What is the difference between FIQ and IRQ interrupt system?

Question

I want to know the difference between FIQ and IRQ interrupt system in any microprocessor, e.g: ARM926EJ.

Answer 1

FIQ is higher priority, and can be introduced while another IRQ is being handled. The most critical resource(s) are handled by FIQ's, the rest are handled by IRQ's.

Answer 2

It Depends how we design interrupt handlers, as FIQ is at last it may not need one branch instruction, also it has unique set of r8-r14 registers so next time we come back to FIQ interrupt we do not need to push/pop up the stack. Ofcourse it saves some cycles, but again it is not wise to have more handlers serving one FIQ and yes FIQ is having more priority but it is not any reason to say it handles the interrupt faster, both IRQ/FIQ run at same CPU frequency, So they must be running at same speed.

Answer 3

A feature of modern ARM CPUs (and some others).

From the patent:

A method of performing a fast interrupt in a digital data processor having the capability of handling more than one interrupt is provided. When a fast interrupt request is received a flag is set and the program counter and condition code registers are stored on a stack. At the end of the interrupt servicing routine the return from interrupt instructions retrieves the condition code register which contains the status of the digital data processor and checks to see whether the flag has been set or not. If the flag is set it indicates that a fast interrupt was serviced and therefore only the program counter is unstacked.

In other words, an FIQ is just a higher priority interrupt request, that is prioritized by disabling IRQ and other FIQ handlers during request servicing. Therefore, no other interrupts can occur during the processing of the active FIQ interrupt.

Answer 4

Chaos has already answered well, but an additional point not covered so far is that FIQ is at the end of the vector table and so it's common/traditional to just start the routine right there, whereas the IRQ vector is usually just that. (ie a jump to somewhere else). Avoiding that extra branch immediately after a full stash and context switch is a slight speed gain.

Answer 5

This may be wrong. All I know is that FIQ stands for Fast Interrupt Request and that IRQ stands for Interrupt Request. Judging from these names, I will guess that a FIQ will be handled(thrown?) faster than an IRQ. It probably has something to do with the design of the processor where an FIQ will interrupt the process faster than an IRQ. I apologize if I'm wrong, but I normally do higher level programming, I'm just guessing right now.