Processors Toolkit

Intel overclocking guide

James McPherson Tech Republic

Published: 11 Feb 2003 14:25 GMT

Overclocking process
Essentially, overclocking Intel processors consists of increasing the FSB in the BIOS, booting the computer, and then testing for stability. You repeat the process until you identify the maximum stable speed. Changing the FSB is a relatively simple matter of entering the computer's BIOS setup screen, switching from automatic to manual configuration, and selecting the FSB speed you want. The crudest form of overclocking is available to Celerons; it consists of telling the motherboard the processor is really a Pentium 4 so it will use the 533-MHz bus, boosting the processor's speed by 33 percent. Most overclocking motherboards support altering the FSB in 5-MHz increments, while the best ones allow 1-MHz increments.

You'll need to check the PCI/AGP clock multiplier or clock speed (Table A). Many motherboards have the PCI speed locked to match the intended processor's FSB. Others have multiple PCI/AGP clock multipliers to select from. The best will feed the correct speed to the PCI and AGP devices automatically. You're trying to keep the PCI bus at 33 MHz and the AGP port at 66 MHz -- the farther you are from these speeds, the more rigorously you'll have to test your peripherals.

Table A

Component Bus Multipliers
	Core FSB
Bus Speeds	100 Mhz	133 Mhz
Memory*	x1	x1
PCI (33 Mhz)	x1/3	x1/4
AGP (66 Mhz)	x2/3	x1/2
*The memory bus has its own multipliers to the core FSB. DDR doubles the core FSB, while RDRAM quadruples it. Most motherboards will show the core FSB, but others will show the multiplied speed. DDR operates at speeds of 200-333 Mhz depending on grade, while RDRAM operates at 400 Mhz or 533 Mhz with a dual-channel 1066 Mhz speed.

Intel Celeron
The Celeron is Intel's processor for low-cost PCs. The majority of Celerons are based on Pentium II/Pentium III cores and run at speeds of 1.4 GHz or less. Overclocking these older processors is the same as a Pentium II/III. However, the Celeron now comes in a Pentium 4 flavor in speeds of 1.7 GHz and 1.8 GHz. Yes, Intel has given the Celeron the heart of its pride and joy, the Pentium 4. But like all Celerons, it has a congenital heart defect: a slow bus and smaller cache.

Celerons use a 400 MHz (4 x 100 MHz) bus and only come with 128 KB of cache. Celeron systems are equipped with PC1600 (2 x 100 MHz) DDR memory and, with the phase-out of the AMD Duron, are the only mass-market processors still using this slow speed. In some ways, this makes the Celeron easier to overclock, since faster PC2100 (2 x 133 MHz) DDR memory is readily available at no significant cost increase. However, if you overclock a Celeron to catch up to its Pentium 4 big brother, the reduced cache will still hamper its performance. In other words, don't purchase a Celeron with the intent to make it the fastest system on the block because it won't happen.

Intel keeps Celeron clock speeds several hundred megahertz behind the Pentium 4 to maintain the "premium" value of its flagship processor. This also lets Intel use older equipment to make Celeron processors. The up side is that we know the maximum accepted performance of this processor, giving us our maximum "safe" ceiling. The down side is that Pentium 4s rated at 2.0 GHz or faster no longer use the 0.18-micron process currently used to manufacture Celerons. In the case of 1.4-GHz Celerons, that's a 40-percent speed increase, while for the 1.8-GHz Celerons, it's a meager 10-percent boost.

The Celeron currently runs on 1.560 to 1.565v, consuming between 63 and 66 watts of power. Given the clock speed, you can see that this older processor is far more power hungry than the current 0.13-micron Pentium 4s. So, pay careful attention to Celeron cooling solutions and power supplies, since these budget systems often have budget components.

Intel Pentium 4
The latest Pentium 4 is based on a 478-pin socket, is equipped with a 512-KB cache, and operates on a 533-MHz (4 x 133MHz) bus. Any Pentium 4 that uses the 423-pin socket or has 256 KB of cache uses a 400-MHz (4 x 100 MHz) FSB. Pentium 4 systems can be equipped either with DDR or Rambus RDRAM. DDR is far cheaper and performs as well as standard RDRAM. High-end Pentium 4s will be equipped with 1066-MHz RDRAM that consists of two 533-MHz RIMM modules in a dual-channel configuration, outperforming DDR or single-channel RDRAM. Dual-channel DDR motherboards aren't yet available for Pentium 4s, but eventually will be, given their cost savings. Voltages for the 478-pin Pentium 4 range from 1.325v to 1.365v with 60 to 70 watts of power consumed.

Testing for stability
Stability testing is essential to keep problems from cropping up in the future. The exact tests you'll need to perform will vary, depending on your operating system and hardware. The goal is to apply a heavy workload to every aspect of your system to ensure that there are no hidden problems. Hardware testing is easy: Simply use every peripheral attached to the PC. Pay particular attention to CD-Rs and CD-RWs, since changes to the FSB can cause problems with drive controllers. USB and FireWire devices are tolerant of overclocking, but test any must-have device before clearing it for general use. For PCs with Microsoft operating systems, I recommend using a full system test suite, such as the freely available WinBench. You should test any important programs in a stressful way by loading the largest and most complex files you have available. Linux systems can compile software to test your new CPU speed. Games like Quake 3 and Unreal Tournament are also capable of uncovering many overclocking problems when put into demo mode and left running in loops. If you use a game to test stability, be sure to test the stability of the unmodified machine to establish a baseline. You should be able to run a game in a loop for at least two hours after a reboot without a problem.

WinBench, kernel compilation, and games are also capable of determining the performance increase of your system. WinBench generates a performance value for various aspects of your system, not all of which are affected by processor speed. The kernel compilation process reports the time required to complete; shorter times reflect increased performance. You can set games to report the frame rate, or the rate the system can update the screen. You'll need to run the test on the unmodified machine and record the results to make the comparisons.

The real reason manufacturers limit overclocking options
The idea of running a device beyond standard operating parameters may seem a bit dangerous. And, yes, to some extent there's a risk. However, you should realize that the same production line can create processors of different speeds on the same day. In many cases, the only difference is the stamp and multiplier lock on the chip.

From a financial standpoint, there's often little reason to limit the potential performance of CPUs at the production level. Higher-performing components command a greater price and theoretically don't cost any more to manufacture. All processors are tested to meet certain standards; CPUs that fail at a given rating are retested at a lower speed. While manufacturers don't detail their performance tests, they know it isn't wise to rate any device at the maximum capability it handled at manufacture in case it degrades over time and becomes a warranty liability.

It also makes sense for manufacturers to offer a wide range of CPU speeds. The low-end components are close to the nominal manufacturing cost, while performance parts can command as much as a 30-percent profit margin. It might seem contradictory to downgrade a processor and reduce the profit on that part, but if the sales channel becomes flooded with chips running at a particular speed, component prices drop across the board and reduce overall profit. Thus, due to profit maximizing and engineering safety margins, most processors can only be increased by at least one speed rating (between 66 and 133 MHz), assuming the motherboard supports speed changes that small.

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