First Level Interrupt Handler - FLIH processing

Enhanced Definition

A First Level Interrupt Handler (FLIH) is a highly optimized, low-level routine in z/OS that gains control immediately after a hardware interrupt occurs. Its primary purpose is to quickly save the current state of the interrupted program, identify the type of interrupt, and then dispatch control to a more specific, higher-level handler (often called a Second Level Interrupt Handler or SLIH) for detailed processing. This rapid initial response minimizes system disruption and ensures efficient handling of critical events.

Key Characteristics

- Speed and Efficiency: Designed for extreme speed, executing in the shortest possible time to minimize the impact on system performance and responsiveness.
- Context Saving: Responsible for saving the critical CPU state, including the Program Status Word (PSW) and general-purpose registers, to allow the interrupted program to resume correctly later.
- Interrupt Identification: Determines the specific type of interrupt that occurred (e.g., I/O, SVC, Program Check, External, Machine Check) using information provided by the hardware.
- Supervisor State Execution: Always runs in supervisor state with a protection key of 0, granting it full access to system resources and privileged instructions.
- Non-Reentrant Design: Typically designed to be non-reentrant to simplify its logic and avoid complex locking mechanisms during the initial, time-critical phase of interrupt handling.
- Minimal Processing: Performs only the essential tasks required to stabilize the system and transfer control, deferring complex logic to subsequent handlers.

Use Cases

- I/O Completion Handling: Responding to an I/O interrupt indicating the completion of a channel program, such as a disk read/write or network transmission, and initiating subsequent I/O processing.
- SVC Instruction Processing: Intercepting and initiating the processing of a System SVC (Supervisor Call) instruction issued by an application program to request a privileged operating system service.
- Program Check Recovery: Handling program check interrupts (e.g., addressing exceptions, data exceptions, protection exceptions) to initiate error recovery or program termination.
- External Event Management: Responding to external interrupts from timers, console attention, or inter-processor communication signals.
- Machine Check Analysis: Processing machine check interrupts that signal hardware malfunctions, initiating diagnostic procedures and potential recovery actions.

Related Concepts

The FLIH is a foundational component of the z/OS interrupt management architecture, working closely with the Program Status Word (PSW), which contains the CPU's state and interrupt masks. It acts as the initial gatekeeper for all z/Architecture interrupt types, determining which Second Level Interrupt Handler (SLIH) or other specialized routine should take over. Its efficiency is critical to the overall system dispatching and multitasking capabilities of z/OS, ensuring that the operating system can rapidly respond to events and manage concurrent workloads.

Best Practices:

Keep it Lean: The code within an FLIH should be as minimal and efficient as possible, performing only essential context saving and dispatching to avoid introducing latency.
Robust Error Handling (for SLIH): While the FLIH itself is minimal, ensure that the subsequent SLIHs and related routines have robust error detection and recovery mechanisms.
Performance Optimization: Due to its frequent execution, any performance bottlenecks in the FLIH path can significantly impact overall system throughput; continuous optimization is crucial.
Security and Integrity: FLIH code resides in highly privileged memory and must be rigorously protected from unauthorized modification to maintain system integrity and security.
Careful Design of SLIHs: Design the SLIHs that the FLIH dispatches to for reentrancy and concurrency, as they will perform the more complex, potentially longer-running processing.