GATE CS 2005 Q69 — I/O Interface: Interrupt and DMA Mode

To find the performance gain of operating the device under interrupt-driven mode over program-controlled mode, we compare the CPU time consumed per byte transfer in both modes.

1. Time Interval per Byte

The data transfer rate of the device is $10\text{ KB/sec}$. Assuming standard decimal notation for data rates ($1\text{ KB} = 10^3\text{ bytes}$):

The time interval between two consecutive byte transfers is:

2. CPU Time Spent in Each Mode

• Program-Controlled Mode (Polling): The CPU continuously polls the device to check if a byte is ready. Consequently, the CPU is fully occupied for the entire duration of $100\,\mu\text{sec}$ per byte.
• Interrupt-Driven Mode: The device interrupts the CPU only when a byte is ready. The CPU is busy only during the interrupt overhead, which is $4\,\mu\text{sec}$ per byte (as the actual transfer time is negligible).

3. Performance Gain

The performance gain is the ratio of the CPU time consumed in program-controlled mode to that in interrupt-driven mode:

Hence, the minimum performance gain is 25.