The SPA is a set of FORTRAN90 subprograms stored in a series of source directories beneath the SPA root. All interactions between the driving program and each of the subprograms are strictly controlled trough interface blocks stored in modules which are use-associated as needed in the code. This interfacing scheme allows for explicit error checking as well as making the subprogram interactions easily-understandable for the developer.
There are no common blocks or global variables in the SPA, although some common variables are declared within the lower-level modules responsible for file functions. These variables, however, are scoped only within the module and the subprogram by which it is associated. We are very insistent on the explicit passing of values between routines, since the data flow becomes rapidly untraceable once common blocks and global variables dominate in an extendible archive such as the SPA.
The subroutines contained in the SPA are primarily responsible for interactions with the datafiles and configuration scripts. Functions are generally used for calculation of diagnostics (the essence of the SPA) and are mostly generic, accepting either single value or array inputs for each argument, and producing single value or array output, respectively. Interfacing with array-valued functions is advisable, since they run many times faster than a nested single-valued function call.