The new component of ngspice, the Code Model Subsystem, provides the tools needed to model the various parts of your system. While NGSPICE is targeted primarily at integrated circuit (IC) analysis, XSPICE includes features to model and simulate board-level and system-level designs as well. The Code Model Subsystem is central to this new capability, providing XSPICE with an extensive set of models to use in designs and allowing you to add your own models to this model set.

The NGSPICE simulator at the core of XSPICE includes built-in models for discrete components commonly found within integrated circuits. These `model primitives' include components such as resistors, capacitors, diodes, and transistors. The XSPICE Code Model Subsystem extends this set of primitives in two ways. First, it provides a library of over 40 additional primitives, including summers, integrators, digital gates, controlled oscillators, s-domain transfer functions, and digital state machines. See Chapt. 12 for a description of the library entries. Second, it adds a code model generator to ngspice that provides a set of programming utilities to make it easy for you to create your own models by writing them in the C programming language.

The code models are generated upon compiling ngspice. They are stored in shared libraries, which may be distributed independently from the ngspice sources. Upon runtime ngspice will load the code model libraries and make the code model instances available for simulation.