====== Subprogram declaration ======
===== subprogram_declaration =====
<code vhdl>
subprogram_specification ;
</code>

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

===== Further definitions =====
==== subprogram_specification ====
''procedure [[.:bnf#designator]] [ ( [[.:bnf#formal_parameter_list]] ) ]''

''[ pure | impure ] function [[.:bnf#designator]] [ ( [[.:bnf#formal_parameter_list]] ) ]'' \\
''return [[.:bnf#type_mark]]''

===== Comment =====
A pure function always returns the same value when called different times with the same actual parameters. Impure functions can return different values when called several times with the same actual parameters. This is the case when for example a impure function reads in a text file, which content changes during simulation.

===== Examples =====
Simple declaration of the procedure //thank_you// without transfer values
<code vhdl>
PROCEDURE thank_you ;
</code>

----

Declaration of the procedure //test// with the transfer value //A// .
<code vhdl>
PROCEDURE test( A : Bit ) ;
</code>

----

Declaration of the function //convert// with the transfer value //B// and the result type //fuzzy_bit// .
<code vhdl>
FUNCTION convert( B : Bit )
      RETURN fuzzy_bit ;
</code>

----

Declaration of the procedure //regist// with the signals //D// , //CK// and //Q// being transferred.
<code vhdl>
 PROCEDURE regist(
      SIGNAL D : IN Bit;
      SIGNAL CK : IN Bit;
      SIGNAL Q : OUT Bit ) ;
</code>

----

Declaration of the procedure //p// with the variable //COL// and the constant //C// being transferred.
<code vhdl>
PROCEDURE p(
      VARIABLE COL : INOUT color;
      CONSTANT C : IN choice ) ;
</code>