The definitive reference for VHDL is the VHDL language reference manual ( LRM ), currently IEEE Std 1076-1993. 1 References here such as [93LRM 1.1], for example, refer to Section 1.1 of the VHDL-93 LRM [IEEE 1076-1993]. According to IEEE bylaws all standards are updated (reaffirmed, reballoted, or dropped) every five years, and thus VHDL-87 (the original standard) is superceded by VHDL-93. However, some software systems (and some IEEE standards, notably VITAL) are based on VHDL-87 [IEEE 1076-1987]. Both VHDL-93 and VHDL-87 are covered in this Appendix.