Interrupt Handler - Interrupt processor

Enhanced Definition

An interrupt handler, also known as an interrupt processor, is a specialized routine within the z/OS operating system's supervisor that is invoked asynchronously in response to an interrupt. Its primary purpose is to identify the cause of the interrupt, perform necessary actions to address it, and then return control, often to the interrupted program or a different dispatchable unit.

Key Characteristics

- Asynchronous Execution: Interrupt handlers are not explicitly called by application programs; they are activated by hardware or software events that cause an interrupt, making their execution asynchronous to the flow of the interrupted program.
- Privileged State: They typically execute in a highly privileged state (supervisor state, Problem State bit in PSW is 0), allowing them direct access to system resources and control over the CPU.
- Context Switching: Upon an interrupt, the CPU's current state (including the Program Status Word - PSW, general purpose registers, etc.) is automatically saved by hardware into a designated area, and a new PSW is loaded, pointing to the appropriate interrupt handler.
- Specific Types: z/OS recognizes several types of interrupts, each with its own set of handlers: I/O, Supervisor Call (SVC), Program, External, Machine Check, and Restart.
- Minimal Processing: Best practice dictates that interrupt handlers perform the absolute minimum processing required to handle the event, deferring more complex work to lower-priority tasks or Service Request Blocks (SRBs) to minimize the time spent in the critical interrupt context.

Use Cases

- I/O Completion: When an I/O operation (e.g., reading from a disk, writing to a printer) completes, an I/O interrupt is generated. The I/O interrupt handler processes this, updates the status of the I/O request, and potentially posts the waiting task.
- Supervisor Call (SVC) Processing: When an application program needs to request an operating system service (e.g., allocate memory, perform a file I/O, create a task), it issues an SVC instruction. The SVC interrupt handler identifies the requested service and dispatches the appropriate system routine.
- Program Error Handling: If a program encounters an error condition (e.g., division by zero, invalid address, protection exception), a Program Interrupt occurs. The Program Interrupt handler typically attempts recovery or terminates the offending program.
- External Event Management: External interrupts can be generated by timers expiring, console commands, or inter-processor communication. Handlers for these events manage system clocks, process operator requests, or coordinate activities across multiple CPUs.
- Hardware Fault Detection: Machine Check interrupts signal hardware malfunctions. The Machine Check interrupt handler attempts to diagnose the problem, log the error, and potentially initiate recovery or system shutdown procedures.

Related Concepts

Interrupt handlers are fundamental to the operation of z/OS, acting as the bridge between hardware events or software requests and the operating system's response. They are tightly coupled with the Program Status Word (PSW), which dictates the CPU's state and points to the next instruction, and is automatically swapped during an interrupt. Supervisor Calls (SVCs) are a direct mechanism for user programs to intentionally trigger a specific type of interrupt, allowing them to request services from the z/OS kernel/supervisor. Efficient I/O operations heavily rely on I/O interrupts to signal completion, preventing the CPU from busy-waiting. Ultimately, interrupt handlers enable multitasking and resource management by allowing the OS to respond to critical events and switch contexts effectively.

Best Practices:

Keep it Short and Fast: Interrupt handlers should be as concise and efficient as possible to minimize the time spent in a high-priority, non-preemptible (or minimally preemptible) state, ensuring system responsiveness.
Defer Complex Work: Avoid performing lengthy or complex operations within an interrupt handler. Instead, schedule a lower-priority task, SRB, or TSR (Task-Switchable Routine) to complete the work once the interrupt context has been exited.
Careful Serialization: Implement robust serialization mechanisms (e.g., ENQ/DEQ, LOCK macros) to protect shared data structures from concurrent access by multiple interrupt handlers or between an interrupt handler and a regular task.
Minimize Interrupt Disablement: While some critical sections require disabling interrupts, this should be done for the shortest possible duration to avoid delaying other critical system events.
Thorough Testing: Due to their asynchronous nature and privileged execution, interrupt handlers are prone to subtle timing issues and race conditions. Rigorous testing is essential to ensure stability and correctness under various system loads and conditions.