An interrupt is usually defined as an event that alters the sequence of instructions executed by a processor. Such events correspond to electrical signals generated by hardware circuits both inside and outside the CPU chip.
Interrupts are often divided into synchronous and asynchronous interrupts:
• Synchronous interrupts are produced by the CPU control unit while executing instructions and are called synchronous because the control unit issues them only after terminating the execution of an instruction.
• Asynchronous interrupts are generated by other hardware devices at arbitrary times with respect to the CPU clock signals.
Intel microprocessor manuals designate synchronous and asynchronous interrupts as exceptions and interrupts, respectively. We'll adopt this classification, although we'll occasionally use the term "interrupt signal" to designate both types together (synchronous as well as asynchronous).
Interrupts are issued by interval timers and I/O devices; for instance, the arrival of a keystroke from a user sets off an interrupt. Exceptions, on the other hand, are caused either by programming errors or by anomalous conditions that must be handled by the kernel. In the first case, the kernel handles the exception by delivering to the current process one of the signals familiar to every Unix programmer. In the second case, the kernel performs all the steps needed to recover from the anomalous condition, such as a Page Fault or a request (via an int instruction) for a kernel service.
We start by describing in the next section the motivation for introducing such signals. We then show how the well-known IRQs (Interrupt ReQuests) issued by I/O devices give rise to interrupts, and we detail how 80 x 86 processors handle interrupts and exceptions at the hardware level. Then we illustrate, in Section 4.4, how Linux initializes all the data structures required by the Intel interrupt architecture. The remaining three sections describe how Linux handles interrupt signals at the software level.
One word of caution before moving on: in this chapter, we cover only "classic" interrupts common to all PCs; we do not cover the nonstandard interrupts of some architectures. For instance, laptops generate types of interrupts not discussed here.
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