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2.5 Interrupts
2.5.1 Legacy PIC System 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 2-3 along with their functions. Table 2-4 on page 38 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 2-4 on page 38.
The interrupt hardware implementation on the V7768/V7769 is standard for
computers built around the PC 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 Programmable Interrupt Controller (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 2-1 on page 42.
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 2-3 Interrupt Line Assignments
IRQ
AT Function
NMI
Parity Errors
(Must be enabled in BIOS Setup)
0
System Timer
1
Keyboard
2
Duplexed to IRQ9
3
COM2
4
COM1
5
Unused
6
Floppy Controller
7
Unused
8
Real-Time Clock
9
Old IRQ2
10
Not Assigned
11
Not Assigned
12
Mouse
13
Math Coprocessor
14
AT Hard Drive
15
Flash Drive
Comments
Used by V7768/V7769 PCI bus Interface
Set by BIOS Setup
Set by BIOS Setup
SVGA or Network I/O
Determined by BIOS
Determined by BIOS
Standard Features 37
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