Although a single common data format is highly desirable for the integration of heterogeneous simulation tools, enforcing the conversion to a common data format after each simulation step causes a significant overhead in computing time and a loss in numerical stability of the overall data and can be avoided if two subsequently executed simulators share a common data format, or if, as it is often the case, only tools of a single family are used in one project. To take advantage of this potential increase in speed and numerical accuracy, native tool formats are also fully supported, with conversions taking place as necessary. A prominent example for a native tool format that is supported by the VISTA/SFC environment is the TIF format used by TSUPREM4 [Tec95]. In addition to predefined data types, new ones may be defined to specify appropriate conversion routines and tools (cf. Section 7.1.3).