 EXCP devices are OPENed (that is, are made available to the application) by specifying the Data Control Block (DCB) for OS and the DTFPH for DOS.A comprehensive list of "exits" (called, in the EXCP context, "appendages") allows authorized programs to override or augment many of the system security and data integrity checks.Most of these appendages (really, closed subroutines) are supported for compatibility with earlier instances of the OS, but the functions of several have been modified or extended for MVS.