26.2. Lecture – 34

In our last lecture, we discussed issues related to input/output and I/O modules. We mentioned three ways to transfer information: programmed I/O, interrupt driven I/O, and DMA. This lecture will focus on interrupt driven I/O.

This chunk introduces the topic of interrupt driven I/O as part of a broader discussion on I/O subsystems. We learned that in the previous lecture, the instructor posited three methods for transferring information, with the current lecture dedicated to the interrupt driven method. This teaching format helps us to frame our understanding of I/O interactions by distinguishing between different techniques.

Imagine deciding how to send messages: you can either send them continuously checking for a response (programmed I/O), send them but wait to hear back while not doing anything else (busy waiting), or send them and carry on with your tasks until you get notified (interrupt driven I/O). The latter is more efficient, paralleling how workflows operate in many workplaces.

The objectives of this unit include: 1. Discussing the need for interrupt driven I/O transfer at a comprehension level. 2. Specifying the control signals needed for interrupt driven I/O transfer at an analysis level. 3. Explaining the design issues of interrupt driven I/O transfer at a design level.

This chunk outlines the objectives of the lecture on interrupt driven I/O, highlighting a structured approach to learning the topic. The first objective emphasizes understanding the need for this method, the second focuses on analyzing the signal controls necessary for operation, and the third involves discussing design considerations that engineers face when implementing interrupt driven I/O.

Think of these objectives as steps in developing a new smartphone. First, you need to understand users’ needs (comprehension), then decide what buttons and interfaces will control it (analysis), and finally create a prototype based on those controls (design). Each step builds on the previous one.

In programmed I/O, processors continuously check if the device is ready, leading to wasted CPU time. In interrupt driven I/O, the CPU can continue other tasks while it waits for the I/O module to signal that the device is ready.

This chunk illustrates a critical advantage of interrupt driven I/O — minimizing CPU idle time. In programmed I/O, the processor must check the status of the I/O device repeatedly, which can waste processing power and time. In contrast, interrupt driven I/O allows the CPU to execute other tasks until the I/O module interrupts it, indicating readiness for data transfer.

Consider a scenario where you're cooking. In programmed I/O, you'd be standing by the oven, checking every minute if the food is done. With interrupt driven I/O, you could do something else, like setting the table, and then your oven alerts you once the food is ready. This way, you're not stuck doing nothing but waiting.

When the processor requests an I/O transfer, it can continue performing other tasks. Once all data is ready, the I/O module sends an interrupt signal to the processor for transfer.

This section explains the process by which the CPU can multitask during an I/O operation. After the CPU issues a request for I/O, instead of idling, it executes other processes. Once the I/O module prepares the data for transfer, it sends an interrupt signal to notify the CPU that the data is ready, allowing for a seamless transition to the next operation.

Imagine you're using a printer connected to your computer. You send a document to be printed and instead of waiting, you continue working on other tasks. Once the document is printed, the printer sends a signal (like beeping) to your computer that the job is complete. You can then go back to check and see the printed output without wasting time.

Context switching refers to saving the state of the current program before servicing an interrupt. This process ensures that once the interrupt is handled, the CPU can resume the original program without loss of data.

This chunk describes what context switching entails, highlighting its importance in managing CPU tasks effectively. When an interrupt occurs, the system saves the current operational state, including the program counter and other register values, so that it can return to this precise point after servicing the interrupt.

Think of context switching like pausing a movie to answer a call. Before you answer, you make a mental note of where you paused, so you can pick up right where you left off once you're done. In computing, context switching serves the same function, allowing seamless transitions between tasks.

--