System Resources; Interrupts - HP Compaq dc7900 USDT Technical Reference Manual

4.3 System Resources

This section describes the availability and basic control of major subsystems, otherwise known as
resource allocation or simply "system resources." System resources are provided on a priority
basis through hardware interrupts and DMA requests and grants.

4.3.1 Interrupts

The microprocessor uses two types of hardware interrupts; maskable and nonmaskable. A
maskable interrupt can be enabled or disabled within the microprocessor by the use of the STI
and CLI instructions. A nonmaskable interrupt cannot be masked off within the microprocessor,
but may be inhibited by legacy hardware or software means external to the microprocessor.
The maskable interrupt is a hardware-generated signal used by peripheral functions within the
system to get the attention of the microprocessor. Peripheral functions produce a unique INTA-H
(PCI) or IRQ0-15 (ISA) signal that is routed to interrupt processing logic that asserts the
interrupt (INTR-) input to the microprocessor. The microprocessor halts execution to determine
the source of the interrupt and then services the peripheral as appropriate.
Most IRQs are routed through the I/O controller of the super I/O component, which provides the
serializing function. A serialized interrupt stream is then routed to the ICH component.
Interrupts may be processed in one of two modes (selectable through the F10 Setup utility):
■
8259 mode
■
APIC mode
These modes are described in the following subsections.
8259 Mode
The 8259 mode handles interrupts IRQ0-IRQ15 in the legacy (AT-system) method using
8259-equivalent logic. If more than one interrupt is pending, the highest priority (lowest number)
is processed first.
APIC Mode
The Advanced Programmable Interrupt Controller (APIC) mode provides enhanced interrupt
processing with the following advantages:
■
Eliminates the processor's interrupt acknowledge cycle by using a separate (APIC) bus
■
Programmable interrupt priority
■
Additional interrupts (total of 24)
The APIC mode accommodates eight PCI interrupt signals (PIRQA-..PIRQH-) for use by PCI
devices. The PCI interrupts are evenly distributed to minimize latency and wired as shown in
Table 4-5.
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