Interrupt Requests (IRQs): How Hardware Communicates with the CPU

Learn how Interrupt Requests (IRQs) allow hardware devices to communicate with the CPU and request service. Understand the process of interrupt handling, its importance in computer systems, and how it enables efficient multitasking.

Interrupt Request (IRQ)

An Interrupt Request (IRQ) is a signal used to temporarily stop the processor’s activities. A hardware device, such as a printer, can send this signal to the CPU to pause its current process, allowing the device to continue operating independently. For instance, when a printer finishes printing, it sends an interrupt signal to the CPU to pause, so the processor can decide what task to perform next.

IRQs are essential when multiple devices are connected to a computer. These devices need to request the CPU’s attention to complete their operations and process data. An interrupt handler notifies the CPU to stop its current task and handle the device's request, such as processing mouse movements when a user moves the mouse.

The Purpose of the IRQ

IRQs are used for actions like keyboard presses, mouse movements, and printer operations. When a device needs the CPU’s attention, it sends an interrupt request to pause the processor. For example, each time a key is pressed, the interrupt handler notifies the CPU to stop what it’s doing and process the keystrokes.

Each device uses a specific data channel, called an IRQ number. For example, one device may use IRQ 4, while another uses IRQ 7. The CPU can respond accordingly to each device based on these IRQ numbers.

Types of Interrupt Requests

1. Software Interrupts

A software interrupt is generated by a system or software, rather than hardware. These interrupts are often called traps or exceptions. They signal the operating system to handle specific tasks or address an issue. Software interrupts are initiated using a special instruction that pauses the CPU’s current task and activates a specific interrupt handler. Once the necessary task is completed, the interrupt handler returns control to the original application.

2. Hardware Interrupts

In a hardware interrupt, every device is connected to the Interrupt Request Line. Each device can generate an interrupt by activating a switch connected to it. When the interrupt occurs, the CPU pauses its current task to process the interrupt. After processing, the device stops signaling the interrupt, and the CPU resumes its interrupted program.

IRQ Errors

IRQ errors can occur when two devices attempt to use the same IRQ channel, leading to conflicts. While it is theoretically possible for multiple devices to use the same channel, it is uncommon in practice. Conflicts typically arise when IRQ signals overlap on the same channel.

An example of an IRQ error is IRQL_NOT_LESS_OR_EQUAL, which happens when a system process or device driver tries to access a memory address without the proper permissions. This error often leads to a system crash and a Blue Screen of Death (BSOD) in Windows, which displays a message like "Your computer encountered an issue and needs to restart."

Modern devices, especially Plug and Play (PnP) devices, rarely experience IRQ conflicts, as they are automatically configured.

Plug and Play and IRQ

Plug and Play (PnP) devices automatically configure their IRQ settings, eliminating the need for manual intervention. Before PnP devices, users had to manually assign IRQ values for each new device. With PnP, the computer configures these values automatically, saving users time and effort. If a non-PnP device is installed, the manufacturer typically provides the necessary IRQ settings.

IRQ Numbers and Channels

Each device connected to a computer has a unique IRQ number. For example, a printer might use IRQ 1, a keyboard IRQ 4, and a mouse IRQ 7. These IRQ numbers help prioritize tasks and manage how the CPU responds to different devices. Devices with lower IRQ numbers have higher priority. For example, the system timer usually uses IRQ 0, while primary and secondary IDE channels use IRQ 14 and IRQ 15, respectively.

How to View and Change IRQ Settings

In Windows, IRQ settings can be viewed and changed through Device Manager. To access IRQ settings:

• Open Device Manager.
• Go to the Resources tab and select “View by type” to see IRQ assignments.
• For detailed information, run msinfo32.exe from the Run dialog box and navigate to Hardware Resources > IRQs.

You can change IRQ settings in Device Manager as follows:

• Double-click the device to open its Properties window.
• Uncheck the "Use automatic settings" option in the Resources tab.
• Select the hardware configuration to change and modify the IRQ value using the “Change Setting” button.

Note: Be careful when modifying IRQ settings, as errors could lead to system instability. Always note the original settings before making any changes.

Common IRQ Channels

Conclusion

An Interrupt Request (IRQ) is a critical component in computer systems that allows hardware devices to communicate with the CPU. By temporarily halting the processor, IRQs ensure that the system can multitask effectively and respond to real-time events. Proper management of IRQs is essential for system stability and preventing device conflicts. Modern systems rely heavily on IRQs for efficient performance.