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10-26 Vol. 3
ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)
x2APIC After RESET
The valid transitions from the xAPIC mode state are:
• to the x2APIC mode by setting EXT to 1 (resulting EN=1, EXTD= 1). The physical
x2APIC ID (see Figure 10-6) is preserved across this transition and the logical
x2APIC ID (see Figure 10-21) is initialized by hardware during this transition as
documented in Section 10.7.2.4. The state of the extended fields in other APIC
registers, which was not initialized at RESET, is not architecturally defined across
this transition and system software should explicitly initialize those program
-
mable APIC registers.
• to the disabled state by setting EN to 0 (resulting EN=0, EXTD= 0).
The result of an INIT in the xAPIC state places the APIC in the state with EN= 1,
EXTD= 0. The state of the local APIC ID register is preserved (the 8-bit xAPIC ID is in
the upper 8 bits of the APIC ID register). All the other APIC registers are initialized as
a result of INIT.
A RESET in this state places the APIC in the state with EN= 1, EXTD= 0. The state of
the local APIC ID register is initialized as described in
Section 10.5.6.1. All the other
APIC registers are initialized described in Section 10.5.6.1.
Figure 10-11. Local x2APIC State Transitions with IA32_APIC_BASE, INIT, and RESET
xAPIC Mode
EN =1
Illegal
Transition
Init
EN=1, Extd=1
Extended
Invalid
State
Mode
Reset
Extd = 1
Illegal
Transition
EN = 0
EN = 0
Illegal
Transition
Extd = 0
Illegal
Transition
Extd = 0
EN=1, Extd=0
EN = 0
Extd = 1
Reset
Reset
Init
Init
Disabled
EN = 0
Extd = 0
Extd = 1
EN = 0