Package
Primary Library Unit

Syntax

package package_name is
	declarations
end package_name;

See LRM section 2.5

Rules and Examples

Declarations may typically be any of the following: type, subtype, constant, file, alias, component, attribute, function, procedure 
package DEMO_PACK is
  constant SOME_FLAG : bit_vector := "11111111";
  type STATE is (RESET,IDLE,ACKA);
  component HALFADD 
    port(A,B : in bit;
         SUM,CARRY : out bit);
  end component;
end DEMO_PACK;

Items declared in a package are visible wherever selected via a use clause. For instance, assume DEMO_PACK is analysed into library work: 
use work.DEMO_PACK.all;
entity DEMO is
port
 (Z: out bit_vector(7 downto 0));
end DEMO;

architecture BEHAVE of DEMO is 
begin
  Z <= SOME_FLAG; 
end BEHAVE;

When a procedure or function is declared in a package, its body (the algorithm part) must be placed in the package body.

A constant declared in a package may be deferred. This means that its value may be changed by re-analysing omly the package body: 
package P is
  constant C : integer;
end P;

package body P is
  constant C : integer := 200;
end P;

Synthesis Issues

Packages are usually supported by synthesis tools, provided all the items they declare are compatible with synthesis

Synthesisable declarations and non-synthesisable declarations (e.g.for a test bench) should therefore be placed in separated packages.

Design Libraries are often not supported, so design files containing packages must either by analysed first for synthesis, or be present in the local directory.

If a package has a body, it must usually be in the same design file as the package itself.

Whats New in '93

In VHDL-93, the keyword end may be followed by the keyword package, for clarity and consistancy.

Shared variables and groups may also be declared in a package.