The APIC, the MPS and ACPI

When the BIOS boots the OS startup code into memory and gives control to it, the OS may be capable of utilizing multiple processors (in other words, it may be an MP-capable OS). An MP OS must have some way of determining the number and type of processors present in the system. Most MP-capable OSs presume that the BIOS has already built a resource table in memory that list each of the processors, their Local APICs, any IO APICs and the capabilities of each of the processors. The OS reads the entries to determine what it has to work with. In the PC-compatible world, some OSs expect to find a Multiprocessing (MP) Table in memory. The Intel® version 1.4 MP spec (sometimes abbreviated as MPS) defines the exact structure and use of this table. Other OSs (actually, the preponderance of MP OSs today) expect to find the ACPI Table in memory. The industry standard ACPI version 2.0 spec defines the exact structure and use of this table.

As mentioned in “Introduction to the BSP Selection Process” on page 1595, during system startup the program executing on the BSP (i.e., the system BIOS code) is responsible for building the Multiprocessing Table in memory and for making entries in the ACPI table in memory. More information can be found in “AP Detection and Configuration” on page 888.