Intel SL22T Specifications download pdf (Page 7)

2-1

LOCAL X2APIC ARCHITECTURE

CHAPTER 2

LOCAL X2APIC ARCHITECTURE

2.1 X2APIC ENHANCEMENTS

The key enhancements provided by the x2APIC architecture over xAPIC are the

following:

• Support for two modes of operation to provide backward compatibility and exten-

sibility for future platform innovations:

— In xAPIC compatibility mode, APIC registers are accessed through memory

mapped interface to a 4K-Byte page, identical to the xAPIC architecture.

— In x2APIC mode, APIC registers are accessed through Model Specific Register

(MSR) interfaces. In this mode, the x2APIC architecture provides significantly

increased processor addressability and some enhancements on interrupt

delivery.

• Increased range of processor addressability in x2APIC mode:

— Physical xAPIC ID field increases from 8 bits to 32 bits, allowing for interrupt

processor addressability up to 4G-1 processors in physical destination mode.

A processor implementation of x2APIC architecture can support fewer than

32-bits in a software transparent fashion.

— Logical xAPIC ID field increases from 8 bits to 32 bits. The 32-bit logical

x2APIC ID is partitioned into two sub-fields: a 16-bit cluster ID and a 16-bit

logical ID within the cluster. Consequently, ((2^20) -16) processors can be

addressed in logical destination mode. Processor implementations can

support fewer than 16 bits in the cluster ID sub-field and logical ID sub-field

in a software agnostic fashion.

• More efficient MSR interface to access APIC registers.

— To enhance inter-processor and self directed interrupt delivery as well as the

ability to virtualize the local APIC, the APIC register set can be accessed only

through MSR based interfaces in the x2APIC mode. The Memory Mapped IO

(MMIO) interface used by xAPIC is not supported in the x2APIC mode.

• The semantics for accessing APIC registers have been revised to simplify the

programming of frequently-used APIC registers by system software. Specifically

the software semantics for using the Interrupt Command Register (ICR) and End

Of Interrupt (EOI) registers have been modified to allow for more efficient

delivery and dispatching of interrupts.

The x2APIC extensions are made available to system software by enabling the local

x2APIC unit in the "x2APIC" mode. The rest of this chapter provides details for

detecting, enabling and programming features of x2APIC.

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Intel SL22T Specifications Page 7