IVT - Interrupt Vector Table

Enhanced Definition

In the context of IBM mainframe systems and z/OS, the Interrupt Vector Table (IVT) is a critical system control block containing the entry points (addresses) of the various interrupt handler routines. When a hardware or software interrupt occurs, the CPU uses the IVT to quickly locate and transfer control to the appropriate operating system service routine, enabling the system to respond to events such as I/O completion, program errors, or supervisor calls. It is a fundamental mechanism for asynchronous event processing and maintaining system responsiveness.

Key Characteristics

- Fixed Location: The IVT typically resides in a fixed, protected area of main storage, accessible only by the operating system kernel in supervisor state to ensure system integrity.
- Vector Array: It is structured as an array of pointers (vectors), where each entry corresponds to a specific interrupt type (e.g., I/O interrupt, program check interrupt, external interrupt, SVC interrupt).
- CPU Dispatching: The CPU's hardware interrupt logic uses the interrupt type code to index into the IVT and fetch the address of the corresponding Interrupt Service Routine (ISR) to execute.
- Privileged Access: Modification of the IVT is a highly privileged operation, strictly controlled by the operating system to prevent unauthorized access or malicious alteration.
- System Integrity: The integrity of the IVT is paramount; any corruption can lead to severe system instability, including ABENDs (abnormal ends) or system crashes requiring an IPL (Initial Program Load).
- Core System Component: It is a foundational element of the z/OS kernel, essential for managing hardware interactions, software requests, and error conditions.

Use Cases

- I/O Completion Handling: When an I/O operation (e.g., data transfer from a DASD or tape drive) completes, an I/O interrupt is generated, and the IVT directs the CPU to the I/O supervisor routine to process the completion status and dispatch the next I/O.
- Program Check Error Recovery: If a user program encounters an invalid operation (e.g., division by zero, protection exception, invalid instruction), a program check interrupt occurs, and the IVT points to the program check handler for error diagnosis and potential ABEND processing.
- Supervisor Call (SVC) Processing: When an application program issues an SVC instruction to request an operating system service (e.g., allocate memory, perform a file operation), an SVC interrupt is generated, and the IVT directs control to the appropriate SVC routine.
- Timer Management: The system timer generates external interrupts at regular intervals, which are handled via the IVT to facilitate time-slicing for task scheduling, updating the system clock, and managing time-dependent processes.
- External Event Handling: Processing interrupts from external sources, such as console interrupts, inter-processor communication signals, or hardware error notifications.

Related Concepts

The Interrupt Vector Table is fundamental to the z/OS Supervisor Call (SVC) mechanism, Program Check handling, and the I/O Supervisor (IOS). It works in close conjunction with the Program Status Word (PSW), which contains the current instruction address, control bits, and the interrupt mask. When an interrupt occurs, the current PSW is automatically saved, and a new PSW (containing the address of the interrupt handler obtained from the IVT) is loaded, effectively transferring control to the appropriate routine. This entire process is critical for the operating system's ability to manage resources, handle errors, and provide services to user programs asynchronously.

Best Practices:

System Integrity: Never attempt to directly modify or access the IVT from unprivileged user programs or unauthorized system utilities; this is a highly protected system area.
Error Diagnosis: When diagnosing system ABENDs or unexpected behavior, understanding the interrupt flow and the role of the IVT can help pinpoint the root cause, especially for program checks or SVC-related issues.
OS Maintenance: Ensure that operating system maintenance and updates are applied correctly, as they often involve updates to system control blocks, including potential changes or additions to interrupt handlers.
Security: Implement robust hardware memory protection mechanisms to safeguard the system's core memory where the IVT resides,