Technical data

PCI to ISA Bus Interrupt Mapping

The ISA bridge (Intel 82371SB) provides the sixteen conventional ISA interrupts, plus four interrupt

request pins for PCI peripheral interrupts (PIRQ0 through PIRQ3). For PC-AT architecture

compatibility reasons, the PCI interrupts are routed to the ISA interrupts within the ISA bridge. The

assertion of a PCI interrupt concludes in an ISA interrupt being asserted.

Bit 7 of each PIRQ registers enable (Low) or disable (High) the routing of the PIRQ to an ISA

interrupt. The lowest four bits (3:0) of each PIRQ register determines to which ISA interrupt the

PIRQ will be routed. The PIRQs can be mapped to the following ISA interrupts: IRQ 5, 9, 10, 11,

and 15.

Each PCI slot on the riser card has four available interrupt lines: INTA, INTB, INTC, and INTD.

These are connected to the PCI interrupts, PIRQ0 through PIRQ3, as shown below:

Interrupt Line

Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6

INTA PIRQ0 PIRQ1 PIRQ2 PIRQ3 PIRQ0 PIRQ1

INTB PIRQ1 PIRQ2 PIRQ3 PIRQ0 PIRQ1 PIRQ2

INTC PIRQ2 PIRQ3 PIRQ0 PIRQ1 PIRQ2 PIRQ3

INTD PIRQ3 PIRQ0 PIRQ1 PIRQ2 PIRQ3 PIRQ0

ISA Bus

The ISA slots on the MSMT463 riser card accommodate ISA based option boards and a 16-bit wide

expansion bus. The ISA bus interrupt (IRQ) assignments are defined below.

IRQ

Name IRQ Name

0 System Timer 8 Real Time Clock

1 Keyboard 9 H/W Monitor

2 Cascade input for IRQ8 - IRQ15 10 Spare

3 COM2, COM4 11 Spare

4 COM1, COM3 12 Mouse

5 Crystal CS4236B Controller 13 Floating Point Unit

6 Floppy Controller 14 EIDE CDROM

7 Parallel Port 15 MIDI Port

The spare interrupts listed above may be assigned to PCI and ISA devices. When you add a PCI

device to the system, the interrupt will be automatically assigned by the system BIOS. However, at

least one interrupt must be available for the PCI bus. For increased performance, one interrupt should

be left available for each PCI based controller used in the system. When you add a ISA board, you

must assign the interrupt using the System Configuration Utility and jumpers on the option board.

The Sound Controller can be configured for either IRQs 5, 7, 9, 11, 12, or 15. The MIDI Port can be

configured for either IRQs 9, 10, 11, 15, or no interrupt, in which case it will act as a write-only

device.