This component of the VISTA Base System libraries is the basis on which the whole PAI is built upon. It separates operating-system dependencies from all other PAI layers, and defines simple access routines to the file input and output services. In ANSI C only the buffered file I/O is defined and standardized, but buffering is not needed by the PAI since this is done in the caching layer above. If the unbuffered UNIX style file I/O exists in a specific operating system, this is used instead. This is the only component of the whole VISTA system which is system dependent and implements also basic functions for network access.