Chapter 4 System Support
8259 Mode
The 8259 mode handles interrupts IRQ0-IRQ15 in the legacy (AT-system) method using 8259-
equivalent logic. Table 4-8 lists the standard source configuration for maskable interrupts and
their priorities in 8259 mode. If more than one interrupt is pending, the highest priority (lowest
number) is processed first.
Table 4-8. Maskable Interrupt Priorities and Assignments
Table 4-8.
Maskable Interrupt Priorities and Assignments
Priority Signal Label Source (Typical)
1 IRQ0 Interval timer 1, counter 0
2 IRQ1 Keyboard
3 IRQ8- Real-time clock
4 IRQ9 Unused
5 IRQ10 Unused
6 IRQ11 Unused
7 IRQ12 Mouse
8 IRQ13 Coprocessor (math)
9 IRQ14 IDE primary I/F
10 IRQ15 IDE secondary I/F
11 IRQ3 Serial port (COM2)
12 IRQ4 Serial port (COM1)
13 IRQ5 Unused
14 IRQ6 Diskette drive controller
15 IRQ7 Parallel port (LPT1)
-- IRQ2 NOT AVAILABLE (Cascade from interrupt controller 2)
APIC Mode
The Advanced Programmable Interrupt Controller (APIC) mode provides enhanced interrupt
processing with the following advantages:
♦ Eliminates the processor’s interrupt acknowledge cycle by using a separate (APIC) bus
♦ Programmable interrupt priority
♦ Additional interrupts (total of 24)
The APIC mode accommodates eight PCI interrupt signals (INTA-..INTH-) for use by PCI
devices. The PCI interrupts are evenly distributed to minimize latency and wired as follows:
ICH2
Int. Cntlr.
PCI
Slot 1
PCI
Slot 2
PCI
Slot 3
PCI
Slot 4
PCI
Slot 5
AGP
Slot
NIC
I/F [1]
USB
I/F #2
Direct AGP
Cntlr. [2]
INTA- INTA- INTD- INTC- INTB- INTD- -- -- -- INTA-
INTB- -- -- -- -- -- -- -- -- INTB-
INTC- INTB- INTA- INTD- INTC- INTA- INTA- -- -- --
INTD- -- -- -- -- -- INTB- -- -- --
INTE- -- -- -- -- --- -- INTA- -- --
INTF- INTC- INTB- INTA- INTD- INTB- -- -- -- --
INTG- INTD- INTC- INTB- INTA- INTC- -- -- -- --
INTH- -- -- -- -- -- -- -- INTC- --
NOTES:
Wired
to
[1] Connection internal to the ICH2. Will be reported by BIOS as using INTA but is NOT shared with
other functions using INTA.
[2] Internal graphics controller of the 82815 GMCH, internally wired.
Deskpro SDT and CMT systems only.
Compaq Deskpro and Evo Personal Computers
Featuring Intel Celeron and Pentium III Processors
Fifth Edition – March 2002
4-16
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