Interrupt Vector - Handler address

Enhanced Definition

In the context of IBM z/Architecture and z/OS, an interrupt vector handler address is a specific entry point within a system-defined table that the CPU uses to locate and transfer control to the appropriate routine (the interrupt handler) when a particular type of interrupt occurs. This mechanism is crucial for the operating system to respond to events like I/O completion, program errors, external signals, and system calls, ensuring system stability and efficient resource management.

Key Characteristics

- Hardware-Driven: The z/Architecture hardware defines the various classes of interrupts (e.g., I/O, Program Check, Supervisor Call, External, Machine Check) and the process of saving the current CPU state and loading the new state (including the handler address) from the Program Status Word (PSW).
- System-Managed: The z/OS kernel is responsible for initializing and maintaining the addresses of these critical interrupt handlers, ensuring they point to the correct system routines.
- Critical for OS Functionality: Without this mechanism, z/OS would be unable to effectively manage I/O devices, handle program exceptions, process system service requests, or perform core multitasking functions.
- Context Switching: An interrupt causes the CPU to save the state of the interrupted program (its PSW, registers) and load a new PSW that points to the interrupt handler's entry point, effectively performing a context switch.
- Fixed Interrupt Types: While the handler addresses can vary dynamically, the *types* of interrupts and their conceptual mapping to specific handler functions are fixed by the architecture.

Use Cases

- I/O Completion: When a channel program completes an I/O operation (e.g., a READ from a DASD volume or a WRITE to a printer), an I/O interrupt occurs, and the CPU vectors to the I/O interrupt handler to process the completion status and dispatch any waiting tasks.
- Program Check Handling: If a COBOL program attempts an invalid operation (e.g., division by zero, addressing an invalid memory location, or an instruction format error), a Program Check interrupt occurs, leading to the Program Check handler for error diagnosis and potential abnormal termination (abend).
- Supervisor Calls (SVCs): When an application program issues an SVC instruction (e.g., to request a system service like OPEN a dataset, GETMAIN storage, or WAIT for an event), an SVC interrupt occurs, and the CPU vectors to the SVC handler to process the requested service.
- External Interrupts: Used for events like timer expiration, inter-CPU communication, or operator intervention (e.g., pressing the RESTART button on the console), which are processed by specific external interrupt handlers.

Related Concepts

The interrupt vector handler address is fundamental to the CPU's interrupt mechanism and the z/OS kernel's core functionality. It directly supports multitasking and resource management by allowing the OS to regain control from application programs. It's intrinsically linked to the Program Status Word (PSW), which is loaded with the handler's address upon an interrupt. This mechanism underpins Supervisor Calls (SVCs), I/O operations, and error handling (abends), as these all rely on specific interrupt types being vectored to their respective handlers to ensure system integrity and responsiveness.

Best Practices:

Minimize Interrupt Latency: z/OS system programmers and developers strive to optimize interrupt handlers to be as efficient as possible, reducing the time spent in interrupt processing to improve overall system throughput and responsiveness.
Protect System Integrity: The memory regions containing interrupt handlers and the interrupt vector table are highly protected by z/OS using storage keys and address space separation to prevent unauthorized modification, which could lead to system crashes.