EECBT - Extended Exit Control Block Table

Enhanced Definition

The Extended Exit Control Block Table (EECBT) is a crucial z/OS control block that defines and manages **extended exits**. It serves as a central repository for information about multiple exit routines associated with a single system exit point, allowing z/OS to dispatch various user-written or IBM-provided routines when a specific system event occurs. The Extended Exit Control Block Table (EECBT) is a critical z/OS system control block that provides a centralized directory of all currently active dynamic system exits. It allows z/OS to manage and dispatch control to user-written or IBM-provided exit routines at predefined points during system processing. This table is essential for the dynamic customization and extension of the operating system's behavior.

Key Characteristics

- Multi-Exit Management: Unlike older, single-routine exits, EECBT enables multiple exit routines to be active for the same exit point, each with its own entry in the table.
- Dynamic Registration: Entries in the EECBT can be dynamically added, deleted, or modified by authorized programs using the IEAEXRET macro.
- Exit Routine Information: Each EECBT entry contains vital details about an exit routine, such as its entry point address, attributes (e.g., AMODE, RMODE), environment requirements, and parameters passed during its registration.
- System-Wide Scope: Many extended exits managed by EECBT are system-wide, meaning they affect all address spaces and run in a highly privileged state (e.g., supervisor state, system key).
- Storage Location: EECBT entries and the table itself typically reside in system-managed storage areas like ECSA (Extended Common System Area) or SQA (System Queue Area) to be accessible system-wide.

Use Cases

- System Monitoring and Auditing: Implementing custom routines to intercept system events (e.g., SMF record creation, SVC calls) for enhanced logging, security auditing, or performance data collection.
- Security Policy Enforcement: Developing exits to enforce specific security policies beyond standard RACF/ACF2/TSS capabilities, such as validating specific resource access patterns or user actions.
- Resource Management Customization: Modifying default z/OS resource management behavior, for instance, by intercepting allocation requests or task creation to apply custom rules.
- Performance Optimization: Injecting code to gather detailed performance metrics for specific system functions or to dynamically adjust system parameters based on workload analysis.
- Problem Determination and Debugging: Using exits to trace specific system calls, capture diagnostic data, or alter execution paths during complex problem analysis.

Related Concepts

The EECBT is fundamental to the extended exit architecture in z/OS, providing the mechanism for the system to locate and invoke user-supplied code at predefined points. It works in conjunction with the IEAEXRET macro, which is the primary interface for programs to interact with the EECBT to register or deregister exit routines. Many extended exits are triggered by SVCs (Supervisor Calls) or other low-level system events, and the EECBT allows the system to efficiently dispatch the appropriate exit routines. It is a type of control block, similar to ASCB or TCB, used by z/OS to manage critical system resources and processes, contrasting with the simpler IEAEXIT macro used for older, single-routine exits.

Best Practices:

Minimize Code Complexity: Keep extended exit routines as concise and efficient as possible, as they run in a highly privileged state and can impact overall system performance and stability.
Ensure Reentrancy: Design exit routines to be fully reentrant, especially if they can be invoked concurrently by multiple tasks or address spaces, to prevent data corruption.
Thorough Testing: Rigorously test all extended exit routines in a controlled, non-production environment before deployment to production, given their potential to cause system-wide outages.
Proper Registration/Deregistration: Always ensure that exit routines are correctly registered upon activation and deregistered upon deactivation using IEAEXRET to prevent resource leaks or orphaned exits.
Robust Error Handling: Implement comprehensive error handling within exit routines to gracefully manage unexpected conditions and prevent system abends or loops.
Document All Exits: Maintain detailed documentation for every extended exit, including its purpose, entry/exit conditions, parameters, and any dependencies.