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10-16 Vol. 3
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
10.5 EXTENDED XAPIC (X2APIC)
The x2APIC architecture extends the xAPIC architecture (described in Section 9.4) in
a backward compatible manner and provides forward extendability for future Intel
platform innovations. Specifically, x2APIC
• Retains all key elements of compatibility to the xAPIC architecture:
— delivery modes,
— interrupt and processor priorities,
— interrupt sources,
— interrupt destination types;
• Provides extensions to scale processor addressability for both the logical and
physical destination modes;
• Adds new features to enhance performance of interrupt delivery;
• Reduces complexity of logical destination mode interrupt delivery on link based
platform architectures.
• Uses MSR programming interface to access APIC registers in x2APIC mode
instead of memory-mapped interfaces. Memory-mapped interface is supported
when operating in xAPIC mode.
10.5.1 DETECTING AND ENABLING x2APIC
Processor support for x2APIC mode can be detected by executing CPUID with EAX=1
and then checking ECX, bit 21 ECX. If CPUID.(EAX=1):ECX.21 is set , the processor
supports the x2APIC capability and can be placed into the x2APIC mode.
• System software can place the local APIC in the x2APIC mode by setting the
x2APIC mode enable bit (bit 10) in the IA32_APIC_BASE MSR at MSR address
01BH. The layout for the IA32_APIC_BASE MSR is shown in
Figure 10-8.
Figure 10-7. Local APIC Version Register
31
0
Reserved
2324 15
Version
Max. LVT
Value after reset: 000N 00VVH
V = Version, N = # of LVT entries minus 1
Entry
7
Address: FEE0 0030H
16 8
Reserved