Package Body
Secondary Library Unit

Syntax

package body package_name is
	declarations
	deferred constant declarations
	subprogram bodies
end package_name;

See LRM section 2.6

Rules and Examples

When a procedure or function is declared in a package, its body (the algorithm part) must be placed in the package body: 
package REF_PACK is
  procedure PARITY
   (signal X : in bit_vector;
    signal Y : out bit);
end REF_PACK;

package body REF_PACK is
  procedure PARITY
   (signal X : in bit_vector;
    signal Y : out bit) is  
  begin
    -- procedure code
  end PARITY;
end REF_PACK;

Other declarations made in a package body may be used within that body, but are not visible outside. declarations may typically be any of the following: type, subtype, constant, file, alias, attribute, function, procedure.

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

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

A package body cannot be analysed unless a matching package exists in the same design library.

Each package can only have one body.

Synthesis Issues

Package bodies are usually supported by synthesis tools, provided all the items they declare are compatible with synthesis.

A package body 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 body, for clarity and consistancy.