Achieves direct visibility of declarations that are visible by selection.
The use clause makes visible items specified as suffixes in selected names listed in the clause. In practice, the use clause makes visible declarations specified in packages and has the following form:
If a designer wants to have all declarations in a package visible, then the 'item' clause should be substituted by the reserved word all.
The use clause is valid for the design unit immediately following it and for all secondary design units assigned to this design unit (if it is a primary design unit).
In the first example, all declarations specified in the package Std_Logic_1164 (which belongs to the library IEEE) have been made visible.
The second example makes visible the Rising_Edge function, which is declared in the same package. The function uses the type Std_ulogic, therefore declaration of this type is also made visible.
Using multiple value logic and resolution functions requires using library clause and use clause like in the first example.