Operating System #14 What is an Interrupt? Types of Interrupts

Summary

This video delves into the concept of interrupts in modern operating systems, explaining how they allow the processor to respond to external events efficiently. It covers the different types of interrupts—hardware interrupts, software traps, and processor-generated exceptions—along with the role of interrupt controllers like the 8259 and APIC. The video also explores the interrupt descriptor table (IDT), which links interrupt requests to their respective handler routines. Overall, it provides a comprehensive understanding of interrupt handling, which is crucial for multitasking and resource management in today's complex computing systems.

Takeaways

• 😀 Interrupts are essential in operating systems, allowing the system to handle events as they occur, rather than continuously checking for them.
• 😀 There are three main types of interrupts: hardware interrupts (from external devices), traps (software interrupts), and exceptions (processor-generated errors).
• 😀 Hardware interrupts are triggered by external devices like keyboards, network cards, or USB devices and can occur asynchronously at any time during program execution.
• 😀 Traps are software-generated interrupts that allow user programs to request OS services, such as writing data to the screen or accessing system resources.
• 😀 Exceptions are errors generated by the processor due to illegal operations, such as dividing by zero, which may lead to program termination.
• 😀 The interrupt handler is responsible for managing the event triggered by an interrupt and ensures the system resumes its original task after handling the interrupt.
• 😀 Modern systems use an interrupt controller to manage multiple interrupt requests from various devices, ensuring that they are properly prioritized and handled.
• 😀 The 8259 Programmable Interrupt Controller (PIC) was used in legacy systems to manage up to 8 interrupt sources, while newer systems use the Advanced Programmable Interrupt Controller (APIC) for more advanced and scalable interrupt handling.
• 😀 The Interrupt Descriptor Table (IDT) stores information about interrupt handlers, mapping IRQ numbers to specific handler routines in memory.
• 😀 In multi-core systems, each CPU has its own Local APIC, and interrupts are distributed efficiently between processors using an I/O APIC and inter-processor interrupts (IPIs).

Q & A

What are interrupts in operating systems and why are they important?

Interrupts are events that trigger the operating system to execute specific code in response to certain hardware or software events. They are important because they enable an operating system to be responsive and handle asynchronous events like hardware signals or software system calls, ensuring efficient resource management.

How does the processor handle interrupts in terms of privilege levels?

When an interrupt occurs, the processor switches from user space (Ring 3) to kernel space (Ring 0), which is a higher privilege level. After the interrupt is serviced, the processor switches back to the original user process.

What are the three main types of events that can trigger interrupts?

The three main types of events that trigger interrupts are: 1) Hardware Interrupts, generated by external devices (e.g., keyboard, network card), 2) Traps, or software interrupts, initiated by user programs for system calls, and 3) Exceptions, which are errors caused by illegal instructions (e.g., division by zero).

What is the difference between a fault and an abort in exceptions?

A fault is a recoverable error, like a page fault, where the operating system can intervene and resolve the issue. An abort is an irrecoverable error, such as a divide-by-zero exception, which results in the termination of the program or process.

How does the processor know which interrupt handler to execute?

The processor uses the Interrupt Descriptor Table (IDT), which contains entries with information on where the interrupt handler routines are located in memory. When an interrupt occurs, the processor uses the IRQ number to look up the corresponding handler address in the IDT.

What role does the interrupt controller play in managing interrupts?

The interrupt controller manages multiple devices sending interrupts to the processor. It ensures that interrupts from different devices are properly prioritized and routed to the processor, which then triggers the correct interrupt handler routine.

What is the function of the 8259 Programmable Interrupt Controller (PIC)?

The 8259 PIC is a hardware component used in legacy systems to manage up to 8 interrupt requests (IRQs) from devices. It routes these interrupts to the processor and helps prioritize them. In systems with more than 8 devices, multiple PICs can be used in a master-slave configuration.

How does the Advanced Programmable Interrupt Controller (APIC) differ from the 8259 PIC?

The APIC is used in modern multi-core systems to support local interrupt management for each CPU, providing more advanced interrupt handling, including support for multi-processor systems. Unlike the 8259 PIC, the APIC can route interrupts across different processors and has more flexibility in managing priorities and distribution.

What is the Interrupt Descriptor Table (IDT) and how does it function?

The IDT is a table stored in memory that contains the addresses of interrupt handler routines. Each entry in the table corresponds to an interrupt vector (IRQ number) and provides the segment selector and offset needed to locate the handler routine. The processor uses the IRQ number to find the appropriate entry in the IDT when handling an interrupt.

How do modern x86 systems handle multiple interrupts across processors?

In modern x86 systems, each processor has its own local APIC, and external devices communicate with the system through an I/O APIC. The I/O APIC routes interrupts to the local APICs, which handle distribution across processors, enabling effective interrupt management in multi-core systems.