What is the difference between FIQ and IRQ interrupt system?

Question

hi

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

Renjith G

Answer 1

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.

Answer 2

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 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

I believe this is what you are looking for:

http://newsgroups.derkeiler.com/Archive/Comp/comp.sys.arm/2005-09/msg00084.html

Essentially, FIQ will be of the highest priority with multiple, lower priority IRQ sources.

Answer 5

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 6

another reason is in case of FIQ, lesser number of register is needed to push in the stack, FIQ mode has R8 to R14_fiq registers