Interrupts
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An I/O bus is similar to bus between the CPU, mainboard control logic, and memory. Both types of bus structure have address wires, data wires, and a similar set of housekeeping wires. Both bus structures must determine if an operation refers to memory or an I/O address. Both must distinguish between 8-bit, 16-bit, and 32-bit operations. Both must be able to introduce "Wait States" to slow down the CPU when a device needs more time to complete an operation. The most important difference between the CPU-memory local bus and the I/O bus is the presence of Interrupt Request (IRQ) wires. The I/O bus has 15 separate IRQ wires. The CPU has only one interrupt pin. The chip set on the mainboard has to provide a translation between the two. Without interrupts, the CPU must start an operation to a device and then spin in a loop asking, "Is it done yet? Is it done yet? Is it done yet?" After a few hundred thousand tests, the device will signal that the operation is complete. Interrupts allow the CPU (particularly on a more advanced operating system like Windows 95, OS/2, or NT) to do some other work until the operation is complete. When a device generates an interrupt, the CPU hardware stops running an ordinary program and jumps to an interrupt handling routine in the Device Driver. The interrupt may signal that:
Any device on the I/O bus can request an interrupt by placing a signal on one of the 15 IRQ wires. If more than one IRQ signal is received at the same time, the chip set on the mainboard has to select the one with highest priority to process first. The CPU is interrupted (by sending a signal on its one wire) and the chip set then transfer the identity of the IRQ level to be processed. Each IRQ wire goes to every slot in the I/O bus. An adapter card is configured, physically with switches or logically with a utility) to use a specific IRQ value. The I/O bus on the first PC assumed that each device would have its own IRQ line, so the circuit to drive an interrupt request was made very simple. Unfortunately, when two adapter cards are incorrectly configured with the same IRQ value, and if both try to generate IRQ's at the same time, the result is to produce a short in the I/O bus. Usually there is no damage, but the effect can be to burn out either card or to trash the mainboard. Later bus architectures (MCA, EISA, PCI) use safer circuitry that allows two devices to share the same interrupt. When an interrupt is shared, the system responds to an interrupt by calling the device driver for each device associated with that IRQ. The drivers poll their respective adapter cards to determine if there is any pending activity which requires a response. There is a slight loss of efficiency when interrupts are shared, but not enough to cause worry. |
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