EXCPVR - Execute Channel Program Virtual

Enhanced Definition

EXCPVR (Execute Channel Program Virtual) is a low-level z/OS macro instruction that allows an authorized program to perform direct I/O operations to a device using a channel program, with the crucial capability of addressing data buffers located in **virtual storage**. It provides a highly efficient, direct interface to the I/O subsystem, bypassing standard access methods for specialized requirements.

Key Characteristics

- Direct I/O Access: Bypasses conventional z/OS access methods (like VSAM, QSAM, BSAM), giving programs granular control over I/O operations directly to physical devices.
- Virtual Storage Addressing: The "VR" suffix signifies that the channel program's data addresses (e.g., for Read or Write commands) can refer to buffers located in virtual storage, which are then translated to real storage by the system during I/O execution.
- Privilege Requirement: Typically requires the executing program to be in supervisor state or to be APF-authorized (Authorized Program Facility) due to its direct interaction with the I/O subsystem and potential for system disruption.
- Channel Program Dependency: Relies on a channel program, a sequence of Channel Command Words (CCWs) that define the specific I/O operations (e.g., seek, read, write, control) to be performed by the channel and device.
- Performance Potential: Offers the potential for extremely high-performance I/O for specialized applications by minimizing system overhead associated with higher-level access methods.
- Complexity: Programming with EXCPVR is significantly more complex than using standard access methods, requiring deep knowledge of device architecture, channel programs, and I/O error handling.

Use Cases

- Custom Device Drivers: Implementing specialized device drivers for non-standard hardware or for devices that require unique I/O protocols not supported by standard z/OS access methods.
- High-Performance Utilities: Developing performance-critical utilities, such as fast data movers, backup/restore tools, or database recovery utilities that need to read/write large volumes of data directly and efficiently.
- System-Level Tools: Used by z/OS itself and other system components (e.g., certain parts of DFSMS, VTAM, or specialized monitoring tools) for low-level interaction with storage and communication devices.
- Research and Diagnostics: For advanced system programmers or researchers to diagnose I/O problems, analyze device behavior, or experiment with new I/O techniques.

Related Concepts

EXCPVR is a specialized variant of the fundamental EXCP (Execute Channel Program) macro, differing primarily in its support for virtual storage addressing for I/O buffers. Both EXCP and EXCPVR are at the heart of how z/OS performs I/O, directly interacting with channels and control units to execute channel programs. They bypass the higher-level access methods like VSAM, QSAM, or BSAM, which abstract away the complexities of channel programming for application developers. Its use often necessitates APF authorization or supervisor state, linking it closely to z/OS security and privilege mechanisms.