26.5. Basic Operations of Interrupt Driven I/O

In this unit, we are going to discuss about the interrupt driven I/O. So, what are the objectives of this particular unit? There are three objectives: 1) Discuss the need for interrupt driven I/O transfer. 2) Specify the control signal needed for interrupt driven I/O transfer and their use. 3) Explain the design issues of interrupt driven I/O transfer.

This chunk introduces the key concepts of interrupt driven I/O. It outlines the three main objectives for the unit, which are understanding the need for interrupt driven I/O, identifying the necessary control signals for this type of I/O operation, and addressing the design considerations involved in implementing such a system.

Imagine a waiter in a restaurant. Instead of standing by each table waiting for customers to call for their order (which wastes their time), the waiter checks in with each table periodically. This way, they can attend to customers only when needed, similar to how interrupt driven I/O allows the CPU to perform other tasks while waiting for input/output operations.

In interrupt driven I/O, we are removing busy waiting. The processor can request an I/O transfer and then, while waiting for the I/O module to be ready, perform other computations. When the I/O module is ready, it interrupts the processor to notify it.

One of the primary advantages of interrupt driven I/O is the elimination of busy waiting. In traditional programmed I/O, the CPU continuously checks if an I/O device is ready, wasting processing time. With interrupt driven I/O, the CPU can focus on other tasks while the I/O device prepares data. Once the I/O device is ready, it sends an interrupt signal to the CPU, which then handles the I/O operation.

Think of a teacher who is grading papers. Instead of just waiting by the door for a student to drop off their assignment (busy waiting), the teacher can grade other assignments. When a student arrives, they ring a bell to get the teacher's attention (the interrupt), allowing the teacher to handle each task more effectively.

The CPU issues a read command. While the I/O module gets data from the peripheral, the CPU does other work. Once the data is ready, the I/O module interrupts the CPU, which then requests the data transfer to occur.

This chunk describes the sequential steps involved in interrupt driven I/O. Initially, the CPU issues a read command instructing the I/O module to fetch data. Instead of waiting idly, the CPU continues with its tasks. After the I/O module has completed the data retrieval, it sends an interrupt to the CPU. Following this, the CPU will request the data from the I/O module, completing the I/O operation without unproductive waiting.

Imagine a chef preparing a meal while waiting for a pot of water to boil. Instead of standing by the pot, the chef is chopping vegetables (doing other work). When the water begins to boil (data is ready), the stove signals an alert (interrupt), and the chef can return to the pot to continue the cooking process.

When servicing an interrupt, the CPU saves its current state (like the program counter and registers) to avoid losing progress. This saved state allows the CPU to return to its previous operation after handling the interrupt.

In the context of interrupt driven I/O operations, context switching refers to the process of saving the current operational state of the CPU before addressing the interrupt. This includes saving the program counter, which tells the CPU where to resume after the interrupt service routine execution. Once the interrupt has been serviced, the CPU can restore its earlier state and continue from where it left off.

Consider a person multitasking by cooking and answering phone calls. When the phone rings (interrupts), they write down their recipe or pause the cooking process (saving state) before focusing on the call. Once they finish the call, they can easily pick up where they left off in cooking (return to the previous task).

The ISR is a special routine executed in response to an interrupt. Each device may have a unique ISR tailored to its needs. Once the ISR completes its task, control is returned to the CPU to continue from where it was interrupted.

The Interrupt Service Routine (ISR) is a specific piece of software that the CPU executes when an interrupt occurs. Each device connected to the system can have its own ISR depending on the type of request it generates. After the ISR completes the necessary operations (like reading data), control flows back to the CPU, allowing it to resume its previous work seamlessly.

Think of an emergency alarm in a factory. When an alarm goes off, a designated safety officer follows a set protocol (the ISR) to assess the situation. Once the alarm is handled, the officer can return to their regular duties, just like the CPU resumes execution after an interrupt.

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