Programmed I/O vs interrupt-driven I/O in Operating Systems - Quick Revision & Key Differences

Recall & Review

beginner

What is Programmed I/O?

Programmed I/O is a method where the CPU actively waits and checks the status of an I/O device to see if it is ready for data transfer. The CPU controls the data transfer directly by repeatedly polling the device.

beginner

What is Interrupt-driven I/O?

Interrupt-driven I/O is a method where the I/O device sends a signal (interrupt) to the CPU when it is ready for data transfer. The CPU can perform other tasks and only responds when interrupted by the device.

intermediate

How does CPU usage differ between Programmed I/O and Interrupt-driven I/O?

In Programmed I/O, the CPU is busy waiting and checking the device status, which wastes CPU time. In Interrupt-driven I/O, the CPU can do other work and only stops when interrupted, making better use of CPU time.

intermediate

Which I/O method is more efficient for slow devices and why?

Interrupt-driven I/O is more efficient for slow devices because the CPU does not waste time waiting. It can continue other tasks and only handles the device when it signals readiness.

beginner

What is a disadvantage of Programmed I/O?

A disadvantage of Programmed I/O is that it wastes CPU resources by constantly checking the device status, which can slow down overall system performance.

In Programmed I/O, how does the CPU know when the device is ready?

A By waiting for a timer

B By receiving an interrupt signal

C By using direct memory access

D By continuously checking the device status

Programmed I/O uses polling, where the CPU repeatedly checks the device status to know when it is ready.

What triggers the CPU to handle data transfer in Interrupt-driven I/O?

A A timer event

B A signal from the I/O device called an interrupt

C Continuous polling by the CPU

D Manual user input

In Interrupt-driven I/O, the device sends an interrupt signal to the CPU when it is ready.

Which I/O method allows the CPU to perform other tasks while waiting for the device?

A Interrupt-driven I/O

B Programmed I/O

C Both methods

D Neither method

Interrupt-driven I/O lets the CPU work on other tasks until interrupted by the device.

What is a main drawback of Programmed I/O?

A It cannot handle slow devices

B It requires complex hardware

C It wastes CPU time by busy waiting

D It uses interrupts

Programmed I/O wastes CPU time because the CPU keeps checking the device status instead of doing other work.

Which I/O method is generally better for improving CPU efficiency?

A Interrupt-driven I/O

B Both are equally efficient

C Programmed I/O

D Neither improves CPU efficiency

Interrupt-driven I/O improves CPU efficiency by allowing the CPU to do other work until interrupted.

Explain the main difference between Programmed I/O and Interrupt-driven I/O.

Think about how the CPU learns that the device is ready to transfer data.

Describe one advantage and one disadvantage of Programmed I/O.

Consider CPU usage and complexity.

Practice

1. Which of the following best describes programmed I/O?

easy

A. CPU ignores the device until a scheduled time

B. Device sends a signal to CPU only when it needs attention

C. CPU and device communicate using direct memory access

D. CPU continuously checks the device status until it is ready

Solution

1. Step 1: Understand programmed I/O behavior

   Programmed I/O means the CPU actively checks the device repeatedly to see if it is ready.

2. Step 2: Compare options with definition

   CPU continuously checks the device status until it is ready matches this behavior exactly, while others describe interrupt or other methods.

3. Final Answer:

   CPU continuously checks the device status until it is ready -> Option D

4. Quick Check:

   Programmed I/O = CPU polls device [OK]

Hint: Programmed I/O means CPU keeps checking device status [OK]

Common Mistakes:

• Confusing programmed I/O with interrupt-driven I/O
• Thinking device signals CPU in programmed I/O
• Mixing up direct memory access with programmed I/O

2. Which of the following is the correct statement about interrupt-driven I/O?

easy

A. Device interrupts CPU only when it needs service

B. CPU waits in a loop checking device status

C. CPU ignores device signals completely

D. CPU uses programmed polling to check device

Solution

1. Step 1: Recall interrupt-driven I/O definition

   Interrupt-driven I/O means the device sends a signal (interrupt) to the CPU when it needs attention.

2. Step 2: Match options to definition

   Device interrupts CPU only when it needs service correctly states that the device interrupts the CPU only when service is needed.

3. Final Answer:

   Device interrupts CPU only when it needs service -> Option A

4. Quick Check:

   Interrupt-driven I/O = device signals CPU [OK]

Hint: Interrupt-driven means device signals CPU when ready [OK]

Common Mistakes:

• Choosing options describing CPU polling
• Assuming CPU ignores device signals
• Confusing programmed I/O with interrupt-driven I/O

3. Consider this scenario: A CPU uses programmed I/O to read data from a device. What is the main effect on CPU usage?

medium

A. CPU can perform other tasks while waiting for device

B. CPU is busy checking device status and cannot do other work

C. CPU ignores the device until data is ready

D. CPU receives an interrupt when device is ready

Solution

1. Step 1: Understand CPU behavior in programmed I/O

   In programmed I/O, CPU repeatedly checks device status, which keeps it busy and unable to do other tasks efficiently.

2. Step 2: Analyze options for CPU usage

   CPU is busy checking device status and cannot do other work correctly states CPU is busy polling and cannot perform other work effectively.

3. Final Answer:

   CPU is busy checking device status and cannot do other work -> Option B

4. Quick Check:

   Programmed I/O = CPU busy polling device [OK]

Hint: Programmed I/O keeps CPU busy waiting, no multitasking [OK]

Common Mistakes:

• Thinking CPU can multitask during programmed I/O
• Confusing interrupt-driven I/O effects with programmed I/O
• Assuming CPU ignores device in programmed I/O

4. A system uses interrupt-driven I/O but the CPU never responds to device interrupts. What is the likely problem?

medium

A. Interrupts are disabled or masked in the CPU

B. CPU is using programmed I/O instead

C. Device is not sending any interrupts

D. CPU is stuck in a loop polling the device

Solution

1. Step 1: Identify why CPU ignores interrupts

   If CPU never responds to interrupts, a common cause is that interrupts are disabled or masked, preventing CPU from handling them.

2. Step 2: Evaluate other options

   Interrupts are disabled or masked in the CPU correctly identifies this cause. Other options either contradict the interrupt-driven setup or describe different issues.

3. Final Answer:

   Interrupts are disabled or masked in the CPU -> Option A

4. Quick Check:

   CPU ignoring interrupts = interrupts disabled [OK]

Hint: CPU ignoring interrupts usually means interrupts are disabled [OK]

Common Mistakes:

• Assuming device is not sending interrupts
• Confusing programmed I/O with interrupt-driven I/O
• Thinking CPU polls device in interrupt-driven mode

5. A system designer wants to maximize CPU efficiency while handling multiple I/O devices. Which approach is best and why?

hard

A. Use programmed I/O because CPU controls all device checks

B. Use programmed I/O with longer polling intervals to reduce CPU load

C. Use interrupt-driven I/O so CPU works on other tasks until interrupted

D. Use neither; rely on manual device checks by the user

Solution

1. Step 1: Analyze CPU efficiency needs

   Maximizing CPU efficiency means CPU should not waste time waiting or polling devices unnecessarily.

2. Step 2: Compare I/O methods for efficiency

   Interrupt-driven I/O allows CPU to do other work and only respond when device signals, improving efficiency.

3. Step 3: Evaluate other options

   Programmed I/O wastes CPU cycles; manual checks are impractical. Longer polling reduces load but still wastes CPU time.

4. Final Answer:

   Use interrupt-driven I/O so CPU works on other tasks until interrupted -> Option C

5. Quick Check:

   Interrupt-driven I/O maximizes CPU efficiency [OK]

Hint: Interrupt-driven I/O lets CPU multitask efficiently [OK]

Common Mistakes:

• Choosing programmed I/O for efficiency
• Ignoring CPU multitasking benefits
• Thinking manual checks improve performance