Hacker's Web Docs for the FreeBSD SMP kernel

The MP Specification:

The MP spec, v1.1
The MP spec, v1.4

Intel PCIsets-Datasheets on the WEB

The 82439HX System Controller (TXC) (1st half of 430HX, ie. Triton II)
The 82371SB (PIIX3) (2nd half of 430HX, ie. Triton II)
The 82371AB (PIIX4)
Intel 440FX PCIset (ie. Natoma)
Intel 450KX/GX

APIC & IO Chips:

The 82093AA I/O Advanced Programmable Interrupt Controller (IOAPIC)
The 82378ZB System I/O (SIO) and 82379AB System I/O APIC (SIO.A)
The 82489DX APIC datasheet is NOT available on WEB (look here for hardcopy)

Intel Errata

Intel Pentium errata
Intel Pentium Pro errata
Intel Pentium II errata

Pentium Pro Processor BIOS Writer's Guide V2.0

Multi-processor locking in 4.4BSD Lite2, by Thorsten Lockert (tholo@sigmasoft.com)

[missing in action] X86 Multiprocessing Basics , includes discussion of Open APIC (rumored to be dead, by the way...).