An interrupt is a condition that causes the microprocessor to temporarily work on a different task, and then later return to its previous task. Interrupts can be internal or external. Internal interrupts, or "software interrupts," are triggered by a software instruction and operate similarly to a jump or branch instruction. An external interrupt, or a "hardware interrupt," is caused by an external hardware module. As an example, many computer systems use interrupt driven I/O, a process where pressing a key on the keyboard or clicking a button on the mouse triggers an interrupt. The processor stops what it is doing, it reads the input from the keyboard or mouse, and then it returns to the current program.
The image below shows conceptually how an interrupt happens:
The grey bars represent the control flow. The top line is the program that is currently running, and the bottom bar is the interrupt service routine (ISR). Notice that when the interrupt (Int) occurs, the program stops executing and the microcontroller begins to execute the ISR. Once the ISR is complete, the microcontroller returns to processing the program where it left off.
What happens when external hardware requests another interrupt while the processor is already in the middle of executing the ISR for a previous interrupt request?
When the first interrupt was requested, hardware in the processor causes it to finish the current instruction, disable further interrupts, and jump to the interrupt handler.
The processor ignores further interrupts until it gets to the part of the interrupt handler that has the "return from interrupt" instruction, which re-enables interrupts.
If an interrupt occurs while interrupts were turned off, some processors will immediately jump to that interrupt handler as soon as interrupts are turned back on. With this sort of processor, an interrupt storm "starves" the main loop background task. Other processors execute at least one instruction of the main loop before handling the interrupt, so the main loop may execute extremely slowly, but at least it never "starves".