2.6 Interrupt operations

INTERRUPT

 

1. What is the primary purpose of an interrupt in a computer system?

a) To signal an event requiring immediate attention from the CPU, temporarily interrupting the current program execution.

b) To provide a mechanism for communication between different programs running on the system. (Incorrect)

c) To allocate memory resources to programs during their execution. (Incorrect)

d) To synchronize data access between multiple processors in a multi-core system. (Can be used for synchronization, but not the primary purpose)

Answer: a) To signal an event requiring immediate attention from the CPU, temporarily interrupting the current program execution.

Explanation: Interrupts are a fundamental mechanism in computer systems:

• Event notification: They notify the CPU about an event requiring immediate attention, like an I/O device needing data transfer or a timer expiring.
• Temporary pause: The CPU temporarily pauses the current program execution, saves its state (registers), and services the interrupt.
• Resuming execution: After handling the interrupt, the CPU restores the saved state and resumes the interrupted program.

2. What are the two main categories of interrupts based on their origin?

a) Hardware interrupts: Generated by external devices or hardware components within the system.

b) Software interrupts: Initiated by software programs themselves using specific instructions.

c) Both hardware and software interrupts are the two main categories.

d) Priority interrupts and non-priority interrupts

Answer: c) Both hardware and software interrupts are the two main categories.

Explanation: Interrupts can be categorized based on their origin:

• Hardware interrupts: Triggered by external devices (like keyboard input) or hardware components (timers) signaling the CPU.
• Software interrupts: Initiated by software programs themselves using specific instructions to request the CPU's attention for a particular task (e.g., system calls).

3. What is the role of an Interrupt Service Routine (ISR) in handling interrupts?

a) An ISR is a dedicated software program that defines how the CPU handles a specific interrupt type.

b) The ISR is responsible for saving the state of the interrupted program before servicing the interrupt. (Incorrect)

c) ISRs are hardware components that manage interrupt requests to the CPU. (Incorrect)

d) The operating system is solely responsible for handling interrupts. (Incorrect)

Answer: a) An ISR is a dedicated software program that defines how the CPU handles a specific interrupt type.

Explanation: Interrupt Service Routines (ISRs) are crucial for handling interrupts:

• Specific handler: Each interrupt type has a corresponding ISR that defines the actions the CPU takes to service that interrupt.
• ISR execution: When an interrupt occurs, the CPU jumps to the corresponding ISR, saving its state (registers) beforehand.
• Interrupt handling: The ISR performs the necessary actions to address the interrupt event (e.g., reading data from a device).
• Resuming execution: After handling the interrupt, the ISR restores the saved CPU state and allows the CPU to resume the interrupted program.

4. What are some key characteristics of interrupt handling in a computer system?

a) Interrupts can be nested, meaning one interrupt can be serviced while another interrupt is already being handled.

b) Interrupt handling should be as fast as possible to minimize disruption to the running program.

c) A mechanism is required to determine the priority of different interrupts to ensure timely handling of critical events.

d) All of the above

Answer: d) All of the above

Explanation: Interrupt handling involves several key aspects:

• Nesting: Interrupts can be nested, allowing the CPU to service a new interrupt while handling an existing one. However, this requires careful management to avoid conflicts.
• Speed: Interrupt handling should be fast to minimize disruption to the running program.
• Priority: A mechanism is needed to prioritize interrupts. Critical events like timer expirations might need higher priority than keyboard input to ensure timely handling.

5. How do maskable interrupts differ from non-maskable interrupts (NMI)?

a) Maskable interrupts can be temporarily disabled by the CPU to prevent them from interrupting the current program.

b) Non-maskable interrupts (NMI) have the highest priority and cannot be disabled by the CPU.

c) Maskable interrupts are typically used for handling hardware events, while NMIs are software-generated. (Not always true)

d) Both maskable and non-maskable interrupts have the same priority level.

Answer: a) and b)

6. What are some potential challenges associated with interrupt handling in a computer system?

a) Ensuring proper nesting of interrupts to avoid conflicts during servicing multiple interrupt requests simultaneously.

b) Defining efficient Interrupt Service Routines (ISRs) that handle interrupts quickly and minimize disruption to the running program.

c) Implementing a robust priority scheme to ensure critical events are serviced promptly even in the presence of lower-priority interrupts.

d) All of the above

Answer: d) All of the above

Explanation: Interrupt handling involves several challenges:

• Nesting complexity: Proper management of nested interrupts is crucial to avoid conflicts and ensure proper handling of all requests.
• ISR efficiency: Efficiently designed ISRs minimize the time spent servicing interrupts, reducing disruption to the running program.
• Priority management: A well-defined priority scheme ensures critical events like timer expirations or hardware failures are handled promptly even when lower-priority interrupts are pending.

7. How do trap gates differ from interrupt gates in x86 architecture?

a) Trap gates are used for software-generated interrupts (like system calls), while interrupt gates are for hardware interrupts.

b) Trap gates offer additional features like privilege level checks, making them more secure for handling system calls.

c) Both trap gates and interrupt gates have identical functionality in x86 architecture. (Incorrect)

d) Interrupt gates are a newer technology compared to trap gates in x86 architecture. (Incorrect)

Answer: b) Trap gates offer additional features like privilege level checks, making them more secure for handling system calls.

Explanation: In x86 architecture, both gates manage interrupts but differ in functionality:

• Interrupt gates: Used for typical hardware interrupts, they provide a basic mechanism for transferring control to the ISR.
• Trap gates: Designed for software-generated interrupts (like system calls), they offer additional features like privilege level checks. This ensures that only authorized programs can initiate system calls, enhancing system security.

8. What is the role of an Interrupt Descriptor Table (IDT) in interrupt handling?

a) The IDT stores information about each interrupt type, including the memory address of the corresponding ISR.

b) It defines the priority levels for different interrupt types, allowing the CPU to prioritize interrupt handling. (Can be part of the information stored)

c) The IDT also manages the nesting of interrupts and ensures proper handling of simultaneous interrupt requests. (Can be part of the functionality)

d) All of the above

Answer: d) All of the above

Explanation: The Interrupt Descriptor Table (IDT) is a crucial data structure in interrupt handling:

• ISR location: The IDT stores information about each interrupt type, including the memory address of the corresponding ISR.
• Priority levels: In some architectures, the IDT might also hold information about interrupt priorities (though this can be managed elsewhere).
• Nesting management: The specific mechanism for interrupt nesting management can vary, but the IDT plays a role in facilitating this process.

9. Briefly explain the concept of interrupt latency in a computer system.

a) Interrupt latency refers to the time it takes for the CPU to acknowledge an interrupt request and start servicing it.

b) Lower interrupt latency is desirable for real-time systems that require prompt handling of time-sensitive events.

c) Hardware limitations and complex interrupt handling procedures can contribute to higher interrupt latency.

d) All of the above

Answer: d) All of the above

Explanation: Interrupt latency is a critical factor in system responsiveness:

• Time to acknowledge: It refers to the time taken by the CPU to acknowledge an interrupt request and begin servicing it. This can involve saving the current program's state and preparing to execute the ISR.
• Real-time systems: Lower interrupt latency is crucial for real-time systems that handle time-sensitive events like audio or video processing, where delayed responses can lead to noticeable glitches.
• Factors affecting latency: Hardware limitations (e.g., bus access times) and complex interrupt handling procedures (e.g., nested interrupts) can contribute to higher interrupt latency.

10. How do modern operating systems utilize interrupts to manage hardware devices and resources?

a) Operating systems rely on hardware interrupts to signal the completion of I/O operations, allowing the CPU to handle other tasks.

b) Interrupts are used to implement device drivers, which provide a software interface for interacting with hardware devices.

c) Modern operating systems can prioritize interrupt requests based on their criticality, ensuring timely handling of essential events.

d) All of the above

 

INTERRUPT SERVICE ROUTINE 

 

1. What is the primary function of an ISR?

a) Manage memory allocation

b) Handle device requests or exceptions

c) Perform complex calculations

d) Store user data

Answer: b) Handle device requests or exceptions

Explanation: ISRs are short pieces of code that temporarily halt the main program's execution to service an interrupt, which can be a device requesting attention or an exception that needs handling.

2. When an interrupt occurs, what happens to the program counter (PC)?

a) It remains unchanged.

b) It is loaded with the address of the ISR.

c) Its value is saved on the stack.

d) It is overwritten with the interrupt source.

Answer: c) Its value is saved on the stack.

Explanation: Before jumping to the ISR, the CPU needs to remember where it was in the main program. This is done by pushing the current PC value onto the stack.

3. Which of the following statements about ISR execution is TRUE?

a) ISRs can take a long time to execute.

b) ISRs should be kept short and efficient.

c) Multiple ISRs can execute concurrently.

d) Only one ISR can exist in a system.

Answer: b) ISRs should be kept short and efficient.

Explanation: Long ISRs can significantly delay the main program's execution. Keeping them short ensures quick handling of interrupts and minimizes disruption.

4. How does the CPU prevent other interrupts while servicing an existing one?

a) It disables all interrupt requests.

b) It masks specific interrupt sources.

c) It prioritizes incoming interrupts.

d) It ignores all interrupts until the ISR finishes.

Answer: a) It disables all interrupt requests (or d) It ignores all interrupts until the ISR finishes.

Explanation: Two common approaches exist: disabling all interrupts entirely or using an interrupt mask register to selectively block lower priority interrupts.

5. What happens after the ISR finishes its execution?

a) The program restarts from the beginning.

b) The system halts and waits for further instructions.

c) The PC is restored from the stack, and the main program resumes execution.

d) The ISR is called again for further processing.

Answer: c) The PC is restored from the stack, and the main program resumes execution.

Explanation: The CPU pops the saved PC value from the stack, effectively returning to the point where the interrupt occurred in the main program.

6. What is the difference between a hardware interrupt and a software interrupt (trap)?

a) There is no difference; they are the same.

b) Hardware interrupts are generated by devices, while software traps are initiated by programs.

c) Hardware interrupts are slower than software traps.

d) Software traps cannot be disabled.

Answer: b) Hardware interrupts are generated by devices, while software traps are initiated by programs.

Explanation: Hardware interrupts are triggered by external events (e.g., device finished operation), while software traps are intentional calls from the program itself (e.g., system call).

7. Which data structure is commonly used to manage nested interrupts (multiple interrupts occurring simultaneously)?

a) Linked list

b) Array

c) Stack

d) Queue

Answer: c) Stack

Explanation: A stack is a LIFO (Last-In- First-Out) data structure. When servicing nested interrupts, the return addresses for each ISR are pushed onto the stack, ensuring proper handling and return order.

8. What is the role of interrupt priority levels in an ISR system?

a) To determine the order in which ISRs are executed.

b) To allocate memory for different ISRs.

c) To identify the source of the interrupt.

d) To enable or disable specific interrupts.

Answer: a) To determine the order in which ISRs are executed.

Explanation: Interrupt priority levels dictate which interrupt request gets serviced first. Higher priority interrupts preempt (interrupt) lower priority ones, ensuring critical events are addressed promptly.

9. What can happen if an ISR accidentally modifies critical data used by the main program?

a) The system might experience a runtime error.

b) The program might produce incorrect results.

c) The CPU could become permanently locked.

d) All interrupts might be disabled permanently.

Answer: b) The program might produce incorrect results.

Explanation: Modifying shared data without proper synchronization (e.g., using semaphores) can lead to data corruption and unexpected program behavior.

 

INTERRUPT PROCESSING

 

1. What is the main purpose of interrupt processing?

a) To manage memory allocation

b) To handle device requests or exceptions efficiently

c) To perform complex calculations in the background

d) To store user data securely

Answer: b) To handle device requests or exceptions efficiently

Explanation: Interrupt processing allows a CPU to temporarily halt the current program execution and respond to asynchronous events like device requests (e.g., key press) or exceptions (e.g., division by zero).

2. When an interrupt occurs, what happens to the program counter (PC)?

a) It remains unchanged.

b) It is loaded with the address of the interrupt service routine (ISR).

c) Its value is saved on the stack for later reference.

d) It is overwritten with the interrupt source identifier.

Answer: c) Its value is saved on the stack for later reference.

Explanation: Before jumping to the ISR, the CPU needs to remember where it was in the main program. This is done by pushing the current PC value onto the stack.

3. How should an ISR be designed for optimal performance?

a) It should be complex and handle multiple tasks.

b) It should prioritize long-running operations.

c) It should be kept short and focused on handling the immediate interrupt.

d) It should wait for user input before proceeding.

Answer: c) It should be kept short and focused on handling the immediate interrupt.

Explanation: Long ISRs can significantly delay the main program's execution. Keeping them short ensures quick response to interrupts and minimizes disruption.

4. How does the CPU manage multiple interrupt requests simultaneously?

a) It processes them all at once.

b) It employs a first-come, first-served approach.

c) It utilizes interrupt priority levels to determine the order of execution.

d) It ignores all additional interrupts until the first one finishes.

Answer: c) It utilizes interrupt priority levels to determine the order of execution.

Explanation:  Interrupt priority levels dictate which interrupt request gets serviced first. Higher priority interrupts can preempt (interrupt) lower priority ones, ensuring critical events are addressed promptly.

5. After completing its task, how does the ISR return control to the main program?

a) It restarts the program from the beginning.

b) It waits for further instructions from the device.

c) It restores the program counter (PC) from the stack and resumes program execution.

d) It calls itself again for further processing.

Answer: c) It restores the program counter (PC) from the stack and resumes program execution.

Explanation: The CPU pops the saved PC value from the stack, effectively returning to the point where the interrupt occurred in the main program.

6. What is the difference between a hardware interrupt and a software interrupt (trap)?

a) There is no difference; they are the same mechanism.

b) Hardware interrupts are generated by internal CPU events, while software traps are triggered by external devices.

c) Hardware interrupts are faster than software traps.

d) Software traps cannot be disabled.

Answer: b) Hardware interrupts are generated by external devices, while software traps are triggered by programs themselves.

Explanation: Hardware interrupts are triggered by external events (e.g., device finished operation), while software traps are intentional calls from the program itself (e.g., system call).

7. Which data structure is commonly used to manage nested interrupts (multiple interrupts occurring consecutively)?

a) Linked list

b) Array

c) Stack

d) Queue

Answer: c) Stack

Explanation: A stack is a LIFO (Last-In, First-Out) data structure. When servicing nested interrupts, the return addresses for each ISR are pushed onto the stack, ensuring proper handling and return order based on the order of interrupt occurrence.

8. What can happen if an ISR modifies critical data used by the main program without proper synchronization?

a) The system might crash completely.

b) The program might produce unexpected results due to data corruption.

c) The CPU could become permanently locked in an infinite loop.

d) All interrupt processing will be disabled permanently.

Answer: b) The program might produce unexpected results due to data corruption.

Explanation: Modifying shared data without proper synchronization (e.g., using semaphores or mutexes) can lead to data corruption and unexpected program behavior.