26.5.2. I/O Module Interrupts CPU

In interrupt driven I/O, we are trying to eliminate busy waiting or idle cycles. The processor requests for the I/O transfer and moves on to perform other tasks while waiting for the I/O module to make the data ready.

This chunk describes the primary concept of interrupt driven I/O. Unlike programmed I/O, where the CPU continuously checks to see if the I/O device is ready (which wastes CPU time), interrupt driven I/O allows the CPU to issue a request for I/O transfer and then continue with other computations or tasks. The I/O module will recognize when it's ready to transfer data and will notify the CPU via an interrupt signal. This effectively reduces idle time and improves CPU utilization.

Think of interrupt driven I/O like a waiter in a restaurant. Instead of standing by the kitchen door waiting for food to be ready (busy waiting), the waiter takes the customer's order and moves on to serve other tables. When the food is ready, the kitchen sends a notification (the interrupt) to the waiter, who then returns to pick up the food. This way, the waiter maximizes their time by attending to multiple tasks, just like a CPU can handle different operations while waiting for an I/O operation to complete.

The I/O module interrupts the CPU when the device is ready. The basic operations involve the CPU issuing a read or write command while the I/O module prepares the data.

In this chunk, we learn about the specific functions of the I/O module during interrupt driven I/O operations. When the CPU issues a command, like a read command to get data, the I/O module steps in to handle the data transfer process. While the I/O module is busy preparing the data for transfer, the CPU is free to perform other tasks, thereby enhancing efficiency. Once the data is ready, the I/O module interrupts the CPU, signaling that the CPU can now retrieve or send data. This reflects the modular design of systems where I/O operations do not halt CPU processing.

Imagine a teacher who has asked a student to fetch a book from the library. While the student is away (I/O module processing), the teacher is able to prepare for the next class (CPU performing other tasks). When the student returns with the book (I/O interrupt), the teacher can immediately start the lesson without having to wait idle for the student to come back (busy waiting).

After receiving an interrupt signal, the CPU completes the current instruction, saves its state, and then executes the interrupt service routine (ISR).

This portion covers the steps the CPU must take once it receives an interrupt signal. First, it finishes executing the current instruction to maintain a consistent state. Next, it saves essential information, such as the address of the next instruction (program counter) and the current operational status (program status word), onto a stack. Then, the CPU will load the address of the interrupt service routine and begin processing this routine, which dictates how to handle the interrupt (like fetching data from an I/O device). After completing the ISR, the CPU restores the saved state and resumes normal operation.

This resembles a person talking on the phone who receives a text message. They finish their current conversation (the current instruction) to ensure nothing is missed. They note the context of their conversation (saving state), check the text message (running the ISR), and once they've read it, they return to the phone call as if nothing interrupted their flow (resuming normal operation).

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