Interrupts

Hardware interrupts were introduced as a way to reduce wasting the processor's valuable time in polling loops, waiting for external events. They may be implemented in hardware as a distinct system with control lines, or they may be integrated into the memory subsystem.
If implemented in hardware, an interrupt controller circuit such as the IBM PC's Programmable Interrupt Controller (PIC) may be connected between the interrupting device and the processor's interrupt pin to multiplex several sources of interrupt onto the one or two CPU lines typically available. If implemented as part of the memory controller, interrupts are mapped into the system's memory address space.
Interrupts can be categorized into these different types:

• Maskable interrupt (IRQ): a hardware interrupt that may be ignored by setting a bit in an interrupt mask register's (IMR) bit-mask.
• Non-maskable interrupt (NMI): a hardware interrupt that lacks an associated bit-mask, so that it can never be ignored. NMIs are used for the highest priority tasks such as timers, especially watchdog timers.
• Inter-processor interrupt (IPI): a special case of interrupt that is generated by one processor to interrupt another processor in a multiprocessor system.
• Software interrupt: an interrupt generated within a processor by executing an instruction. Software interrupts are often used to implement system calls because they result in a subroutine call with a CPU ring level change.
• Spurious interrupt: a hardware interrupt that is unwanted. They are typically generated by system conditions such as electrical interference on an interrupt line or through incorrectly designed hardware.

Processors typically have an internal interrupt mask which allows software to ignore all external hardware interrupts while it is set. Setting or clearing this mask may be faster than accessing an interrupt mask register (IMR) in a PIC or disabling interrupts in the device itself. In some cases, such as the x86 architecture, disabling and enabling interrupts on the processor itself act as a memory barrier; however, it may actually be slower.
An interrupt that leaves the machine in a well-defined state is called a precise interrupt. Such an interrupt has four properties:

• The Program Counter (PC) is saved in a known place.
• All instructions before the one pointed to by the PC have fully executed.
• No instruction beyond the one pointed to by the PC has been executed (that is no prohibition on instruction beyond that in PC, it is just that any changes they make to registers or memory must be undone before the interrupt happens).
• The execution state of the instruction pointed to by the PC is known.

An interrupt that does not meet these requirements is called an imprecise interrupt.
The phenomenon where the overall system performance is severely hindered by excessive amounts of processing time spent handling interrupts is called an interrupt storm.