In practice, developing models often involves using a combination of NGSPICE passive devices, device models, subcircuits, and XSPICE Code Models. XSPICE's Code Models may be seen as an extension to the set of device models offered in standard NGSPICE. The collection of over 40 predefined Code Models included with XSPICE provides you with an enriched set of modeling primitives with which to build subcircuit models. In general, you should first attempt to construct your models from these available primitives. This is often the quickest and easiest method.

If you find that you cannot easily design a subcircuit to accomplish your goal using the available primitives, then you should turn to the code modeling approach. Because they are written in a general purpose programming language (C), code models enable you to simulate virtually any behavior for which you can develop a set of equations or algorithms.