Intel® 64 Architecture x2APIC

The x2APIC architecture extends the xAPIC architecture that provides key mechanisms for interrupt delivery. This extension is primarily intended to increase processor addressability.

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 the performance of interrupt delivery
• Reduces the complexity of logical destination mode interrupt delivery on link based architectures

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

• Support for two modes of operation to provide backward compatibility and extensibility 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 the x2APIC mode, APIC registers are accessed through the 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 x2APIC mode. The Memory Mapped IO (MMIO) interface used by xAPIC is not supported in 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. To benefit from x2APIC capabilities, operating system support and a new BIOS are both needed, with special support for the x2APIC mode.
• The x2APIC architecture provides backward compatibility to the xAPIC architecture and forwards extensible for future Intel platform innovations.

For more information, refer to Intel® 64 Architecture x2APIC Specification at http://www.intel.com/products/processor/manuals/