External Interrupt - Hardware signal
An external interrupt is a hardware-generated signal that causes the CPU to stop its current execution and transfer control to a specific interrupt handler routine within the z/OS kernel. These interrupts are crucial for handling asynchronous events originating from outside the CPU, such as timer expirations, operator intervention, or inter-processor communication. They are a fundamental mechanism for the operating system to manage system resources and respond to critical external events.
Key Characteristics
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- Asynchronous Nature: Occurs independently of the currently executing program's instruction stream, triggered by events external to the CPU.
- Hardware-Generated: Initiated by system hardware components like timers, the operator console, or other CPUs in a multi-processor configuration.
- Program Interruption: Causes the CPU to suspend the currently executing program, saving its state (primarily the Program Status Word -
PSW). - PSW Manipulation: The old
PSW(containing the address of the interrupted instruction and machine state) is stored, and a newPSWis loaded, directing the CPU to the appropriate external interrupt handler. - Specific Interrupt Classes: External interrupts are one of the six primary interrupt classes in z/Architecture (alongside I/O, SVC, Program, Machine Check, and Restart interrupts).
- Priority: External interrupts have a higher priority than SVC or Program interrupts, ensuring timely response to critical system events.
Use Cases
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- Timer Expiration: The system interval timer or CPU timer reaching its set value, used by z/OS for time-slicing, dispatching tasks, updating the system clock, and enforcing time limits.
- Operator Intervention: An operator pressing an attention key on the system console, signaling the need for operating system intervention or to interrupt a running job.
- Inter-Processor Communication (IPC): One CPU signaling another CPU in a multi-processor environment (e.g., using the
SIGP- Signal Processor instruction) to coordinate activities or deliver messages. - External Call (EC) Interrupts: Used by hardware features or other systems to signal the z/OS system for specific purposes, often related to hardware configuration changes or special conditions.
- Channel Path Not Operational: While primarily an I/O related event, certain channel path status changes or errors can trigger external interrupts to notify the system.
Related Concepts
External interrupts are a core component of the z/OS interrupt handling mechanism, working alongside I/O, Program, SVC, and Machine Check interrupts to provide a robust event-driven operating system. They directly influence the system scheduler by signaling timer expirations, enabling time-slicing and multi-tasking. The Program Status Word (PSW) is fundamental to external interrupt processing, as it saves the context of the interrupted program and directs execution to the appropriate interrupt handler routine within the z/OS kernel. They are distinct from SVC interrupts (software-generated requests for OS services) and Program interrupts (error conditions within a program).
- Efficient Interrupt Handlers: Ensure that external interrupt service routines are highly optimized and execute quickly to minimize system overhead and maintain overall system responsiveness.
- Proper Masking: Use the Program Status Word (PSW) mask bits to selectively enable or disable specific types of external interrupts when critical operations require uninterrupted execution, but do so judiciously to avoid missing vital events.
- Resource Management: Design applications and system components to gracefully handle the asynchronous nature of interrupts, especially when dealing with shared resources or critical sections of code.
- Error Logging and Analysis: Implement robust logging for unexpected or unhandled external interrupts to aid in problem determination, system debugging, and identifying potential hardware or configuration issues.
- System Monitoring: Regularly monitor external interrupt rates and types using tools like RMF (Resource Measurement Facility) to identify potential hardware problems, performance bottlenecks, or unusual system activity.