The Undocumented PC: A Programmer's Guide to I/O, CPUs, and Fixed Memory Areas (2nd Edition)
"The ultimate source for low-level details on the PC."Mike Schmit, author of Pentium Processor Optimization Tools  "...a category-killer for PC-hardware junkies...hard-core  technical information." Circuit Cellar INK "...a bible for system-level programmers." PC Magazine