- Inactive-Withdrawn. Replaces IEEE Std 1076.6-2004. This document specifies a standard for use of very high-speed integrated circuit hardware description language (VHDL) to model synthesizable register-transfer level digital logic. A standard syntax and semantics for VHDL register-transfer level synthesis is defined. The subset of the VHDL language, which is synthesizable, is described, and nonsynthesizable VHDL constructs are identified that should be ignored or flagged as errors.