Programmable interrupt controller

Programmable Interrupt Controller

A Programmable Interrupt Controller (PIC) is a device used in computer systems to manage interrupt signals from various sources, allowing the CPU to prioritize and handle these interrupts efficiently. The PIC is essential in systems where multiple devices can generate interrupts, as it helps in determining the priority of each interrupt and ensures that the CPU processes them in an orderly manner.

Overview

In a computer system, an interrupt is a signal to the processor emitted by hardware or software indicating an event that needs immediate attention. The processor responds by suspending its current activities, saving its state, and executing a function called an interrupt handler to deal with the event. After the interrupt handler finishes, the processor resumes its previous activities.

A PIC is responsible for managing these interrupts by:

1. Prioritizing Interrupts: It assigns priority levels to each interrupt request (IRQ) line, ensuring that higher priority interrupts are handled before lower priority ones. 2. Masking Interrupts: It can enable or disable specific interrupts, allowing the system to ignore certain interrupts when necessary. 3. Vectoring Interrupts: It provides the CPU with the address of the interrupt handler that should be executed for a particular interrupt.

Types of Programmable Interrupt Controllers

There are several types of PICs, including:

• 8259A PIC: One of the most common PICs used in older x86 architecture systems. It can handle up to 8 interrupt lines and can be cascaded to handle more.
• Advanced Programmable Interrupt Controller (APIC): Used in modern systems, APICs can handle more interrupts and provide more advanced features such as interrupt distribution in multi-processor systems.

Operation

The operation of a PIC involves several steps:

1. Interrupt Request: A device sends an interrupt request to the PIC. 2. Priority Resolution: The PIC determines the priority of the interrupt and checks if it is masked. 3. Interrupt Acknowledgment: If the interrupt is not masked, the PIC sends an acknowledgment to the device and signals the CPU. 4. Interrupt Vectoring: The PIC sends the interrupt vector to the CPU, which points to the interrupt handler. 5. Interrupt Handling: The CPU executes the interrupt handler. 6. End of Interrupt: The CPU signals the PIC that the interrupt has been handled, allowing the PIC to process the next interrupt.

Applications

PICs are used in various applications, including:

• Embedded Systems: Where multiple peripherals need to communicate with the CPU.
• Real-Time Systems: Where timely processing of interrupts is critical.
• Personal Computers: To manage interrupts from devices like keyboards, mice, and network cards.

Also see

• Interrupt
• Advanced Programmable Interrupt Controller
• Central Processing Unit
• Embedded System
• Real-Time Operating System