Much of this stuff involves features that can give designers a look at how the chips are actually working. For example, the model-specific registers (MSRs) provide a window into a vast array of hidden microprocessor features. They include a set of performance-monitoring registers that tally data on 40 aspects of chip operation, including cache hit rates, memory accesses and processor pipeline operation-information that can be crucial for developers attempting to optimize their systems or applications software.

However, until a hacker reverse-engineered the function of the MSRs and posted that information on the Internet in mid-1995, their operation remained a mystery. Only selected developers under tight non-disclosure restrictions from Intel were granted access to "Appendix H," the closely held document that detailed the MSRs.

Following the hacker's posting, Intel made A ppendix H public. To access the info, which today appears in a different document, download Intel's 1997 Pentium Processor Family Developer's Manual, and look at Chapter 16: "Model Specific Registers and Functions."

The latest chunk of inside CPU information that's come to light concerns Intel's bug-busting "BIOS Update Feature." The capability, which is hidden in the Pentium MMX, Pentium II and Pentium Pro, allows the chip's microcode to be patched, effectively providing a software workaround for certain classes of bugs.

Despite such disclosures, Intel continues to hold back some tasty information on CPU internals. Most likely, the company is seeking to protect its trade secrets and to make life tougher for X86 cloners like AMD and Cyrix.

For engineers, however, this can sometimes become a roadblock to a successful design. An undocumented feature can provide the missin g link that's needed to get that peripheral chip, subsystem, or piece of software up and running. Indeed, since there are so many ambiguities inherent in the kludge we call the IBM PC architecture, any added information can only help engineers work their way around the constant conflicts that crop up.

But most of all, there's something just plain fun about being able to uncover information that few living souls--unless they're in the employ of Intel--have a line on.

Fortunately, some help is available. While Intel tries to keep its hatches tightly battened, there's a community of engineers and Internet rats that aims to keep the company on its toes.

Some of these experts have found that there's information enough to fill a book--and that's just what they've done. I have three particular favorites, which belong in the reference library of every X86 addict.

"The Indispensable PC Hardware Book," by Hans-Peter Messmer (Addison-Wesle y) is a treasure-trove of information on how and why PCs do the inelegant things they do.

"The Undocumented PC: A Programmer's Guide to I/O, CPUs and Fixed Memory Areas," by Frank van Gilluwe (Addison-Wesley Developers Press) is a much drier (it reads a lot like documentation!), but no less valuable piece of work. If you're stumped by an undocumented instruction or an interrupt question, this is the place to go.

For sheer page-turning pleasure, my favorite is "Upgrading and Repairing PCs," by Scott Mueller (Que) . Now in the sixth edition, the book is particularly strong on hard-disk operation, formats and interfaces--areas not covered as accessibly in the other books.

But be forewarned: all three of these tomes exceed 1,000 pages in length and would make truly gut-crushing beach-reading. Better to keep them in the lab, c lose to the floor, so they can't fall off a shelf and hurt anybody.

Along with books, there are several "renegade" Web sites that traffic in undocumented information. According to the grapevine, Intel isn't too happy with these sites.

The granddaddy of the undocumented Web sites is Robert Collins's "Intel Secrets" site. Christian Ludloff, the former East German who uncovered the workings of the Appendix H MSRs back in 1995, runs the "Sandpile" X86 site.

If you're grappling with a tough technical issue, it might pay to go right to the horse's mouth. The "Intel newsgroup forums" on the company's Web site provide bulletin boards for anyone to post comments and queries. Like Ann Landers, Intel won't promise to answer every question. However, technical-support personnel monitor the site and they seem to be willing to serve up short replies to serious questions.

What's your take on the X86 "undocumented" debate? Come on over to our Wintel Watch Forum and let's talk microprocessor innards.

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