IRQ - Interrupt Request

Enhanced Definition

In the context of IBM mainframes and z/OS, an interrupt is a hardware-generated or software-generated signal that causes the CPU to suspend its current execution, save its state (primarily the Program Status Word), and transfer control to a predefined interrupt handler routine within the operating system. This mechanism is crucial for managing asynchronous events, I/O operations, error conditions, and system calls, ensuring the efficient and responsive operation of the mainframe.

Key Characteristics

- Asynchronous Event Handling: Interrupts are the primary mechanism for the CPU to respond to events that occur asynchronously to the currently executing program, such as I/O completion, timer expiration, or external signals.
- Program Status Word (PSW) Manipulation: Upon an interrupt, the current PSW is automatically saved in a designated storage area, and a new PSW (containing the address of the interrupt handler and new control bits) is loaded, effectively changing the CPU's state and execution path.
- Categorized Interrupts: z/OS classifies interrupts into several distinct types, including I/O interrupts (for device activity), External interrupts (timers, inter-processor communication), Program interrupts (exceptions like invalid operations or data errors), SVC interrupts (supervisor calls), Machine Check interrupts (hardware failures), and Restart interrupts.
- Hardware-Software Interface: Interrupts serve as the fundamental interface between the mainframe's hardware components (e.g., channel subsystem, processors, timers) and the z/OS operating system, allowing hardware to signal the need for software intervention.
- Prioritization and Masking: The z/OS kernel manages interrupt priorities, and specific bits in the PSW (e.g., the I/O interrupt mask, external interrupt mask) can be set to temporarily disable or "mask" certain types of interrupts to ensure critical operations are not interrupted.

Use Cases

- I/O Completion Notification: When a disk read/write operation or a network transmission completes, the channel subsystem generates an I/O interrupt to inform z/OS, allowing the system to process the completed operation and release resources.
- Timer-Driven Task Switching: The system timer generates external interrupts at regular intervals, enabling z/OS to perform time-slicing, dispatch different tasks, and manage system resources efficiently.
- Program Error Handling: A COBOL program attempting an invalid instruction or accessing protected memory will trigger a Program interrupt, leading to an abnormal termination (abend) or a system-managed recovery process.
- Supervisor Call (SVC) Execution: An application program issues an SVC interrupt to explicitly request services from the z/OS kernel, such as allocating memory, opening a dataset, or creating a new task.
- Inter-Processor Communication: In a multi-processor (MP) or Parallel Sysplex environment, one CPU might generate an external interrupt to signal another CPU about a critical event or to request a specific action.

Related Concepts

Interrupts are foundational to the z/OS Supervisor, which is the core component responsible for handling all interrupt types, managing system resources, and dispatching work. They are inextricably linked to the Program Status Word (PSW), which defines the CPU's operational state and controls interrupt enablement/disablement. The Channel Subsystem heavily relies on I/O interrupts to communicate the status and completion of I/O operations to the CPU. Furthermore, SVCs (Supervisor Calls) are a specific type of program-initiated interrupt that allows user programs to request privileged operating system services.

Best Practices:

Efficient Interrupt Handlers: Design and implement interrupt handlers within the z/OS kernel to be as concise and efficient as possible to minimize the time spent in privileged mode and maintain system responsiveness.
Minimize Interrupt Disablement: The operating system should minimize periods during which interrupts are globally disabled to prevent delays in handling critical events and potential system deadlocks.
Robust Error Recovery: Implement comprehensive error recovery routines for Program and Machine Check interrupts to gracefully handle exceptions, prevent system crashes, and provide detailed diagnostic information (e.g., dumps).
Proper PSW Management: Ensure that the PSW is correctly saved and restored during interrupt processing to preserve the state of the interrupted program and allow for its correct resumption.
Performance Monitoring: Utilize z/OS performance monitoring tools (e.g., RMF, SMF) to track interrupt rates and processing