Interrupts
Interrupts allow hardware to communicate with the processor. For example, as you type, the keyboard controller (the hardware device that manages the keyboard) issues an electrical signal to the processor to alert the operating system to newly available key presses. These electrical signals are interrupts. The processor receives the interrupt and signals the operating system to allow the OS to respond to the new data. Hardware devices generate interrupts asynchronously with respect to the processor clockthey can occur at any time. Consequently, the kernel can be interrupted at any time to process interrupts.
An interrupt is physically produced by electronic signals originating from hardware devices and directed into input pins on an interrupt controller. The interrupt controller, in turn, sends a signal to the processor. The processor detects this signal and interrupts its current execution to handle the interrupt. The processor can then notify the operating system that an interrupt has occurred, and the operating system can handle the interrupt appropriately.
Different devices can be associated with unique interrupts by means of a unique value associated with each interrupt. This way, interrupts from the keyboard are distinct from interrupts from the hard drive. This enables the operating system to differentiate between interrupts and to know which hardware device caused which interrupt. In turn, the operating system can service each interrupt with a unique handler.
These interrupt values are often called interrupt request (IRQ) lines. Typically, they are given a numeric valuefor example, on a PC, IRQ zero is the timer interrupt and IRQ one is the keyboard interrupt. Not all interrupt numbers, however, are so rigidly defined. Interrupts associated with devices on the PCI bus, for example, generally can be dynamically assigned. Other non-PC architectures have similar dynamic assignments for interrupt values. The important notion is that a specific interrupt is associated with a specific device, and the kernel knows this. The hardware then issues interrupts to get the kernel's attention: Hey, I have new key presses waiting; read and process these bad boys!
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In OS texts, exceptions are often discussed at the same time as interrupts. Unlike interrupts, exceptions occur synchronously with respect to the processor clock. Indeed, they are often called synchronous interrupts . Exceptions are produced by the processor while executing instructions either in response to a programming error (for example, divide by zero) or abnormal conditions that must be handled by the kernel (for example, a page fault). Because many processor architectures handle exceptions in a similar manner to interrupts, the kernel infrastructure for handling the two is similar. Much of the discussion of interrupts (asynchronous interrupts generated by hardware) in this chapter also pertains to exceptions (synchronous interrupts generated by the processor itself).
You are already familiar with one exception: In the previous chapter, you saw how system calls on the x86 architecture are implemented by the issuance of a software interrupt, which traps into the kernel and causes execution of a special system call handler. Interrupts work in a similar way, you shall see, except hardwarenot softwareissues interrupts. |
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