What is Interrupt in Operating System? A Comprehensive Guide

As someone who has worked as a software engineer for a while, I've encountered the concept of interrupts when dealing with operating systems. This topic can be quite confusing for newcomers to the field. In this piece, I'll guide you through everything you need to know about interrupts in operating systems so you can confidently address any queries or thoughts related to this concept. So, if you have been wondering what is interrupt – you’re in the right place! Let’s get started.

What is Interrupt?

An interrupt in an OS is a signal that momentarily pauses the operation of a program or task, allowing the operating system to carry out an action or respond to a request. It serves as a mechanism utilized by hardware devices, software programs, or even the operating system itself to draw the CPU’s attention and prioritize handling events.

When an interrupt takes place, the CPU saves its status, which includes the program counter and registers, and hands over control to a routine known as an interrupt handler or interrupt service routine (ISR). The ISR deals with the interrupt by executing tasks and then returns control back to the interrupted program so it can continue running from where it was paused.

Before we move on to understand the different types of interrupt in OS, it’s good to keep in mind that the concepts of interrupts and so on come in extremely handy in the field of software development. So, if you’re looking for a MS in Computer Science course, or anything of that nature, having a better understanding of interrupts and some other core concepts will put you ahead of your peers! 

Types of Interrupts

Now that you’ve understood what is interrupt in OS, let’s look at the different types of interrupts. Interrupts can be classified into several types based on their origin and characteristics. Let's explore the major types of interrupts:

1. Hardware Interrupts:
2. • Hardware interrupts in OS are generated by physical devices or peripherals connected to your computer, such as your keyboard, mouse, printer, or network card. 
   • When a device needs attention or has data to send, it sends an interrupt signal to the processor through a dedicated interrupt request (IRQ) line.
   • Examples of hardware interruptions include keystrokes, mouse clicks, and the arrival of network packets.
     

2. Software Interrupts:
3. • Software interrupts, also called interrupts or exceptions, are generated by software instructions or exceptional conditions within a program.
   • These are intentionally triggered by the software itself to request specific services or to deal with unusual situations. 
   • Examples of software interrupts include system calls, divide by zero, and illegal memory access.
     

3. Timer Interrupts:
4. • Timer interrupts in the operating system are generated periodically by a system timer or clock. 
   • These are used to support time-related functions such as scheduling processes, updating system time, and running periodic tasks.
   • Timer interrupts ensure a fair distribution of CPU time between processes and allow the implementation of time-sharing systems.
     

4. Inter-processor Interrupts (IPI):
5. • Multiprocessor systems use interprocessor interrupts for communication and synchronization between processors.
   • A processor can send an interrupt to another processor to notify it of a particular event or to request its cooperation in processing a task.
   • IPI is essential to enable parallelism and coordination across multiple cores or processors.
     

Interrupt Handling Process

For all types of interrupt that occur, the operating system has a particular course of action that it follows. In my view and understanding, here are the various interrupt handling processes that are invoked: 

1. Interrupt Recognition:
2. • The CPU receives an interrupt signal from a device, software, or timer.
   • Compare the priority level of the current task with that of this interrupt for immediate service to be done.
     

2. Saving the Program State:
3. • If it is accepted, then the CPU saves the state of the present program, stores the program counter, and registers values on the stack.
   • Doing this ensures that when needed, one can resume execution from where it was interrupted before.
   • The vector table contains unique entries for each type of interrupt, which points out its respective ISR address.
     

3. Interrupt Vectoring:
4. • The CPU uses the interrupt vector table, a predefined table in memory, to determine the address of the appropriate interrupt service routine (ISR) for the specific interrupt.
   • Each interrupt in OS type has a unique entry in the vector table, pointing to its corresponding ISR.
     

4. Executing the ISR:
5. • The control then goes to an interrupt service routine pointed by the vector table entry corresponding to this sort of interrupt.
   • ISRs are specialized codes meant to specifically handle given interrupts and subsequently perform duties required by them.
   • Typically, ISRs are designed as brief and fast routines so as not to spend too long time in servicing interrupts
     

5. Restoring the Program State:
6. • After the ISR finishes its execution, the CPU retrieves the saved program state from the stack.
   • The register values and program counter are restored, allowing the interrupted program to resume its execution from where it left off.
     

6. Returning from the Interrupt:
7. • The CPU returns control to the interrupted program, and normal execution resumes.
   • If multiple interrupts are pending, the CPU may immediately service the next highest-priority interrupt before returning to the main program.
     

Significance of Interrupts in Operating Systems

So far, you have understood the different types of interrupts in OS as well as the different interrupt handling processes that OS invokes. Now, in case you are wondering what exactly is the significance of interrupts in an operating system, let’s look at that in this section. Here are some key reasons why interrupts are important:

1. Handling Time-Critical Events: Interrupts play an important role in allowing the operating system to process important events quickly and efficiently. In a computer system, various hardware devices such as keyboards, mice, printers, and network cards may require immediate attention from the processor. When a device needs repair or is ready to process data, it sends an interrupt signal to the processor. Interrupts allow the operating system to respond to these emergency events without delay.

Without interruptions, the processor must constantly poll the device to check its status or wait for data. This approach, known as busy-wait or polling, wastes valuable CPU cycles and leads to inefficient use of computing resources. 

2. Enabling Multitasking: Interrupts are fundamental to multitasking in operating systems. Multitasking allows you to share CPU time and run multiple programs or processes at the same time. Operating systems accomplish this by using timer interrupts to periodically interrupt the execution of the current process and switch to another process. 

These timer interruptions enable the operating system to evenly share CPU time among different processes, preventing any one process from taking over the CPU entirely. 

3. Handling Exceptions and Errors:  Interrupts provide a reliable mechanism for handling exceptions and errors that may occur during program execution. An exception is an abnormal condition that interrupts the normal flow of a program, such as a division by zero, an invalid memory access, or an invalid instruction. When an exception occurs, the processor generates an interrupt so that the operating system can take appropriate action. 

By using interrupts to handle exceptions, operating systems can avoid crashes, protect system resources, and provide a controlled environment for program execution. Exception handling through interrupts enables the operating system to detect and respond to errors, ensuring overall system reliability and stability.

4. Facilitating System Services: When it comes to facilitating seamless communication between user programs and operating systems, interrupts in OS play a crucial role. Essentially, whenever a user program requires any OS service, a special system call is issued. System calls are simply a special set of instructions that cause a software interrupt and transfers control of the user program to the operating system kernel.

When a system interrupt occurs through a system call, the operating system first saves the current state of the user program and then switches to kernel mode. Fully protected mode as a kernel provides the operating system with direct access to system resources and automatically runs services on behalf of user applications. Once the job is complete, the operating system resets the user program's performance and returns control to the user program.

Device drivers that base their communication on interrupt-based protocols can be easily ingrained into the operating system. Such integration results in plug-and-play functionality of the system and eventually vaunts both flexibility and extensibility of the system.

The significance of interrupts in operating systems will become more apparent to you when you undertake any full stack development endeavor – either in the form of a full stack development course, or a project! 

Wrapping Up

Interrupts are the primary way the operating system does its job, keeps track of resources, does multitasking, and behaves rapidly. Critical events such as software interruption, communications with hardware devices, and system services are handled by them on the CPU’s behalf. Engineers analyze types of interrupts and interrupt handling processes so as to apply them carefully to build faultless and rapid operating systems.

Check out software and tech courses offered by upGrad if you’re interested in further exploring the intricate world of computer science and OS. 

Frequently Asked Questions (FAQs)

1. What are interrupts and their types?
   
   The interruptions are indicators which forbid the normal flow of the program temporarily, thus the operating system is able to resolve urgent issues. There are three main types of interrupts. These are the hardware interrupts, the software interrupts (traps or exceptions), timer interrupts and inter-processor interrupts (in multiprocessor systems).
   

2. Why are interrupts used?
   
   Interrupts in OS are meant to help deal with these problems as they promote effective resource management, responsiveness, and task multiplexing in operating systems. They allow the CPU to handle real-time events, multi-task, handle exceptions and errors, provide system services, and support peripheral communication.
   

3. What is a maskable and non-maskable interrupt?
   
   A maskable interrupt is an interrupt that may be ignored or stopped by the processor for a time using an interrupt mask. An exemption can be assigned to the CPU to produce the request for maskable interrupts. In contrast, an NMI (non-maskable interrupt) cannot be ignored by the CPU and must be serviced instantaneously.
   

4. What are interrupts in 8086?
   
   In the 8086 microprocessor, interrupts are instructed to jump to a specific memory segment where an interrupt service routine (ISR) is stored. The 8086 has a built-in interrupt vector table and stores the addresses of different ISRs for different types of interrupts. On taking the place of an interrupt, the CPU saves the current state and jumps to the particular ISR in order to secure its processing.
   

5. Why are interrupts good?
   
   Interrupts are useful as they let CPUs perform tasks simultaneously, respond to crucial events in time, and allocate resources accordingly. These facilitate the operating system to react to hardware devices, handle exceptions and provide system services without continuously checking or wasting CPU cycles.
   

6. How are interrupts generated?
   
   Interrupts can originate from the hardware devices, software procedures as well as special circumstances. As new hardware devices appear, a dedicated cycle called interrupt request lines (IRQs) is used to sort out these requests. The group of software interrupts is created by intents of software instructions or exceptional circumstances such as the system calls or the division by zero.
   
7. How are interrupts stored?
   
   Interrupts are being saved in a special data structure called the interrupt vector table (IVT). The IVT (interrupt vector table) is a part of memory that holds the addresses of the various ISRs associated with various interrupt types. When the interrupt happens, the CPU uses the interrupt number as an index into the IVT to get the ISR address where the compensating action could be performed.
   

8. Which interrupt has least priority?
   
   The arrangement of interrupt priority may be customized by the particular system or architecture. That being said, hardware interrupts in general, have higher priority than software interrupts. The priority logic often relies on the interrupt request line (IRQ) number. Low IRQ numbers have higher priority than high numbers.
   

9. What is an example of interruption?
   
   One instance of the interruption is when the user presses a key on the keyboard. The keyboard controller sends an interrupt signal to the CPU, when it registers that a key has been pressed. The CPU stops its execution, saves the state, and jumps to the interrupt service routine which will be in charge of the key press event.
   

10. What is the difference between trap and interrupt?
    
    Traps are a type of synchronous interrupt generated either by software instructions or internal program exceptions. Traps are machine-produced services specially generated by software to handle special situations and request specific services. Meanwhile, an interrupt is a more general definition that covers both synchronous and asynchronous events, for example, hardware interrupts, software interrupts, and traps.