====== Subtype declarations ======
===== subtype_declaration =====
<code vhdl>
subtype identifier is subtype_indication ;
</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 ====
  * [[.:bnf#basic_identifier]]
  * [[.:bnf#extended_identifier]]

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

===== Examples =====
These are standard predefined subtypes. //natural// is an //integer// -type which is restricted to the range of 0 to the maximum //integer// -value. //positive// is also an //integer// -type which is restricted to the range of 1 to the maximum //integer// -value.
<code vhdl>
SUBTYPE natural IS integer
      RANGE 0 TO integer'high ;

SUBTYPE positive IS integer
      RANGE 1 TO integer'high ;
</code>

----

These are common examples such as subtypes which are created from basic types by means of range-constraints.

The ranges of //integer// - and //real// subtypes are declared with the keyword //RANGE// .

The ranges of //character// -, //string// - and //enumeration// subtypes are declared by explicitly stating a certain section.

<code vhdl>
SUBTYPE primary_color IS
      color RANGE yellow TO blue ;
SUBTYPE same_color IS primary_color ;

SUBTYPE address_integer IS
      integer RANGE 0 TO 127 ;
SUBTYPE human_size IS
      real RANGE 0.50 TO 2.50 ;

SUBTYPE byte IS
      bit_vector (7 DOWNTO 0);
SUBTYPE name IS string ( 1 TO 31 );
SUBTYPE color_10 IS colors (1 TO 10);
</code>

----

The two subtypes //sbit// and //xbit// are declared as subtypes of //zbit// and a [[.:resolution function]] is assigned to both of them. The resolution function makes sure that in a multiple assignment to a signal only definite values are assigned.
<code vhdl>
SUBTYPE sbit IS wiredX zbit ;
SUBTYPE xbit IS wiredX zbit
      RANGE 'X' TO '1' ;
</code>