Section 8: CPU and Device Power States

The Big Picture: Saving Power Inside the Machine

Sections 2 to 5 were about the whole PC going to sleep. But while you are working (still S0), a lot of power saving happens inside the machine, thousands of times a second. The CPU naps between tasks. It runs slower when the job is light. Each device powers down when idle.

In Chapter 4, Section 5.5, you met C-states and P-states in two lines. In Chapter 2, Section 10, you learned how frequency, voltage and power are tied together (higher speed costs far more power). Now we put names and numbers on it.

8.1 CPU C-States (Idle Power States)

What it is: C-states describe how deeply a CPU core is idling. When Windows has nothing for a core to do, the core does not just spin: it goes to sleep for a moment.

The rule: the deeper the C-state, the more power is saved and the longer it takes to wake. The CPU picks the depth based on how long it expects to stay idle. The exact list of states is different for every CPU model.

Power used            C0 ===== (running)
                      C1 ==
                      C3 =
                      C6 .          deeper = less power
                      C10 .

Wake delay            C0 none
                      C1 tiny
                      C6 longer     deeper = slower to wake

Analogy: A shop assistant with no customers can stand at the counter (C1), sit in the back room (C3), or go home and wait for a phone call (C6). Going home saves the most wages but takes longest to return.

How Windows helps: the Power Manager and the Windows timer (the system tick) try to batch work, so cores stay idle for longer stretches and can reach deep C-states. This is one reason a laptop sips power at idle.

8.2 CPU P-States (Performance / Frequency States)

What it is: P-states are the speed levels of a core while it is running (C0). Each level is a pair: a frequency and a voltage.

Why voltage matters: power rises roughly with frequency × voltage squared (Chapter 2, Section 10). Running at half speed can need much less voltage, so it can use much less than half the power. This is the reason P-states exist.

Idle PC (light work)          Busy PC (heavy work)
speed  800 MHz  low voltage   speed 4.5 GHz  higher voltage
      |                             |
      v                             v
   tiny power                   big power, more heat

Who chooses the P-state?

The minimum and maximum processor state settings in the power plan (Section 7) set those limits as percentages.

See it: open Task Manager → Performance → CPU (Chapter 11). The Speed number changes from moment to moment. At idle it may drop well below the base speed; under load it rises, and may go above the base speed when turbo is active.

Core parking (hedge). Windows can park cores, meaning it avoids scheduling work on them so they can sleep in deep C-states, then unparks them when load grows. How much parking happens depends on the CPU, the Windows version and the power plan.

8.3 Device Power States (D0 to D3)

What it is: Every device (not just the CPU) has its own power levels, called D-states ("Device" states). The Power Manager and the device's driver (Chapter 13) move each device between them.

D3 has two variants (hedge: the exact details depend on the bus):

Most devices only really use D0 and D3. D1 and D2 exist in the standard but are rare.

Two kinds of device power change:

8.4 Examples of Device Power Saving

8.5 Seeing a Device's Power Settings

Open Device Manager (Chapter 14) → double-click a device → Power Management tab (not every device has this tab):

Just look at them. Changing them can stop a device from working properly after sleep, so do not tick boxes here without a reason.

Summary