This appendix describes the file format used to store circuits created by the circuit editor GUI, thereby achieving persistence across several editing sessions. When the user saves a circuit to a file using the Save... or Save As... options of the File pull-down menu, all the details relating to the location of the wire points and the placement and orientation of the components are stored in the file. The netlist labels as well as the signals values on each of the labelled netlists are also saved in the file. After a circuit is saved, it may be restored during subsequent editing sessions using the Open... option.
In order to save circuits to a file, a circuit description language which accurately reflects the detailed structure of the circuit was devised. This description language is similar in style to the component and netlist protocols described in Chapter 5, since they both adopt a stanza format for the circuit description. Unlike the component and netlist protocols, however, the circuit file format is much lower level and contains detailed structural information relating to the placement of components and wire points.
The language which describes the physical layout of the circuit is composed of three types of stanzas; namely component, point and label stanzas. Each of these stanzas are described in subsequent sections.