What happens if another interrupt is raised before the first interrupt action is completed?

Question

This question is from the interrupt handling topic.

Suppose an interrupt is being serviced. What happens if another interrupt is raised even before the first interrupt action is completed?

Answer 1

Well, if interrupts were not disabled after the first interrupt, the second will cause your interrupt service routine to be called again. You must make sure interrupts get disabled to avoid this decidedly undesirable behavior.

So, if your interrupt service routine is doing its thing, and then another interrupt occurs, it will be just as if you were doing anything else: the corresponding interrupt routine will be called.

On the Intel architecture, the "cli" instruction will disable interrupts, and "sti" will enable them again.

Answer 2

It depends on the system. Normally, if the new interrupt is a higher priority than the first, then it is responded to, suspending the handler for the first interrupt. When its handler finishes, then the original interrupt handler resumes. Finally, assuming no more interrupts, the original handler finishes and normal service resumes. Sometimes, the resumed process will be the process that was interrupted; sometimes, it will no longer be the most eligible process and some other one will resume.

Similarly, if a second or subsequent instance of the original interrupt occurs before the first handler completes, or if a lower or equal priority interrupt occurs, it will be held up until the first handler completes. Before normal processing is resumed, the kernel checks for outstanding interrupts that should have been handled but were blocked.

An interrupt handler may block other interrupts.

Answer 3

The following applies to the x86 architecture only, but other architectures might well follow the same pattern:

There is a processor flag called IF (Interrupt Flag) that controls whether hardware interrupts can be processed, or have to be put on hold. When IF = 0, interrupts will be postponed until the flag is reenabled (Except for the NMI, the Non-Maskable Interrupt, which is intended as an 'emergency only' interrupt that cannot be blocked).

The IF is automatically cleared by the processor before an interrupt servicing routine is called. This is necessary to prevent interrupt calls to become reentrant out of control. Note that the interrupt servicing code itself could not do this on its own, because if IF were not disabled before entering the routine, it would be possible for more interrupts to occur before the servicing code has time to execute even a single instruction. Then, a "firehose" of interrupts would immediately result in (of all things) a stack overflow.

So, in answer to your direct question: typically, when a second hardware interrupt occurs while an initial one is being serviced, that interrupt will be put on hold until the first one has finished.

As usual, the full story is a bit more complicated. The Intel Architecture Software Developer’s Manual at Intel's web site gives a more complete description starting on page 10-4.