The reason why FIQ (fast interrupt) in ARM responds faster than IRQ (normal interrupt)

What are FIQ and IRQ

FIQ (fast interrupt) and IRQ (normal interrupt) are two processor modes in ARM processors. ARM has 7 processing modes. Both modes are used to handle interrupts. The concept of interrupts will not be elaborated here. If you are not familiar with ARM's exception handling mechanism and registers, it is recommended to read the blog "Detailed Explanation of ARM's 37 Registers and Exception Handling Mechanism". The following content will involve related concepts.

Difference between FIQ and IRQ

Essentially, both are interrupts, but FIQ has a higher priority than IRQ, and the response time of FIQ is also faster than IRQ. Simply put, FIQ is the VIP in IRQ and enjoys privileges. In ARM, you can generally set an interrupt as FIQ. When an interrupt is set as FIQ, it can jump the queue, and the next interrupt to be processed is FIQ. Some can even directly interrupt the currently processed IRQ. This is divided into preemptive and non-preemptive.

Why are interrupts divided into FIQ and IRQ

Today's systems need to interact with users frequently, and the user experience depends largely on the response time. Imagine how you would feel if the system takes a minute to respond after you click the mouse. The system does not know when an interrupt will occur. When multiple interrupts occur, the system usually handles the interrupts in the order in which they occur. However, the urgency of the interrupts is different. For example, two interrupts are generated now, one is generated by the background program, and the other is when the user clicks the mouse. If you handle the interrupt generated by the background program first and then respond to the mouse, the user will feel that the system is stuck and the response is very slow. In fact, it is no big deal if the background program is processed slowly, and the user is not in a hurry now. In this case, we can respond to the mouse first and set the mouse to FIQ to improve the user experience.

The reason why FIQ responds faster than IRQ

1. FIQ has a higher processing priority than IRQ and can even interrupt the executing IRQ;

2. FIQ mode has its own unique registers, while IRQ needs to share registers with other modes, which will be faster in the protection/recovery scene of interrupt processing;

3. In the exception vector table, FIQ is at the very end. In the exception vector table, IRQ can only store the first address of the interrupt handler, and a jump is required when IRQ occurs; while FIQ is at the very end, so the interrupt handler in FIQ mode can be directly stored next to it, so there is one less jump when processing FIQ.