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that directly connects to the INT wire. If
the user presses the J key, the keyboard charges the INT wire. The CPU temporarily
stops running the browser (or whatever program is active) and runs the necessary BIOS
routine to query the keyboard.
This would be fine if the computer had only one device. As you know, however, PCs
have many devices, and almost all of them need to interrupt the CPU at some point.
So the PC needs some kind of traffic cop to act as an intermediary between all of the
devices and the CPU’s INT wire. This traffic-cop chip, called the I/O advanced programmable interrupt controller (IOAPIC), uses special interrupt wires that run to all devices on
the expansion bus (Figure 8-18).
The INT wire Figure 8-18
wires (IRQs) run
from the expansion bus to the
IOAPIC. NOTE IOAPIC functions are usually built into the Southbridge. Many
developers drop the I/O part and simply call them APICs. If a device wants to get the CPU’s attention, it lights the interrupt wires with a special
pattern of ones and zeroes just for that device. The IOAPIC then interrupts the CPU.
The CPU queries the IOAPIC to see which device interrupted, and then it begins to
communicate with the device over the address bus (Figure 8-19).
These unique patterns of ones and zeroes manifest themselves as something called
interrupt requests (IRQs). Before IOAPICs, IRQs were actual wires leading to the previous generation of traffic cops, called PICs. It’s easy to see if your system has a PIC or
an IOAPIC. Go into the Device Manager and select Interrupt request (IRQ) with the
resources set to show by type.
Figure 8-20 shows nearly a dozen IRQs, numbered 0 through 22, making this an
IOAPIC system. IRQ 9 is specialthis IRQ is assigned to the controller itself and is the
IOAPIC’s connection to the CPU. If you look closely, you’ll also notice that some IRQs
aren’t listed. These are unused or “open” IRQs. If you add another device to the system, ch08.indd 304 12/14/09 2:49:26 PM All-In-One / CompTIA Network+ All-in-One Exam Guid...
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This note was uploaded on 04/27/2010 for the course COMPTIA 1201 taught by Professor N/a during the Spring '10 term at Galveston College.
- Spring '10