The problem isn't putting parts of the CPU to sleep, though; it's waking the processor up quickly enough for the napping power capabilities to be useful. After all, if you had to wait a minute every time your laptop always took a minute to wake up after it went to sleep, you'd soon be tempted to throw it against the wall in frustration. Intel CPUs prior to Haswell have two main states: active and sleep. (The technological details are more complicated than that, but that's the general idea.) Over the years, Intel has steadily decreased the amount of time a sleeping CPU takes to wake up. The current Ivy Bridge processor takes several seconds to rouse itself from a deep state of slumber--but several seconds is still not quite "instant on."
Haswell's solution is to add a third power state--something Intel designers call "Active Idle" (aka "SOix"). Active Idle is an extremely low-power active state that uses one-twentieth the power that Ivy Bridge uses. In that state, the PC system considers itself awake, but the CPU is mostly asleep. This trick translates into wake times of no more than a few tenths of a second. A worst-case wake-up time of half a second is considerably better from the user's perspective than the several seconds that today's CPUs take to wake up. Haswell is almost always in this "instant resume" state while running. Much of the technology came directly from Intel's Atom processor power management scheme.
Intel adopted a few other tricks in building Haswell. The new CPU's sleep and Active Idle states are actually divided into multiple smaller states. Each mini-state (known as a "C-state") defines exactly what part of the CPU is turned off. The new C-states permit more-granular power management, which in turn yields longer battery life, since your CPU won't constantly be waking up parts of the CPU that it doesn't need in order to wake up some part of the CPU that it does need.
Intel also examined the way CPU power usage interacts with a system's display. LCD panels take a relatively long time to wake up in today's systems, so Haswell processors will include panel self-refresh. For example, if you're just sitting and staring at your screen, a Haswell CPU will go to sleep, with only a tiny part remaining awake to refresh the monitor. As soon as you move the mouse or press a key, however, the CPU will wake up. You won't notice the wake-up time involved, because the display never went to sleep.
Now that we've discussed how Intel achieved better power efficiency, let's look at the architectural enhancements in the Haswell CPU.
Next: Performance and power efficiency
Improved performance, better power efficiency