How to disable SpeedStep for an Intel Core i7 processor?

I'm taking performance measurements of several bits of code. How can I disable SpeedStep on my Core i7 CPU so that the CPU clock speed is constant? I'm not trying to overclock here, and since SpeedStep only throttles up when it's safe (given the CPU temperature) to do so, I expect turning off SpeedStep will throttle it down to the minimum. That's fine. I just need it constant.

Setting power options to High Performance does not disable SpeedStep.

I am using a Dell Studio 1558 if that's relevant.

This will vary from board to board, but there should be an explicit BIOS option for "Intel SpeedStep" or "EIST" that you can disable.

EDIT: Also, I believe you have it backwards. SpeedStep is responsible for downthrottling, not upthrottling; you'll run at the base clockspeed if you turn it off. Turbo Boost is what upthrottles (but I believe SpeedStep does have to be on for that to work).

DOUBLEEDIT: You shouldn't need to, but you can also try disabling C1E support.

As far as I understand, there should be a BIOS option that might or might not effect the clock throttling.

If the BIOS option doesn't exist and no BIOS firmware allows you to change this, it seems like a no-go...