Hardware Reference
In-Depth Information
Perhaps the most infamous of these bugs is the floating-point division math bug in the
early Pentium processors. This and a few other bugs are discussed in detail later in this
chapter.
Because the processor is the brain of a system, most systems don't function with a defect-
ive processor. If a system seems to have a dead motherboard, try replacing the processor
with one from a functioning motherboard that uses the same CPU chip. You might find
that the processor in the original board is the culprit. If the system continues to play dead,
however, the problem is elsewhere, most likely in the motherboard, memory, or power
supply. See the chapters that cover those parts of the system for more information on
troubleshooting those components. I must say that in all my years of troubleshooting and
repairing PCs, I have rarely encountered defective processors.
A few system problems are built in at the factory, although these bugs or design defects
are rare. By learning to recognize these problems, you can avoid unnecessary repairs or
replacements. Each processor section describes several known defects in that generation
of processors, such as the infamous floating-point error in the Pentium. For more inform-
ation on these bugs and defects, see the following sections, and check with the processor
manufacturer for updates.
Microcode and the Processor Update Feature
All processors can contain design defects or errors. Many times, you can avoid the effects of
any given bug by implementing hardware or software workarounds. Intel documents these
bugs and workarounds well for its processors in the processor Specification Update manual
that is available from Intel's website. Most of the other processor manufacturers also have
bulletins or tips on their websites listing any problems or special fixes or patches for their
chips.
Previously,theonlywaytofixaprocessorbugwastoworkarounditorreplacethechipwith
one that had the bug fixed. Starting with the Intel P6 and P7 family processors, including
the Pentium Pro through Pentium D and Core i7, many bugs in a processor's design can be
fixedbyalteringthemicrocodeintheprocessor.Microcodeisessentiallyasetofinstructions
and tables in the processor that control the way the processor operates. These processors in-
corporate a new feature called
reprogrammable microcode
, which enables certain types of
bugs to be worked around via microcode updates. The microcode updates reside in either
the motherboard ROM BIOS or Windows updates and are loaded into the processor by the
motherboard BIOS during the POST or by Windows during the boot process. Each time the
system is rebooted, the updated microcode is reloaded, ensuring that it will have the bug fix
installed anytime the system is operating.
The updated microcode for a given processor is provided by Intel to either the motherboard
manufacturers or to Microsoft so the code can be incorporated into the flash ROM BIOS for
the board, or directly into Windows via Windows Update. This is one reason it is important
to keep Windows up to date, as well as to install the most recent motherboard BIOS for your
