Introduction - If you have any usage issues, please Google them yourself
VHDL is defined by IEEE Standard 1076, IEEE Standard VHDL Language Reference Manual (the VHDL LRM). The original standard was approved in 1987. IEEE procedures require that standards be periodically reviewed and either reaffirmed or revised. The VHDL standard was revised in 1993, 2000, and 2002. In each revision, new language features were added and some existing features enhanced. The aim in each revision was to improve the language as a tool for design and verification of digital systems.
Since the 2002 revision, there have two parallel efforts to further develop the language. The first was the VHDL Procedural Interface (VHPI) Task Force, a subcommittee of the IEEE P1076 Working Group. The VHPI Task Force prepared an interim amendment to the standard, formally approved by IEEE in March 2007. The amendment is titled IEEE 1076c, Standard VHDL Language Reference Manual—Amendment 1: Procedural Language Application Interface.