Execute Channel Program

In IBM mainframe operating systems, Execute Channel Program (EXCP) is a supervisor call for low-level device access, where the programmer is responsible for providing a list of device-specific CCWs, that is, a channel program, to be executed by I/O channels, control units and devices. EXCP is more specifically described in the OS System Programmer's Guide.

Specifying datasets

Using EXCP, legacy devices and legacy datasets may be operated on with relatively high performance. EXCP devices are OPENed (that is, are made available to the application) by specifying MACRF=(E) in the Data Control Block.

Exits (Appendages)

A comprehensive list of "exits" (called appendages) allows authorized programs to override or augment many of the system security and data integrity checks. Most of these exits (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. The exits are specified in the DCB as the last two characters of the module name IGG019xx. where xx = WA to Z9 inclusive. These module names are reserved for user-written appendages. Any other name of the form IGG019xx is reserved for use by IBM access methods. Appendages must reside in SYS1.SVCLIB (SYS1.LPALIB in SVS or later instances of the OS).