I've never underclocked a CPU, but might, toward the goal of a silent PC. Are unlocked processors better suited for this?

I also haven't overclocked a CPU in many years. With Windows 7, a Z77 chipset motherboard, and an unlocked CPU, can you over/underclock a CPU on the fly, without rebooting? What I'm thinking is overclock the CPU when I need the extra zoom to play a game or do video editing, underclock it for most everything else.

Hi, modern CPUs have idle and load clock speeds. If setup correctly I think you have the best of both worlds, for example my i7 2600k idles at 1.6GHz and is setup to over-clock up to 4.5GHz. Further more the Vcore is reduced at idle settings. If I set it to "AUTO" (stock speed) then it would climb (IIRC) between 3.8GHz with one loaded core and 3.5GHz with all cores loaded, this is Intels "Turbo Boost" in action, there are lots of CPU speeds/voltage that it does automatically switch between from 1.6GHz to the max the CPU allow for locked models or the max set for unlocked models. I think under Windows XP you need to set the power management mode to "laptop", Windows Vista/7 do it automatically. When over-clocking you can set fixed voltage this would give the same high load voltage even at idle so I use the "offset" mode, this increases the Vcore at all speeds, so idle does consume slightly more than at stock but it's not a bit issue, the CPU fans don't need to run at all at idle!
I think AMD had similar technology in their latest CPUs and I think Intel's Ivybridge can take this further than Sandybridge, ie more high clocks when temp / power consumption allow. If you're under-clocking an unlocked Intel CPU gains you nothing as the locked models are only locked at maximum, not minimum. If you have temp controlled fans I'm not sure why you'd want to under-clock a recent processor. Thinking about it I'm not sure the BIOS on my board allows setting a lower maximum, Windows power management does I think allow setting a max CPU state (in %) that may do it for you. The only reason I can see for this is if you have limited cooling capacity and want to limit the CPU power so it never ramps up. I would guess a low-end Sandy/Ivybridge CPU with a decent cooler will not need to ramp up. In fact I'll know soon as I've order parts to build a G840 based PC that's going to get the Freezer 13 cooler I have on it. I'm expecting idle 600rpm (92mm fan) to be enough even under load, and properly quiet too.

My rig idles really quietly, lots of fans but the idle very slowly. 2x140mm CPU fans @500rpm, 2x 92mm case fans @500 rpm, 2x 92 GPU fans, again 500~600rpm and 80 PSU fan also ~500rpm. Playing Batman Arkham City doesn't excite the fans much at all but Folding@home CPU & GPU does quite a lot. Not surprising as it's then nearly 300w DC beast

Does this help?
Seb

All the Intel CPUs, at least the ones I've had since Core2Duo, dynamically adjust multiplier, # cores and Vcc depending on load. My current build is based on an i5-3570k. at idle the the CPU runs at 1.6Ghz & 0.936Vcc. Under load it goes to 4.1Ghz & 1.15Vcc. (I always set my overclocks to only increase the speed without having to raise Vcc. It limits the OC to around 20%, but at no increase in idle power and one that corresponds to the % increase in clock speed. There's very little increase in heat or noise.)

The ability to adjust the OC settings on the fly is really only useful for finding the optimum OC settings and then set the BIOS to them.

People haven't answered the question but speaking as a laptop user who needs the feature, yes you can dynamically underclock... well, sort of anyway.
With ideal software support, you could dynamically set everything your way. But I don't know what software if any gives you full control over Ivy hardware. But you can get easily get most of the benefits of underclocking anyway.

There are several ways to do it.
Let's start with the most obvious: Windows 7 has a power management setting which sets a share of the CPU to idle. It's only a matter of finding a gadjet or a shortcut to adjust this setting without going into the relevant control panel item every time.
But you might prefer more sophisticated schemes. You can for instance launch idle threads with a higher priority than the task you want to put breaks on but with a lower priority than tasks such as your browser.
If you're not satisfied with what you can come up with, there are better places than SPCR to learn about Windows wizardry or software which can fine-tune Ivy hardware.

For the people who don't get it: sometimes you want to put a heavy load on your CPU without making a racket. Let's say you want to compress an archive in the background while you do other stuff for instance... typically you won't mind if it takes a long time. Some software has settings to limit the power it uses but some software will use all the CPU it can get.

Thanks folks. Yeah, I'm not sure the question was answered, but it's good advice and has given me a lot to think about.

I was focused on the underclocking thing and forgot the title question...

You can get the main benefits of underclocking with all CPUs. The overclockable CPUs are not better.

If you can justify paying for an overclockable CPU because you need the power, you could cripple it and make it nearly as quiet as less powerful CPUs when you want to. You woudn't need to reboot but it requires some amount of fiddling (whereas less powerful CPUs are quiet out of the box).

Here is an example for AMD configuration: