Specifications

44 158004.B00
4.5 INTERRUPTS
The Geode GX1 chip set contains the same interrupt controller circuit as is present
on all PC computers. This consists of two 8259 type interrupt circuits, each with eight
interrupt inputs. One 8259 is connected in cascade with the other, leaving 15
interrupts available.
Some of these 15 interrupts are used internally to the Geode GX1 chip set. Other
interrupts are connected to on-board peripherals (serial ports and disk controllers for
example). Further interrupts can come from the PCI bus. In the case of most of the
on-board interrupts, if the on-board peripheral is not programmed to generate
interrupts then the interrupt may be used on the PC/104 or PC/104-Plus buses.
4.5.1 On-Board and PC/104 Bus Interrupts
Table 9 shows how the interrupts are assigned. The sixteen entries in the table are
the “usual” PC interrupts. The next column is either marked “internal”, indicating that
the interrupt is routed entirely within the Geode GX1 chip set, or "on-board",
indicating that it is connected to an on-board TP400 peripheral but may be available
on the PC/104 or PC/104-Plus buses, or else "external", indicating the interrupt has
no on-board use. The next column indicates whether the Plug and play (PnP) BIOS is
aware of the interrupt requirement of the peripheral. The last column indicates
whether the IRQ pin is connected to the PC/104 bus connectors.
Note that this table mentions the on-board interrupt from the USB controller and the
SoundBlaster-compatible audio circuitry. These sub-systems, internal can to the
Geode chipset, can be set to a variety of interrupt levels, or disabled.
The SoundBlaster-compatible audio sub-system uses an interrupt (IRQ5 by default,
but alternatively IRQ9, IRQ5, IRQ7 or IRQ10). This interrupt can be changed, or the
SoundBlaster system can be disabled, which frees the interrupt for other purposes.
The USB controller uses an interrupt (normally IRQ11) but this interrupt can be
changed, or the USB controller can be disabled, which frees the interrupt for other
purposes. Similarly the Ethernet controller can be allocated a different interrupt.
The allocation of interrupts to PCI bus devices, which includes the USB and Ethernet
controllers, is discussed in section 4.5.2.