11.4 Interrupts --Details
Serialized Interrupts
It was previously stated that there was a wire for each interrupt. But the serialized interrupt (or serial interrupt) is an exception. A single wire is used for all interrupt which are multiplexed on that wire. Each interrupt has a time slot on the interrupt line. It's used on the LPC bus and is also for the PCI bus, but it's seldom used for PCI ??
DMA
Before going into interrupt details, there is another way for some devices to initiate communication besides sending out an interrupt. This method is a DMA (Direct Memory Access) request to take control of the computer from the CPU for a limited amount of time. On the PCI bus, it uses no "resources". Not all devices are capable of doing DMA. See DMA Channels.
Soft interrupts
There's also another type of interrupt known as a "soft interrupt" which is not covered in this HOWTO and doesn't use any "resources". While a hardware interrupt is generated by hardware, a soft interrupt is initiated by software. There are a couple of ways to do this. One way is for software to tell the CPU to issue an interrupt (an interrupt instruction). Another way is for the software to send messages to other processes so as to interrupt them although it's not clear that this should be called an interrupt. The ksoftirq process, which you may find running on a Linux PC, is a program which does this kind of interrupt for dealing with device drivers. The device driver starts running due to a hardware interrupt but later on, software interrupts are used for the "bottom half" of the driver's interrupt service routine. Thus, the ksoftirq process is also known as "bottom-half". For more details see the kernel documentation.
Hardware interrupts
Interrupts convey a lot of information but only indirectly. The interrupt request signal (a voltage on a wire) just tells a chip called the interrupt controller that a certain device needs attention. The interrupt controller then signals the CPU. The CPU then interrupts whatever it was doing, finds the driver for this device and runs a part of it known as an "interrupt service routine" (or "interrupt handler"). This "routine" tries to find out what has happened and then deals with the problem. For example, bytes may need to be transferred from/to the device. This program (routine) can easily find out what has happened since the device has registers at addresses known to the driver software (provided the IRQ number and the I/O address of the device has been set correctly). These registers contain status information about the device . The software reads the contents of these registers and by inspecting the contents, finds out what happened and takes appropriate action.
Thus each device driver needs to know what interrupt number (IRQ) to listen to. On the PCI bus (and for some special cases on the ISA bus) it's possible for two (or more) devices to share the same IRQ number. Note that you can't share a PCI interrupt with an ISA interrupt (are there any exceptions ??). When a shared interrupt is issued, the CPU runs all interrupt service routines sequentially for all devices using that interrupt. The first thing such a service routine does is to check its device's registers to see if an interrupt actually happened for its device. If it finds that its device didn't issue an interrupt (a false alarm) then it likely will immediately exit and the next service routine begins for the second device which uses that same interrupt. It checks out the device like described above. This sequence is repeated until the device is found that actually issued the interrupt. All the interrupt routines for an interrupt are said to be constitute a chain. So the chain is traversed until a routine on the chain claims the interrupt by saying in effect: this interrupt was for me. After it handles the interrupt, the interrupt service routines further out on the chain don't run.
The putting of a voltage on the IRQ line is only a request that the CPU be interrupted so it can run a device driver. In almost all cases the CPU is interrupted per the
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