HW/SW co-specification and co-design require a medium both for HW/SW implementation independent description as well as for integration of hardware and software. Since not all design steps can be performed automatically, this medium must be capable of representing results of intermediate design steps from both, CAD-tools and human interaction. VHDL is widely accepted in HW design where it is mostly used on RT level. VHDL implementations of abstract data and control structures which support clock independent communication and synchronization, allow to extend VHDL towards system level and HW/SW co-design.