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.
For DASD the IOB includes a seek address, IOBSEEK, in the format MBBCCHHR, where M is the extent, BB is the bin for a data cell, CCHH is the cylinder and head, and R is the record number.
"An appendage is a programmer-written routine that provides additional control over I/O operations during channel program execution.
"[9] A comprehensive list of appendages[10] (exits in the EXCP context) allows authorized programs[a] to override or augment many of the system security and data integrity checks.
Most of these appendages are supported for compatibility with earlier instances of the OS, but the functions of several have been modified or extended for MVS.
In MVS/370 and subsequent instances of the OS, the EXCP processor invokes STARTIO to schedule execution of the channel program.
Although the EXCP processor's back-end is always in Service Request Block[b] (SRB) mode,[13] the back-end contains emulation code which allows the appendages developed for earlier, pre-MVS instances of the OS, specifically for EXCP, to function largely as before and thereby to appear to be in TCB mode, for which these appendages were originally designed.