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GATE CS 2024 Set 1 — Question 15

MCQ+1 / -0.33EasyProgrammed I/O (Polling)I/O OrganizationComputer Organization & ArchitectureInterrupt-Driven I/ODMA (Operation & Types)

Computer Organization & Architecture → I/O Organization → DMA (Operation & Types)

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Question

Which one of the following statements is FALSE?
A.
In the cycle stealing mode of DMA, one word of data is transferred between an I/O device and main memory in a stolen cycle
B.
For bulk data transfer, the burst mode of DMA has a higher throughput than the cycle stealing mode
C.
Programmed I/O mechanism has a better CPU utilization than the interrupt driven I/O mechanism
D.
The CPU can start executing an interrupt service routine faster with vectored interrupts than with non-vectored interrupts

Correct answer

(C) Programmed I/O mechanism has a better CPU utilization than the interrupt driven I/O mechanism

Solution

To identify the false statement, let's evaluate each option:
  • Statement (A): In cycle stealing mode, the DMA controller requests the bus for a single cycle to transfer one word of data. This is a standard definition of cycle stealing. So, this statement is TRUE.
  • Statement (B): Burst mode (or block transfer mode) transfers a contiguous block of data without releasing the bus until the entire block is moved. This reduces the overhead of bus requests and grants compared to cycle stealing, leading to higher throughput for large data transfers. So, this statement is TRUE.
  • Statement (C): In Programmed I/O, the CPU must constantly poll the status register of the I/O module to check if it is ready. This 'busy-waiting' consumes CPU cycles that could be used for other tasks. In contrast, Interrupt-driven I/O allows the CPU to perform other work and only services the I/O device when an interrupt is received. Therefore, Interrupt-driven I/O has much better CPU utilization. This statement is FALSE.
  • Statement (D): In vectored interrupts, the interrupting device provides a vector (an index or address) that points directly to the Interrupt Service Routine (ISR). In non-vectored interrupts, the CPU must poll multiple devices or execute a general routine to determine the source of the interrupt. Thus, vectored interrupts are faster. So, this statement is TRUE.
Since statement (C) is the only false one, it is the correct answer.

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