IREQ - Interrupt Request

Enhanced Definition

An Interrupt Request (IREQ) in the z/OS environment is a signal to the CPU that an event has occurred requiring immediate attention, causing the CPU to temporarily suspend its current task and transfer control to a specific interrupt handler. This mechanism is fundamental for the operating system to manage asynchronous events, handle I/O completions, respond to program errors, and provide system services.

Key Characteristics

- Asynchronous Nature: Interrupts are typically asynchronous events, meaning they can occur at any time, independent of the CPU's current instruction stream, allowing the system to react to external or internal events dynamically.
- Program Status Word (PSW) Management: Upon an interrupt, the current PSW (containing the instruction address, condition code, and control bits) is automatically saved as the "old PSW," and a new "new PSW" (pointing to the interrupt handler's entry point) is loaded, effectively changing the CPU's state and execution flow.
- Interrupt Classes: z/Architecture defines several classes of interrupts, including I/O, External, Supervisor Call (SVC), Program, Machine Check, and Restart interrupts, each serving distinct purposes.
- Hardware-Software Interface: Interrupts serve as the primary interface between hardware events (e.g., I/O device completion, timer expiry) and the operating system's software routines, enabling efficient resource management.
- Prioritization: Interrupts are often prioritized, allowing more critical events (like machine checks) to preempt less critical ones, ensuring system stability and responsiveness.

Use Cases

- I/O Completion Handling: When a disk read/write or network transfer completes, the I/O device controller generates an I/O interrupt, signaling z/OS to process the completion, update status, and potentially dispatch the waiting task.
- Timer Services and Dispatching: The system clock periodically generates external interrupts, which z/OS uses for time-slicing, task dispatching, and managing time-dependent system services.
- Program Error Detection: An application program attempting an invalid operation (e.g., division by zero, addressing protection violation) triggers a program interrupt, leading to error recovery or an abnormal termination (abend).
- Supervisor Call (SVC) Processing: User programs explicitly issue an SVC instruction to request privileged services from the z/OS kernel (e.g., allocate memory, perform system calls), which is processed as a software-initiated interrupt.
- External Event Notification: Events like console commands, inter-processor communication, or signals from other CPUs can generate external interrupts, prompting z/OS to take appropriate action.

Related Concepts

Interrupts are fundamental to the operation of z/OS, directly interacting with the Program Status Word (PSW) to manage CPU state transitions. They are the core mechanism by which the z/OS kernel gains control to perform task dispatching, handle I/O operations asynchronously, and implement error recovery. The efficiency of interrupt processing directly impacts overall system performance and responsiveness, making it a critical component of the entire mainframe ecosystem.

Best Practices:

Minimize Interrupt Handler Latency: Design interrupt service routines (ISRs) to be as concise and efficient as possible, deferring complex processing to lower-priority tasks to minimize the time spent in interrupt context.
Proper Interrupt Masking: Utilize appropriate interrupt masks within the PSW to temporarily disable specific interrupt classes during critical sections of code, preventing race conditions or data corruption.
Robust Error Handling in ISRs: Implement comprehensive error detection and recovery mechanisms within interrupt handlers to gracefully manage device failures or unexpected conditions without compromising system integrity.
Optimize I/O for Efficiency: While I/O interrupts are necessary, optimize I/O operations (e.g., using larger block sizes, channel programming techniques) to reduce the overall frequency of I/O interrupts and associated CPU overhead.
Understand z/Architecture Interrupt Hierarchy: Be aware of the defined interrupt priorities and classes in z/Architecture when developing system-level code to ensure critical events are handled with appropriate urgency.