195 lines
4.3 KiB
HTML
195 lines
4.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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>How do input devices and interrupts work?</TITLE
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><A
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NAME="devices"
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>6. How do input devices and interrupts work?</H1
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><P
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>Your keyboard is a very simple input device; simple because it
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generates small amounts of data very slowly (by a computer's standards).
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When you press or release a key, that event is signalled up the keyboard
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cable to raise a <I
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CLASS="firstterm"
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>hardware
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interrupt</I
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>.</P
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><P
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>It's the operating system's job to watch for such interrupts. For
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each possible kind of interrupt, there will be an <I
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CLASS="firstterm"
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>interrupt
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handler</I
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>, a part of the operating system that stashes
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away any data associated with them (like your keypress/keyrelease value)
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until it can be processed.</P
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><P
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>What the interrupt handler for your keyboard actually does is post the
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key value into a system area near the bottom of memory. There, it will
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be available for inspection when the operating system passes control to
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whichever program is currently supposed to be reading from the keyboard.</P
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><P
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>More complex input devices like disk or network cards work in a similar
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way. Earlier, I referred to a disk controller using the bus to signal that
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a disk request has been fulfilled. What actually happens is that the disk
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raises an interrupt. The disk interrupt handler then copies the retrieved
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data into memory, for later use by the program that made the request.</P
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><P
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>Every kind of interrupt has an associated <I
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CLASS="firstterm"
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>priority
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level</I
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>.
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Lower-priority interrupts (like keyboard events) have to wait on
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higher-priority interrupts (like clock ticks or disk events). Unix is
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designed to give high priority to the kinds of events that need to be
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processed rapidly in order to keep the machine's response smooth.</P
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><P
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>In your operating system's boot-time messages, you may see references
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to <I
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CLASS="firstterm"
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>IRQ</I
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>
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numbers. You may be aware that one of the common ways to misconfigure
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hardware is to have two different devices try to use the same IRQ, without
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understanding exactly why. </P
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><P
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>Here's the answer. IRQ is short for "Interrupt Request". The operating
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system needs to know at startup time which numbered interrupts each
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hardware device will use, so it can associate the proper handlers with each
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one. If two different devices try use the same IRQ, interrupts will
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sometimes get dispatched to the wrong handler. This will usually at least
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lock up the device, and can sometimes confuse the OS badly enough that it
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will flake out or crash.</P
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>What happens when you run programs
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after boot time?</TD
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>How does my computer do several things at once?</TD
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