====== Variable declarations ======
===== variable_declaration =====
<code vhdl>[ shared ] variable identifier_list : subtype_indication [ := expression ] ;</code>

===== Parents =====
  * entity_declarative_part
  * architecture_declarative_part
  * package
  * package_body
  * block_declarative_part
  * function_declarative_part
  * procedure_declarative_part
  * process_declarative_part

===== Further definitions =====
==== identifier_list ====
''[[.:bnf#identifier]] {, [[.:bnf#identifier]] }''

==== subtype_indication ====
''[ resolution_function_ [[.:bnf#name]] ] [[.:bnf#type_mark]] [ [[.:bnf#constraint]] ]''

==== expression ====
  * [[.:bnf#relation]] { and relation }
  * relation { or relation }
  * relation { xor relation }
  * relation [ nand relation ]
  * relation [ nor relation ]
  * relation { xnor relation }

===== Comment =====
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.

===== Examples =====
The variable //count// of the type //positive// is declared.
<code vhdl>
VARIABLE count : positive ;
</code>

----

The variables //index// and //memory// are declared as subtypes of the types //integer// and //bit_matrix// .
<code vhdl>
VARIABLE index : integer
      RANGE 0 TO 99 := 0 ;
VARIABLE memory : bit_matrix
      ( 0 TO 7, 0 TO 1023 ) ;
</code>

----

  * //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 <nowiki>(2**nb_bit-1)</nowiki>.
  * 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.

<code vhdl>
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 ) ;
</code>

----

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

<code vhdl>
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;
</code>