Intel ARCHITECTURE IA-32 User Manual Page 330

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8-6 Vol. 3A
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
8.4.1 The Local APIC Block Diagram
Figure 8-4 gives a functional block diagram for the local APIC. Software interacts with the local
APIC by reading and writing its registers. APIC registers are memory-mapped to a 4-KByte
region of the processors physical address space with an initial starting address of FEE00000H.
For correct APIC operation, this address space must be mapped to an area of memory that has
been designated as strong uncacheable (UC). See Section 10.3, “Methods of Caching Available.”
In MP system configurations, the APIC registers for IA-32 processors on the system bus are
initially mapped to the same 4-KByte region of the physical address space. Software has the
option of changing initial mapping to a different 4-KByte region for all the local APICs or of
mapping the APIC registers for each local APIC to its own 4-KByte region. Section 8.4.5,
“Relocating the Local APIC Registers,” describes how to relocate the base address for APIC
registers.
NOTE
For P6 family, Pentium 4, and Intel Xeon processors, the APIC handles all
memory accesses to addresses within the 4-KByte APIC register space
internally and no external bus cycles are produced. For the Pentium
processors with an on-chip APIC, bus cycles are produced for accesses to the
APIC register space. Thus, for software intended to run on Pentium
processors, system software should explicitly not map the APIC register
space to regular system memory. Doing so can result in an invalid opcode
exception (#UD) being generated or unpredictable execution.
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