Chapter 7. Bottom Halves and Deferring Work
The previous chapter looked at interrupt handlers as the kernel mechanism for dealing with interrupts. Interrupt handlers are a usefulindeed, requiredpart of any operating system kernel. Because of various limitations, however, interrupt handlers can form only the first half of the interrupt processing solution. The limitations include the following:
It should now be obvious that interrupt handlers are only a piece of the whole solution to managing hardware interrupts. We certainly need a quick, asynchronous, simple handler for immediately responding to hardware and performing any time-critical actions. Interrupt handlers serve this function well; but other, less critical work should be deferred to a later point when interrupts are enabled.
Consequently, managing interrupts is divided into two parts, or halves. The first part, interrupt handlers (top halves), are executed by the kernel asynchronously in immediate response to a hardware interrupt, as discussed in the previous chapter. This chapter looks at the second part of the interrupt solution, bottom halves.