What is the function of the interrupt vector table in an OS, and how does prioritization (including APIC and nested interrupts) work?

• A. It stores the addresses of interrupt handlers; however, it does not support prioritization or masking.
• B. It stores the memory addresses of user-space processes for scheduling.
• C. It stores the mapping of virtual to physical addresses.
• D. The OS may apply priority levels, APICs, and masking for nested interrupts.

Answer: (D)

The interrupt vector table is a fast dispatch mechanism: each interrupt number corresponds to a specific handler routine, so when an interrupt arrives, the OS can jump to the correct piece of code quickly. But simply having a table of addresses isn’t enough to manage multiple sources efficiently on modern hardware. That’s where prioritization and the interrupt controllers come in.

The OS can assign priority levels to interrupts and use mechanisms like the Advanced Programmable Interrupt Controller to enforce them. The Local APIC in each CPU handles per-CPU priority and can deliver higher-priority interrupts even while a lower-priority one is being serviced, enabling nesting of interrupts. The masking capability lets the OS block certain interrupts during critical sections or when handling something more urgent, preventing unwanted preemption or reentry.

So the combination of the vector table for dispatch and the APIC system with masking and priorities provides ordered, preemptive handling of interrupts, including nested interrupts. The other options describe unrelated tables or scheduling mechanisms, which don’t capture how interrupt dispatch is combined with hardware prioritization and masking.