Operating System - Interrupt-Initiated I/O

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Input/Output operations (I/O) are an important part of an operating system. I/O operations allows communication between the CPU and other devices such as keyboards, mouse, printers, and storage drives. There are various I/O techniques like programmed I/O, interrupt I/O and direct memory access(DMA) out of which the interrupt-initiated I/O is an efficient I/O technique compared to programmed I/O as it does not need continuous polling.

What is Interrupt-Initiated I/O?

Interrupt-initiated I/O is an I/O technique in which CPU does not continuously poll the device status. In this technique, if an I/O device is ready or has completed an operation, it will send an interrupt signal to the CPU. When CPU receives an interrupt signal, it stops the current execution of other works, saves its state, and executes the interrupt service routine (ISR) to handle the I/O operation.

After handling the interrupt, CPU resumes its previous task again. The CPU performs other tasks while waiting for the I/O operation to complete instead of continuous polling like programmed I/O.

Characteristics of Interrupt-Initiated I/O

The key characteristics of an Interrupt-Initiated I/O are −

• In this technique, CPU time is not wasted in continuous polling to check device status.
• The device sends interrupt to CPU only when required.
• I/O and CPU work independently.
• CPU responds to device events.

Key Components of Interrupt-Initiated I/O

Components of Interrupt-initiated I/O are mentioned below −

• I/O Device − Devices like keyboards, mouse, and printers that performs input or output operations.
• Device Controller − It is used to manage device operations and generate interrupt signals when needed.
• Interrupt Request Line (IRQ) − Hardware signaling line for . It is a hardware line that connects the devices to the CPU. It sends interrupt to CPU
• Interrupt Service Routine (ISR) − It is also called an interrupt handler. It is a small program that is executed in response to an interrupt.
• Interrupt Vector Table (IVT) − It is a data structure that maps interrupt numbers to ISR addresses. It stores the address of all ISRs. During an interrupt, CPU looks up the corresponding ISR address in this table.

Working of Interrupt-Initiated I/O

Here are the steps how interrupt-initiated I/O works. Below is an animation and steps to demonstrate working of Interrupt-Initiated I/O:

• First, CPU gives an I/O command to the device controller.
• Then, CPU continues to perform other task. It does not wait unlike Programmed I/O.
• The I/O operation completes at device side and the device controller sends an interrupt to the CPU.
• On receiving interrupt, CPU pauses its current task and saves its context.
• Then, CPU executes the corresponding ISR in response to the interrupt.
• After handling I/O, CPU will restore the context and resumes previous task.

Difference between Programmed and Interrupt-Initiated I/O

The differences between Programmed and Interrupt-Initiated I/O are mentioned below −

Types of OS Interrupts

The types of interrupts are given below based on source() and identification() −

• Hardware Interrupts
• Software Interrupts
• Vectored Interrupts
• Non-Vectored Interrupts

Hardware Interrupts

Hardware interrupts are triggered by external or I/O devices whenever a service request is required. For example: In a keyboard if we press a key to do some action, this pressing of the keyboard generates a signal that is given to the processor to do action, such interrupts are called hardware interrupts.

Hardware interrupt are of two types −

• Maskable Interrupt − The maskable interrupts can be disabled or masked by the CPU temporarily when CPU is busy with a higher-priority task.
• Non Maskable Interrupt − The non-maskable interrupts cannot be disabled by the CPU. They are used for high-priority, such as hardware failure.

Software Interrupts

Software interrupts are generated by programs for system calls or exceptions. For example: System call invocation, file operations, and divide by zero.

Software interrupt are of two types −

• Normal Interrupts − The interrupts that are caused by the software instructions are called software instructions.
• Exception − Exceptions are an unplanned interruption while executing any program. For example: while executing a program if we got a value that is divided by zero is called an exception.

Vectored Interrupts

When an interrupt occurs, device sends a unique vector to the CPU that points to ISR address directly. These interrupts are faster.

Non-Vectored Interrupts

In non-vectored interrupts, CPU finds ISR address from interrupt vector table when an interrupt occurs. The ISR address is not provided by the device. These interrupts are Slower and requires identifying the device.

Advantages of Interrupt-Initiated I/O

The advantages of Interrupt-initiated I/O are as follows −

• Interrupt-initiated I/O is efficient in using CPU time as cpu do not waste time in polling.
• It has better system performance.
• It has fast response to device requests.
• It allows multitasking environments.

Disadvantages of Interrupt-Initiated I/O

Below are the disadvantages of Interrupt-initiated I/O −

• It requires interrupt handling hardware.
• There is increased complexity due to ISR.
• Frequent interrupts may cause interrupt overhead.

Conclusion

Interrupt-initiated I/O is an I/O technique that uses interrupt handling hardware rather than continuously polling for device status. This chapter provides a detailed explanation of Interrupt-initiated I/O, its characteristics, key components, working, differences from programmed I/O, types of interrupts, advantages, and disadvantages of Interrupt-initiated I/O.