39
I/O Port Map
3
PCI-to-PCI Bridge
The VMICPCI-7611 uses the Intel 21154 PCI-to-PCI bridge to interface between the
primary PCI bus of unit and the CompactPCI (CPCI) bus. The CompactPCI bus
appears as a secondary PCI bus, and all devices in the seven peripheral slots of the
CPCI chassis are auto detected by the BIOS and respond to normal PCI accesses.
PC/AT Interrupts
In addition to an I/O port address, an I/O device has a separate hardware interrupt
line assignment. Assigned to each interrupt line is a corresponding interrupt vector in
the 256-vector interrupt table at $00000 to $003FF in memory. The 16 maskable
interrupts and the single Non-Maskable Interrupt (NMI) are listed in Table 3-2 along
with their functions. Table 3-3 on page 40 details the vectors in the interrupt vector
table. The interrupt number in HEX and decimal are also defined for real and
protected mode in Table 3-3 on page 40.
The interrupt hardware implementation on the VMICPCI-7611 is standard for
computers built around the PC/AT architecture, which evolved from the IBM
PC/XT. In the IBM PC/XT computers, only eight interrupt request lines exist,
numbered from IRQ0 to IRQ7 at the PIC. The IBM PC/AT computer added eight
more IRQx lines, numbered IRQ8 to IRQ15, by cascading a second slave PIC into the
original master PIC. IRQ2 at the master PIC was committed as the cascade input from
the slave PIC. This architecture is represented in Figure 3-1 on page 44.
To maintain backward compatibility with PC/XT systems, IBM chose to use the new
IRQ9 input on the slave PIC to operate as the old IRQ2 interrupt line on the PC/XT
Expansion Bus. Thus, in AT systems, the IRQ9 interrupt line connects to the old IRQ2
pin (pin B4) on the AT Expansion Bus (or ISA bus).
Table 3-2 PC/AT Hardware Interrupt Line Assignments
IRQ
AT FUNCTION
COMMENTS
NMI
Parity Errors
(Must be enabled in BIOS Setup)
Used by VMICPCI-7611
PCIbus Interface
0
System Timer
Set by BIOS Setup
1
Keyboard
Set by BIOS Setup
2
Duplexed to IRQ9
3
COM2/COM4
4
COM1/COM3
5
LPT2
6
Floppy Controller
7
LPT1
8
Real-Time Clock
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