IO Organization : Comparing Programmed I/O and Interrupt I/O Performance

Question

A device with data transfer rate 20 KB/sec is connected to a CPU. Data is transferred byte-wise. Let the interrupt overhead be 10 microseconds.

1. What is the total time required in programmed I/O for 10 bytes data transfer?

2. What is the total time required in interrupt I/O for 10 bytes data transfer?

3. What is the minimum performance gain of operating the device under interrupt mode over operating it under program controlled mode?

Comments

GATE CSE 2005 | Question: 69

Answer 1

\textbf{1. Programmed I/O Time:}

The data transfer rate is $20~\text{KB/s}$.  
Since no data processing time is given, we assume that the CPU is polling continuously and receives one byte per poll.

\[
\text{Time for 1 byte} = \frac{1}{20 \times 10^3} = 0.00005~\text{s} = 0.05~\text{ms}
\]

\[
\text{Time for 10 bytes} = 10 \times 0.05 = 0.5~\text{ms}
\]

---

\textbf{2. Interrupt-Driven I/O Time:}

Since an interrupt is generated for each byte and no data processing time is given:

\[
\text{Time for 1 byte} = 0.01~\text{ms}
\]

\[
\text{Time for 10 bytes} = 10 \times 0.01 = 0.1~\text{ms}
\]

---

\textbf{3. Performance Gain (Speedup):}

\[
\text{Gain} = \frac{\text{Programmed I/O Time}}{\text{Interrupt I/O Time}}
= \frac{0.5}{0.1} = 5
\]

\[
\boxed{\text{Interrupt-driven I/O is 5 times faster than Programmed I/O}}
\]