Subtype declarations

subtype identifier is subtype_indication ;

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

• basic_identifier
• extended_identifier

[ resolution_function_ name ] type_mark [ constraint ]

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.

SUBTYPE natural IS integer
      RANGE 0 TO integer'high ;
 
SUBTYPE positive IS integer
      RANGE 1 TO integer'high ;

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.

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);

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.

SUBTYPE sbit IS wiredX zbit ;
SUBTYPE xbit IS wiredX zbit
      RANGE 'X' TO '1' ;