The standardized file I/O functions of the VISTA operating system are used by the file layer to handle the physical I/O of PBFs. It guarantees that a PBF is only opened by one application at a time for writing (file locking). Avoiding multiple write accesses to one PBF allows an easier implementation of the data base, since the physical file cannot change during access (unless it is closed); multiple read-only accesses are allowed. The locking of a PBF is not implemented through system functions. It works through a mark in the header of the PBF and a special lock status, where multiple accesses of the same file at the same time are detected. The file layer also allows the creation of temporary PBFs for intermediate storage of simulation data. Temporary files are stored in PBFs without a physical name and deleted automatically upon closing.