Compaq 850 manual Apic Mode, Maskable Interrupt Priorities and Assignments

Chapter 4 System Support

8259 Mode

In 8259-Mode, interrupts IRQ0-IRQ15 are handled in the conventional (AT-system) method using logic that is the equivalent of two 8259 interrupt controllers. Table 4-7 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 4-7.Maskable Interrupt Priorities and Assignments

			Table 4-7.
		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	Microtower, game/MIDI port; desktop or minitower, 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 enhances interrupt-processing performance with the following advantages:

♦Eliminating the processor’s interrupt acknowledge cycle by using a separate APIC bus.

♦Programmable interrupt priority.

♦Additional interrupts (total of 24).

Eight PCI interrupts are available in APIC mode. The PCI interrupts are evenly distributed to minimize latency and are hard-wired as follows:

ICH2

IRQ Cntlr.

INTA-

INTB-

INTC-

INTD-

INTE-

INTF-

INTG-

INTH-

	PCI	PCI	PCI	PCI	PCI	AGP	USB	USB	Audio
	Slot 1	Slot 2	Slot 3	Slot 4	Slot 5	Slot	Cntlr. #1	Cntlr. #2	Cntlr.
	INTA-	INTD-	INTC-	INTB-	INTD-	--	--	--	--
	--	--	--	--	--	--	--	--	--
Wired	INTB-	INTA-	INTD-	INTC-	INTA-	INTA-	--	--	--
	--	--	--	--	--	INTB-	INTD-	--	--
to
	--	--	--	--	--	--	--	--	INTA-
	INTC-	INTB-	INTA-	INTD-	INTB-	--	--	--	--
	INTD-	INTC-	INTB-	INTA-	INTC-	--	--	--	--
	--	--	--	--	--	--	--	INTH-	--

4-16Compaq Deskpro EXS and Workstation 300 Personal Computers Featuring the Intel Pentium 4 Processor

First Edition - December 2000