Hardware Reference
In-Depth Information
• Your system might not boot or might not be able to run Windows until you reset the ACC setting
in your system BIOS to its default mode. More serious core stability problems would cause this
result.
• You could destroy your processor or motherboard. This would be the result if the core were
disabled because it contained a short.
To be able to try unlocking disabled cores, you need a motherboard that has an adjustable ACC
setting in the system BIOS (many motherboards using the AMD750 South Bridge [SB] feature this
option) and a motherboard BIOS that does not include AMD-provided microcode to prevent
unlocking via the ACC adjustment routine.
Although the AMD850 South Bridge used on the latest motherboards does not include an ACC option,
motherboard vendors such as ASUS and Gigabyte have added an ACC chip to their motherboards and
added core unlocking capabilities to their BIOS setups. MSI also offers unlocking but is using a
purely BIOS-based unlock routine for its motherboards with the SB850 processor.
If you do unlock an additional core or two, you might enjoy faster performance and better
multithreaded and multitasking operating for free, or you might find your system to be unstable or
unable to start. Before you assume that a system that runs with an unlocked core is truly stable, use
some of the tests I recommend in
Chapter 20
,
“
PC Diagnostics, Testing, and Maintenance
.
”
Note
The latest four-core and six-core FX processors from AMD cannot have their additional cores
unlocked because the leads have been cut during production.
Bus Speeds and Multipliers
Modern processors run at a multiple of the motherboard speed, and the selected multiple is usually
locked within the processor; therefore, all you can do to change speeds is change the processor bus
speed settings. The processor bus is also called the
CPU bus
,
FSB
, or
processor side bus
(PSB), all
of which are interchangeable terms.
For example, I built a system that uses an Intel Pentium 4 3.2E processor, which typically runs at
3,200MHz on an 800MHz CPU bus. Thus, the processor is locked to run at four times the speed of the
CPU bus. I was able to increase the CPU bus speed from 800MHz to 832MHz, which meant the
processor speed increased from 3,200MHz to 3,328MHz, which is 128MHz faster than the rating.
This took all of about 60 seconds to reboot, enter the BIOS Setup, make the changes in the menu,
save, and reboot again. This was only a 4% increase in overall speed, but it didn't cost a penny, and
testing proved that the system was just as stable as it was before.
Many motherboards allow changes in speed of up to 50% or more, but a processor rarely sustains
speeds that far above its rating without locking up or crashing. Also note that, by increasing the speed
of the processor bus, you may be increasing the speed of the memory bus, PCI bus, or PCI Express (or
AGP) bus by the same percentage. Therefore, if your memory is unstable at the higher speed, the
system will still crash, even though the processor might have been capable of sustaining it. The
lowest common denominator prevails, which means your system will run faster only if all the
components are up to the challenge.
Overclocking Pitfalls
