Predefined libraries

Certain resource libraries are predefined in standard VHDL. The library named std contains the packages standard and textio, which should not be modified. The contents of these packages and other aspects of the predefined language environment are documented in the IEEE Standard VHDL Language Reference Manual, Std 1076-1987 and ANSI/IEEE Std 1076-1993. See also, "Using the TextIO package" .

A VHDL use clause can be used to select specific declarations in a library or package that are to be visible within a design unit during compilation. A use clause references the compiled version of the package-not the source.

By default, every design unit is assumed to contain the following declarations:

LIBRARY std, work;
USE std.standard.all 

To specify that all declarations in a library or package can be referenced, you can add the suffix .all to the library/package name. For example, the use clause above specifies that all declarations in the package standard in the design library named std are to be visible to the VHDL design file in which the use clause is placed. Other libraries or packages are not visible unless they are explicitly specified using a library or use clause.

Another predefined library is work, the library where a design unit is stored after it is compiled as described earlier. There is no limit to the number of libraries that can be referenced, but only one library is modified during compilation.

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