26.3.1. Objective 1: Discuss the need of interrupt driven I/O transfer

In our discussion, we have identified three primary ways to transfer information: programmed I/O, interrupt driven I/O, and DMA. This unit specifically focuses on interrupt driven I/O.

There are three main methods for transferring data between a computer and I/O devices. The first is programmed I/O, where the CPU continuously checks to see if a device is ready, which can waste processing time. The second method is interrupt driven I/O, which allows the CPU to perform other tasks instead of waiting for the device to be ready, making it much more efficient. Lastly, Direct Memory Access (DMA) allows I/O devices to transfer data directly to/from memory without CPU intervention after initial setup.

Think of programmed I/O as a waiter at a restaurant continually checking if a table is ready, which keeps them from serving other customers. Interrupt driven I/O is like a restaurant where the waiter takes your order and moves on to serve other tables, only returning when the food is ready. DMA is like a food delivery service that takes care of delivering large orders directly to the kitchen without needing the waitstaff to check on it.

The major benefit of interrupt driven I/O is the elimination of busy waiting. In this method, the CPU can perform useful work while waiting for I/O operations to complete, significantly improving system efficiency.

In interrupt driven I/O, instead of the CPU wasting cycles waiting for a device, it can execute other instructions. When the I/O device is ready, it generates an interrupt signal that alerts the CPU to stop its current task and handle the input/output operation. This change maximizes CPU usage and minimizes idle time, which can lead to better overall performance of the system.

Imagine a chef who is baking a cake. Instead of standing in front of the oven watching and waiting for the cake to bake (busy waiting), the chef can prepare other dishes. They’ll be signaled by a timer (the interrupt) when it's time to check the cake. This way, the chef's time is used more efficiently.

The process includes the CPU issuing a read command, the I/O module managing data transfer, and interrupting the CPU once the data is ready. This allows the CPU to be busy with other tasks.

When operating with interrupt driven I/O, the CPU begins by sending a read or write command. While the I/O device processes that command, the CPU can perform other tasks. The I/O module checks the device status and, when the data is ready, sends an interrupt signal to the CPU. Upon receiving the interrupt, the CPU suspends its current operations to handle the data transfer, then resumes its previous task. This structured workflow highlights the efficiency of the interrupt driven method.

Consider a skilled technician who is repairing multiple machines simultaneously. They send a maintenance request to a machine (issuing a read command) and while waiting for the machine to indicate it's done fixing (data ready), they attend to another machine. When the first machine is ready, it sends a signal (interrupt), and the technician shifts back to that machine to complete the task.

The CPU must complete the current instruction before it can service the interrupt. After completing the instruction, it checks for the interrupt signal, allowing for effective handling.

When an interrupt occurs, the CPU must finish executing the current instruction. It cannot stop mid-way, as it does not keep track of the intermediate state. Once it has completed the instruction, it checks for any pending interrupts. If there is an interrupt, the CPU processes it by retrieving data as instructed and executing the specific routine associated with that device. This ensures an orderly and efficient processing mechanism.

Imagine a teacher who is grading exams. They must finish grading one paper before they can look at a student's request for help (interrupt). Once they finish the paper, they can address the request, ensuring that nothing is forgotten or left incomplete.

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