• Sof (Signal object file) class is designed because a large part of data-driven programming project is I/O

• using Sof, all objects know how to read and write themselves

• an index is maintained pointing to all objects within the file, but all data is left on the disk until needed

• Sof deals with both ascii and binary files, it can also handle machine architecture differences

• user input parameter files are ascii Sof files, binary mode is typically used for large quantities of data