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Processor Speeds Demystified
by chris

"I like to build and upgrade my computers, but I am confused on upgrading processors. I bought AMD 1800+ and 2400+ processors and they don't run at the speed advertised. The 1800+ runs at 1150 GHz and the 2400+ shows on my computer on boot up as an 1800+. I have these computers running XP. Could you educate me as to what I am doing wrong?"
- PTerry.

Several things could impact what the system reports during startup (text-based Power-On-Self-Test or POST) and within Windows.

The first is based on the Front Side Bus (FSB) and/or CPU Clock frequency; these are usually the same, but not always. The specifics of how you set the CPU clock speed vary by system board manufacturer. Most have either a switch or a jumper setting on the system board to specify a 100MHz or 133MHz CPU Clock Speed. (High speed boards may still have a 100/133 jumper, but only use this setting when one of the slower speed CPUs is installed.) In addition to the CPU Clock Speed, a Core/Bus Ratio or "multiplier" value must be specified. Multiplying the CPU frequency times the Core/Bus Ratio yields the frequency that the CPU is running inside the chip. Current systems configure the multiplier either manually, in the BIOS Setup menus, or automatically, by testing the CPU. Some older systems had rows of jumpers or switches to set the multiplier. To further confuse things, the AMD model number does not directly reflect the actual speed at which the CPU runs, and what is displayed in Windows or during the POST process is usually a function of the BIOS of your computer.

Within the BIOS is a "look-up" table that contains information that will be displayed based on the test results received back from the CPU. If the BIOS of the computer does not have an entry for the CPU that you have installed, or it cannot determine the clock multiplier to implement, then the BIOS will usually pick the lowest speed settings for that class of CPU. The only reason for choosing a slower CPU setting is to prevent possible damage from over-clocking the CPU. Some of these tables only provide the brand name of the processor, and the operating frequency is determined from the clock frequency and multiplier values.

CPU Clock Multipliers for AMD Processors

FSB setting (MHz)
100 133 167 200 266
1100+ 950 9.5 7.1 5.7 4.8 3.6
1200+ 1000 10.0 7.5 6.0 5.0 3.8
1300+ 1100 11.0 8.3 6.6 5.5 4.1
1400+ 1200 12.0 9.0 7.2 6.0 4.5
1500+ 1300 13.0 9.8 7.8 6.5 4.9
1500+ 1333 13.3 10.0 8.0 6.7 5.0
1600+ 1400 14.0 10.5 8.4 7.0 5.3
1700+ 1467 14.7 11.0 8.8 7.3 5.5
1800+ 1500 15.0 11.3 9.0 7.5 5.6
1800+ 1533 15.3 11.5 9.2 7.7 5.8
1900+ 1600 16.0 12.0 9.6 8.0 6.0
2000+ 1667 16.7 12.5 10.0 8.3 6.3
2100+ 1733 17.3 13.0 10.4 8.7 6.5
2200+ 1800 18.0 13.5 10.8 9.0 6.8
2400+ 2000 20.0 15.0 12.0 10.0 7.5
2600+ 2083 20.8 15.7 12.5 10.4 7.8
2600+ 2133 21.3 16.0 12.8 10.7 8.0
2700+ 2167 21.7 16.3 13.0 10.8 8.1
2800+ 2250 22.5 16.9 13.5 11.3 8.5

If you are not getting the proper clock speed being reported, I would check several things. First, look for the CPU Frequency jumper/switch on the system board. Make sure that it is set properly for the CPU you have installed. If you have this switch set wrong, but the multiplier set correctly, the resulting Core frequency would be incorrect. If the clock speed is correct for this CPU, then look in the BIOS Setup menus for a CPU multiplier setting; on some systems this can be manually selected or set to "auto." The table to the right shows what the multiplier would have to be at the different CPU clock speeds. Clock speed multipliers are usually whole numbers or in increments of 0.5, so I grayed out the weird fractions. It is unlikely that boards supporting slower speed CPUs have high-clock speeds (i.e. low-speed 1.x GHz processors won't be supported in boards with a 200+MHz bus.)

If the system does not recognize the CPU type, but can support this processor (after all, you couldn't get the wrong speed if it didn't run), it probably means you need to update the system BIOS. This is a common problem where the system board is engineered to support a family of processors, but the high end ones were not available at the time of manufacture for testing or configuration. As long as the engineering requirements of the CPU still match the system specifications, then the BIOS look-up table probably needs the newer CPU configuration data to properly detect the processor. Check the system board manufacturer's web site for BIOS updates to correct this.

Model and Clock frequency data was taken from AMD White Paper at http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/20734.pdf

Running the CPU at clock speeds higher than it is rated might work and give you a small performance boost, but it also causes the CPU to run at a higher than normal temperature. Having the correct heat sink and conductive thermal compound between the CPU and heat sink is critical to avoiding catastrophic failure from overheating.

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