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3.1i Foundation ISE: Using user defined packages in VHDL files
Keywords: library, package, XST, Project Navigator
The VHDL language is compile order dependent, so users
who want to have packages utilized by entities must consider
If the package is going to be referenced by one entity, the package
must be placed above the entity in the file.
If the package is going to be referenced by more than one entity, the
package must be placed in a separate file and compiled before any
of the entities utilizing the package.