Hardware Reference
In-Depth Information
on a system with APIC enabled. With Windows XP and later, APIC limits IRQ sharing to
enable devices to perform better with fewer conflicts. For example, on one typical system
with APIC enabled, PCI IRQs are assigned as follows:
PCI IRQ 16 —Onboard audio/AGP graphics (shared)
PCI IRQ 17 —Add-on card USB 1.1 controller (nonshared)
PCI IRQ 18 —Add-on card USB 1.1 controller (nonshared)
PCI IRQ 19 —10/100 Ethernet adapter/add-on card USB 2.0 controller (shared)
PCI IRQ 21 —Onboard USB 1.1 controllers (3)/onboard USB 2.0 controller (shared)
The traditional ISA IRQs 0-15 on the sample system are used only for ISA devices, thus
preventing ISA-PCI IRQ conflicts.
Note
APIC must be enabled in the system BIOS before you can install x64 editions of Windows.
IRQ Conflicts
If you manage systems that use ISA slots, you might have the potential for conflicts
between the IRQ requirements of add-on cards and the requirements for built-in devices.
To learn how to deal with common sources of IRQ conflicts and to free up the maximum
number of IRQs, see the section see IRQ Conflicts ” in Chapter 4 , “ Motherboards and
Buses , ” in Upgrading and Repairing PCs, 19 th edition , available in its entirety on the
DVD packaged with this topic.
DMA Channels
Communications devices that must send and receive information at high speeds use direct
memory access (DMA) channels. Conflicts between devices needing to use the same
DMA channel were common on systems using ISA slots, but are no longer an issue on
modern systems.
To learn more about DMA channels and possible conflicts on older systems, see “ DMA
Channels in Chapter 4 , “ Motherboards and Buses , ” in Upgrading and Repairing PCs,
19 th edition , available in its entirety on the DVD packaged with this topic.
I/O Port Addresses
Your computer's I/O ports enable communications between devices and software in your
system. They are equivalent to two-way radio channels. If you want to talk to your serial
port, you need to know on which I/O port (radio channel) it is listening. Similarly, if you
 
 
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