Purpose of an Interrupt in Computer Organization

Interrupts are an essential feature in computer systems, providing a mechanism that allows external devices (like I/O modules and memory) to temporarily interrupt the normal processing of the processor. This mechanism is used to enhance the efficiency of the processor and ensure that I/O operations don't unnecessarily waste valuable CPU cycles.

Why Are Interrupts Needed

Most external devices (e.g., printers, disk drives) are much slower than the processor. Without interrupts, if the processor had to wait for an I/O device to complete its task (like printing data), it would remain idle, unable to perform any other operations during that time. This would lead to inefficient use of the CPU.

For example, imagine the processor is transferring data to a printer. After sending a write command, the processor has to wait until the printer catches up. The processor would remain idle for many instruction cycles, which is wasteful.

Read more about Interrupts here!

How Interrupts Improve Efficiency

Interrupts allow the processor to continue executing other instructions while an I/O operation is in progress. Here’s how it works:

Without Interrupts:

• The processor executes a series of instructions.
• When it encounters an I/O operation (like writing to a printer), the processor must wait until the operation is complete before proceeding to the next instruction.
• This means the processor is essentially idle while the I/O operation is happening.

With Interrupts:

• The processor starts an I/O operation (e.g., writing to a printer).
• The processor doesn’t have to wait for the operation to finish. Instead, it moves on to execute other instructions.
• Once the I/O device is ready, it sends an interrupt request to the processor.
• The processor temporarily stops its current task, handles the interrupt (by executing an interrupt handler to service the I/O device), and then resumes the interrupted program.

Interrupt Handling in Practice

Consider a program that contains a series of WRITE operations. The processor typically invokes an I/O program to perform these tasks. Without interrupts, the processor would sit idle, waiting for the I/O device to finish. With interrupts, after initiating the I/O command, the processor can continue executing other tasks until it receives a signal from the I/O device indicating that it’s ready to continue.

When the I/O device completes its task, it sends an interrupt request to the processor. The processor then pauses its current operation, processes the interrupt, and once the interrupt service routine is complete, it resumes the original task where it left off.

Advantages:

• It increases the efficiency of CPU.
• It decreases the waiting time of CPU.
• Stops the wastage of instruction cycle.
• Enables multitasking by allowing the CPU to quickly switch between different processes.
• Simplifies input/output (I/O) operations by allowing devices to communicate directly with the CPU.

Disadvantages:

• CPU has to do a lot of work to handle interrupts, resume its previous execution of programs (in short, overhead required to handle the interrupt request.).
• Overhead required to handle the interrupt request can reduce the efficiency of the system.
• Interrupt storms can occur when there is high levels of interrupt activity.
• Priority inversion can occur when a low-priority task holds a resource needed by a higher-priority task.