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

Step 1: Evaluate statement S1. In a non-vectored interrupt mechanism, when an interrupt occurs the CPU does not know which device raised it or where its Interrupt Service Routine (ISR) is located. The CPU must execute a polling routine or an interrupt-acknowledge sequence to identify the requesting device and then branch to the correct ISR address. In a vectored interrupt mechanism, the interrupting device itself supplies the vector address (or an identifying code) directly to the CPU, so the CPU can jump almost immediately to the correct ISR. Hence starting the ISR takes more time in the non-vectored scheme, so S1 is TRUE.

Step 2: Evaluate statement S2. In a daisy-chain interrupt mechanism, interrupting devices are connected in a series (chain) using Priority-In (PI) and Priority-Out (PO) signals. When an interrupt request is granted, the acknowledge signal propagates hardware-wise through the chain, and the first device in the chain that is requesting service claims the acknowledgement and places its vector address on the bus. This hardware-based priority resolution means the CPU does NOT poll each device individually; polling of every device is characteristic of a software polling scheme, not daisy chaining. Hence S2 is FALSE.

Step 3: Combine the results: S1 is true and S2 is false, which corresponds to option (C).

\[ \boxed{\text{Option (C): S1 is true and S2 is false}} \]