Hardware Reference
In-Depth Information
and four PCI cards installed, each using PCI interrupt INTA#. In most cases, each of these cards
would be mapped to a different available ISA interrupt request, such as IRQ9, IRQ10, IRQ11, or
IRQ5.
Finding unique IRQs for each device on both the ISA and PCI buses has always been a problem; there
simply aren't enough free ones to go around. Setting two ISA devices to the same IRQ has never been
possible (the so-called
IRQ sharing
of IRQ4 by COM1/3 and IRQ3 by COM2/4 didn't allow both
COM ports to work at the same time), but on most newer systems sharing IRQs between multiple PCI
devices might be possible. Newer system BIOSs as well as plug-and-play OSs support a function
known as
PCI IRQ Steering
. For this to work, both your system BIOS and OS must support IRQ
Steering. Older system BIOSs and Windows 95 or 95A and earlier do not have support for PCI IRQ
Steering.
Advanced Programmable Interrupt Controller
As a replacement for the traditional pair of 8259 interrupt controllers, Intel developed the Advanced
Programmable Interrupt Controller (APIC) in the mid-1990s. Although all processors since the
original Pentium contain an APIC, an APIC must also be present in the motherboard's chipset, and the
BIOS and OS must support APIC. APIC support is present on most recent motherboards and has been
supported in all Windows releases since Windows 2000. In some systems you can enable or disable
APIC support in the system BIOS, but in most it is permanently enabled.
APIC provides support for multiple processors, but it is also used on single-processor computers.
The major benefit of APIC for a single processor is support for virtual PCI IRQs above 15. Most
APIC implementations support virtual IRQs up to at least 24, and some new systems can go higher,
and might even assign IRQs to negative numbers (see
Figure 4.48
). Although Windows 2000 tends to
place PCI IRQs into the traditional ISA range of 0-15, even when APIC is enabled, Windows XP and
later make full use of APIC services when installed on a system with APIC enabled. With Windows
XP and later, APIC limits IRQ sharing to enable devices to perform better with fewer conflicts. The
traditional ISA IRQs 0-15 on the system shown in
Figure 4.48
are used only for ISA devices, thus
preventing ISA-PCI IRQ conflicts.
