FSB overclock calculator: core clock, bus rating and memory speed

On an LGA 775 board the front-side bus (FSB) is the clock that the CPU, the northbridge and, through a ratio, the memory all share. Raise it and everything moves together. This FSB overclock calculator takes four inputs and returns six results, so you can see where a target lands before you open the BIOS. It does arithmetic only. Whether your board holds that bus is a separate question, and the answer comes from testing.

The four inputs

• FSB clock (MHz): the base clock you set in the BIOS, such as 333 or 400. Not the MT/s figure on the box.
• Multiplier: core cycles per bus cycle. Harpertown Xeons cannot go above their stock multiplier (9 on the E5450, 9.5 on the X5460), so the bus is the only way up. Half steps are accepted.
• Stock FSB clock (MHz): the chip's specified bus clock, used for the clock at stock FSB and the percentage change.
• FSB:DRAM ratio: the memory divider as BIOSes print it. The list offers 3:2, 1:1, 5:6, 4:5, 2:3, 3:5 and 1:2.

Results: core clock in GHz (large) and MHz, bus rating, memory clock, DDR rating, clock at stock FSB and the change against it.

Bus clock versus MT/s

Intel rates the bus in MT/s, million transfers per second. The data lines are quad-pumped, moving four transfers per clock tick, so a 333.33 MHz clock is sold as 1333 MT/s and a 400 MHz clock as 1600 MT/s. The calculator multiplies your FSB clock by four and rounds to a whole number. Type 333 and the bus rating reads 1332 MT/s; type 333.33 and it reads 1333. Same bus, different rounding, which is why the field hint gives 333.33 as the exact figure.

Ratios and the DDR rating

The first number of a ratio is the FSB side, the second the memory side, so the memory clock is the FSB clock times the second number divided by the first. DDR memory moves two transfers per clock, which makes the DDR rating twice the memory clock. The tool prints it as DDR-800 and so on, without the generation, because DDR2-800 and DDR3-800 are the same speed on different modules.

At a 400 MHz FSB the seven ratios work out like this:

Your board will not offer all of them. The ratios on the menu depend on the chipset, the memory type and often the northbridge strap, and some BIOSes show the resulting memory speed instead of a ratio. Aim for a DDR rating at or just under your modules' label.

Worked example: the default settings

The calculator opens at an FSB of 400 MHz, a multiplier of 9, a stock FSB of 333 MHz and a 1:1 ratio. That is an E5450 pushed from its stock 3.0 GHz:

Core clock     400 x 9                = 3600 MHz  (3.60 GHz)
Bus rating     400 x 4                = 1600 MT/s
Memory clock   400 x 1 / 1            = 400 MHz
DDR rating     400 x 2                = DDR-800
At stock FSB   333 x 9                = 2997 MHz  (3.00 GHz)
Change         (3600 - 2997) / 2997   = +20.1%

The big figure reads 3.60 GHz, with 3600 MHz, 1600 MT/s, 400 MHz, DDR-800, 3.00 GHz and +20.1% listed beneath it. Intel's real stock bus is 333.33 MHz, which makes the clock at stock FSB 3000 MHz rather than 2997. Type 333.33 in the stock field and the change reads +20%. The gap is rounding, not a different chip.

In plain terms: the CPU gains a fifth of its clock, DDR2-800 modules run at their label speed, and the bus sits at 1600 MT/s. That last number deserves respect. Intel rates the P45, P43, G45 and G41 for 800, 1066 and 1333 MT/s, and among the Intel desktop chipsets for these chips only the X48 is rated for 1600. On most boards a 400 MHz FSB asks the northbridge for a speed it was never specified to run.

Set the multiplier to 9.5 and the same 400 MHz bus gives an X5460 3.80 GHz; with a stock FSB of 333.33 the change is again +20%.

What the arithmetic leaves out

• Northbridge straps. Many P35 and P45 boards have an FSB strap setting that selects internal northbridge timings and changes which memory ratios appear. Intel does not document straps for end users; owners commonly report that a higher strap holds a faster bus at some cost in memory latency.
• VTT and other voltages. FSB termination voltage (VTT), northbridge voltage and core voltage decide whether a bus speed holds. The calculator suggests no voltages.
• Memory limits. A DDR rating on screen is not one your modules or chipset can necessarily run. Check the module label and what your chipset supports; our DDR2 vs DDR3 guide for LGA 775 covers the chipset side.
• Stability. A bus that boots is not a bus that holds. Long stress tests, a memory test and a watch on temperatures decide it, and the heat from a higher clock and voltage is what the cooler TDP headroom check estimates.

Overclocking a Xeon on an LGA 775 board walks through straps, voltages, a careful climb and how to prove the result is stable.

Questions

Why does 333 give 1332 MT/s instead of 1333?

Intel's 1333 MT/s bus runs at 333.33 MHz, a third of a megahertz above 333. The calculator multiplies exactly what you type by four, so 333 x 4 is 1332. Type 333.33 and you get 1333 MT/s; 266.67 likewise gives 1067, the bus Intel sells as 1066 MT/s.

Why does the clock at stock FSB change when I change the multiplier?

That line is the stock FSB times the multiplier you entered, so the change measures the bus overclock alone. If you lower the multiplier to test a faster bus, the comparison is with the stock bus at that lower multiplier, not with the chip's rated clock. For an E5450 at 450 x 7, the tool shows 3.15 GHz against 2.33 GHz, a change of +35.1%; against the real 3.0 GHz it is 5 percent.

Which ratio should I start with?

Whichever puts the DDR rating at or just below your modules' label at the target FSB. For DDR2-800 at a 400 MHz bus that is 1:1, and for DDR3-1600 it is 1:2. Running memory under its rating while you find the CPU's limit keeps one variable out of the way.

Does it work for Core 2 and Pentium 4 chips?

Yes. Every Intel chip on a quad-pumped FSB, from the Pentium 4 through Core 2 to these Xeons, follows the same sums. Enter the multiplier your chip actually runs at.