Since 1994, Siemens Austria has been funding the 'Design Debugging of VHDL Specification' project, aimed at improving the hardware design cycle. Hardware (VLSI) designers use dedicated hardware description language such as VHDL (an internationally standardized language with similarities to Ada). VHDL programs contain a concurrent part consisting of processes that communicate in parallel via signals. The behavior of processes is specified by sequential statements. VHDL programs are compiled and executed, simulating the behavior of the designed circuit. The outcome of the simulation is a waveform trace that records signal changes over time. Waveform traces are then compared with a specification or another trace. In case of discrepancies, the VHDL program contains a bug which has to be fixed.

Two tools were developed over the duration of the project, to be used as an extension of the hardware designer's development environment, which consists out of a VHDL compiler, simulator and a waveform trace viewer. The waveform compare tool (WFCOMP) automates the comparison task and provides several different compare modes. WFCOMP results can be directly used as inputs for the debugging tool VHDLDIAG. Because of the program size (up to 10 MB source code and up to several days of simulation time for large designs), it was decided to use a very abstract model for debugging which would be suitable for focusing on fault candidates even for large programs. The model is based on the dependencies between signals and variables given by the program structure, a concept similar (but not identical) to program slicing. Our group is currently working on improving the diagnosis capability, although these methods are initially intended to be only used with small VHDL programs. In particular, we have examined extensions of models for the description of VHDL programs based on modelling the correct behavior of

• concurrent signal assignments
• the VHDL sequential part
• the VHDL sequential statements considering semantical differences between signal and variable assignments together with the temporal consequences and natural diagnoses ordering criteria.