4.1 Central Processing Unit (CPU) Architecture: Interrupts

Interrupts are a fundamental mechanism in modern computer systems that allow the CPU to temporarily suspend its current operation to handle a higher-priority event. They are crucial for efficient multitasking and responsiveness.

What is an Interrupt?

An interrupt is a signal that indicates a need for the CPU to stop what it's currently doing and attend to a specific event. This event can originate from hardware (e.g., a keyboard press, a disk drive completing a read operation) or software (e.g., a program requesting a system service).

Types of Interrupts

Interrupts are broadly classified into two main types:

• Hardware Interrupts: These are triggered by external hardware devices. For example, a device like a keyboard or a network card can generate an interrupt signal to signal the CPU.
• Software Interrupts: These are triggered by software instructions. They are typically used to request services from the operating system, such as system calls.

How Interrupts Work

1. Interrupt Request: A hardware or software device sends an interrupt request signal to the CPU.
2. Interrupt Handling: The CPU suspends its current execution, saves the current state (including the program counter and registers), and jumps to a pre-defined interrupt handler routine.
3. Interrupt Service Routine (ISR): The ISR is a special routine designed to handle the specific interrupt. It performs the necessary actions to address the cause of the interrupt.
4. Return from Interrupt: Once the ISR has completed, it executes a special instruction to return control to the interrupted program, restoring the saved state.

Interrupt Priority

Not all interrupts are equally important. Operating systems assign priorities to interrupts. Higher-priority interrupts can preempt lower-priority interrupts. This ensures that critical events are handled promptly.

Interrupt Vector Table

The CPU uses an Interrupt Vector Table (IVT) to determine the address of the appropriate ISR for a given interrupt. The IVT is a table in memory that contains the addresses of the ISRs for each possible interrupt.

Table: Interrupt Types and Associated Actions

Interrupt Type	Source	Action Taken
Keyboard Interrupt	Keyboard Device	Signals the CPU when a key is pressed. The ISR reads the keycode and updates the screen.
Disk Interrupt	Disk Drive	Signals the CPU when a disk operation (e.g., read, write) is complete. The ISR updates the file system.
Timer Interrupt	System Timer	Generates a regular interrupt at fixed intervals. Used for timekeeping and scheduling tasks.
Memory Access Error	Memory Controller	Signals the CPU when a memory access error occurs. The ISR handles the error (e.g., by attempting to recover or terminating the program).

Benefits of Using Interrupts

• Improved Efficiency: The CPU doesn't have to constantly poll devices for events. It can continue processing other tasks until an interrupt occurs.
• Responsiveness: Interrupts allow the system to respond quickly to events, such as user input or hardware signals.
• Multitasking: Interrupts are essential for implementing multitasking operating systems, where multiple programs can run concurrently.

Disadvantages of Using Interrupts

• Overhead: Handling interrupts takes time, which can reduce overall system performance.
• Complexity: Implementing interrupt handling can be complex, especially when dealing with multiple interrupt sources.

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