Question

Consider the following two statements about interrupt handling mechanisms in a
CPU.
S1: In non-vectored interrupt mechanism, it usually takes more time to start the
Interrupt Service Routine (ISR) when compared to that in a vectored interrupt
mechanism.
S2: In daisy-chain interrupt mechanism, the CPU polls all the input devices
individually to determine the source of the interrupt.
Which one of the following options is correct with respect to S1 and S2 ?

• A. Both S1 and S2 are true
• B. Both S1 and S2 are false
• C. S1 is true and S2 is false
• D. S1 is false and S2 is true

Hint:

Vectored interrupts let hardware supply the ISR address directly, making them faster to start than non-vectored ones. Daisy chaining resolves priority through hardware chaining (PI/PO signals), not by the CPU polling every device.

Answer 1

The Correct Option is C

Compare the two interrupt-identification schemes side by side.

Non-vectored interrupts give the CPU only a signal that says an interrupt has occurred, with no information about the source. The CPU must then run a software routine, often testing device status registers one by one or issuing an interrupt-acknowledge cycle, before it can locate and start the right ISR. Vectored interrupts skip this search because the requesting hardware itself hands the CPU the address of its ISR (or an index into a vector table), letting execution jump there almost instantly. So starting an ISR is inherently slower with non-vectored interrupts, confirming S1 as correct.

Now consider daisy chaining. This is a purely hardware priority scheme: devices are wired in series, and the acknowledge signal ripples down the chain from highest priority to lowest. Whichever device is asserting a request first intercepts the acknowledge signal and stops it from propagating further, then supplies its own vector. Crucially, the CPU issues one single acknowledge and lets the hardware chain resolve which device responds. There is no repeated CPU-driven interrogation of every device, which is what S2 incorrectly claims. So S2 is wrong.

Putting these together, the true statement is S1 and the false one is S2.

Final answer: Option (C).