The nucleus subroutines and the system macros specify the core operations of the Xtal system. They perform the line input/output, the binary file input/output, the data storage and transfer, and the program scheduling. The requirement that developers use nucleus routines to perform these functions has important implications. It restricts the programmer to using the macro 'tools' designed for this purpose, even though more familiar Fortran 'equivalents' may be available. These tools are essential to the portability of the system and have been designed to be adaptable to the local computing environment. Their function must be learnt, and an Xtal programmer must become disciplined in their use. Some of the macro tools discussed in this manual may seem unnecessary. Be assured that each has a 'reason for being' - either to allow for some variation in the Fortran77 standard, or to provide a compact command to perform a quite complicated task.

Programmers new to Xtal software are advised to use the source code of existing programs, such as FC, as a companion to this guide. This will provide additional working examples and a general overview of the structure of Xtal subroutines.