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Library Clause

Formal Definition

A library clause defines logical names for design libraries in the host environment.

Simplified Syntax

library library_name;


The library clause defines the logical names of design libraries, which are used by the design units. A library is a storage facility for previously analysed design units. In practice, this relates mostly to packages.

When a package is to be used in a design, it has to be made visible to the design. In order to specify it, the library clause (making the library visible) and use clause (making particular declarations visible) must be used. See use clause for more details.

There are two predefined libraries, which are used implicitly in every design: STD and WORK. The first of them contains standard packages STANDARD and TEXTIO. The other is a working library, where all user-created and analysed design units are stored.

User-specified packages are stored in the working library WORK.


library IEEE;
use IEEE.Std_Logic_1164.all;

Such declarations as in the above example must precede each design entity, which uses any of the declarations in the package Std_Logic_1164.

Important Notes

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