8
      := "00110110" ;
VARIABLE integer_address :
      integer := 2**nb_bit - 1 ;
 
VARIABLE bittab : bit_vector (1 TO 9)
      := ( 1 TO 3 => '0', 7 | 9 => 'Z',
          OTHERS => '1' ) ;
 
VARIABLE tab : table_type( 0 TO 4 ) :=
      ( 2, 3, 4, -2, 0 ) ;

An example for the non-deterministic of using shared variables :

A shared variable which is manipulated by two processes is declared in the architecture.

It is not prescribed, whether

• first p1 increments counter by one (counter = 1) and then p2 decrements counter by one (counter = 0)
• first p2 tries to decrement counter by one, which would lead to an error because of the subtype violation

ARCHITECTURE beh OF sv_example IS
   SHARED VARIABLE counter : integer
      RANGE 0 TO 1 := 0 ;
BEGIN
   p1: PROCESS
   BEGIN
      counter := counter + 1 ;
      wait ;
   END PROCESS p1;
   p2: PROCESS
   BEGIN
      counter := counter - 1;
      wait ;
   END PROCESS p2 ;
END ARCHITECTURE beh;