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Chip 101
Lesson 17 of 36 ยท How chips save power

What are P-states and boost?

P-states are a CPU's speed settings. Boost runs it above its base speed while power, current and temperature stay within limits.

  1. P0P1P2P3fastestuses least power

    1. Numbered speed settings

    P-states (performance states) are a CPU's speed settings, each with its own voltage. P0 is the fastest; higher numbers are slower and use less power.

  2. fast, please3.4 GHzoperatingsystemhardwarehintaskspicks

    2. Software asks, hardware picks

    Newer CPUs use CPPC (collaborative processor performance control) instead of a few fixed P-states. The operating system asks for one of many levels, and the hardware picks the exact speed.

  3. save batterymore speedeco modesport modeset by EPP

    3. Choose speed or battery

    EPP (energy-performance preference) tells the chip which matters more: speed or battery life. Choosing battery works like a car's eco mode: it avoids the top speeds, which cost the most energy for each job.

  4. speedbase speedboostlimit reachedtime

    4. Boost, until a limit

    Boost runs cores above their base speed, up to a top speed, while power, current and temperature stay within limits. When a limit is reached, the speed drops.

  5. importantjobcorepreferred core

    5. Some cores are faster

    Some cores can reach higher speeds than others. The operating system sends the most important work to these preferred cores.

Words to know

P-state
A speed and voltage setting of the CPU. P stands for performance.
CPPC
A newer way to set speed: the operating system asks, and the hardware picks.
EPP
Energy-performance preference: a setting that says whether speed or battery life matters more.
Base speed
The speed a CPU is built to hold all the time with normal cooling.
Boost
Running above base speed while power, current and temperature stay within limits.
Current
The flow of electricity, like water flowing through a pipe.
Core
One of several workers inside a CPU. Each core can run at its own speed.
Preferred core
A core that can reach a higher speed than the others.

Where you meet it at work

Tests check that cores reach their expected boost speed, slow down at each limit, and follow the power mode the operating system sets.

Check yourself

When does boost stop raising the speed?Think first, then tap to see the answer.

At the chip's top boost speed, or when power, current or temperature reaches its limit.

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