Technical Reference Guide www.hp.com 4-11
System Support
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,
although it may be inhibited by hardware or software means external to the microprocessor.
Maskable Interrupts
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.
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