Pci To Isa Bus Interrupt Mapping; Isa Bus - Intergraph Rack-Mount Workstation/Server System Reference Manual

60

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
INTA
INTB
INTC
INTD

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
0 System Timer
1 Keyboard
2 Cascade input for IRQ8 - IRQ15
3 COM2, COM4
4 COM1, COM3
5 Crystal CS4236B Controller
6 Floppy Controller
7 Parallel 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.
Slot 1 Slot 2 Slot 3
PIRQ0 PIRQ1 PIRQ2
PIRQ1 PIRQ2 PIRQ3
PIRQ2 PIRQ3 PIRQ0
PIRQ3 PIRQ0 PIRQ1
Slot 4 Slot 5
PIRQ3 PIRQ0
PIRQ0 PIRQ1
PIRQ1 PIRQ2
PIRQ2 PIRQ3
IRQ Name
8 Real Time Clock
9 H/W Monitor
10 Spare
11 Spare
12 Mouse
13 Floating Point Unit
14 EIDE CDROM
15 MIDI Port
Slot 6
PIRQ1
PIRQ2
PIRQ3
PIRQ0