PC Power Management Guide

Discussion in 'Reviews & Articles' started by Dashken, Jul 8, 2007.

  1. Dashken

    Dashken Administrator!

    Have you ever wondered what all those C-state, P-state, S-state terms mean? Sure, you know they are related to power management in your PC, but what exactly do they do?

    The PC Power Management Guide is here to answer all your power management questions. If you want to know what those terms mean, this is the guide for you.


    Link : PC Power Management Guide
    Last edited: Jul 8, 2007
  2. belikethat

    belikethat Just Started

    wow pretty hardcode stuff
    nice guide
  3. charge-n-go

    charge-n-go Newbie

    You guys can shoot me questions here. I'll answer them whenever I am free.

    Dashken, is it possible to update my conclusion with this link?
    This is my main reference for this article:

    Thanks dude.
  4. neverthar

    neverthar Newbie

    Congrats on your first chargey. Damned good job at that too

    LIQNIT Newbie

    Very intresting reading.
    Congrats on your frst article .
  6. charge-n-go

    charge-n-go Newbie

    Thanks for the feedbacks guys :dance:
  7. jojo4u

    jojo4u Newbie

    I do not agree about your view of C1E. You only see in the light of one core C0 and on core C1. But does plain C1 really contradicts FID/VID mangement?
    I'd say it's an P-state management "light" and a subset of EIST. It's a binary P-state mangement with actions based on C-states whereas EIST actions are based on decisions of software driver. Indeed, EIST also works the same way when C1E is disabled in the BIOS.
    C1E must be seen independently of EIST and there are CPUs by Intel which only have C1E but no EIST[1].

    [1] http://processorfinder.intel.com/details.aspx?sSpec=SL9XL

    Btw, the forum activation mails get caught by SA because of a mailformed line(_ instead +):
    Date: Wed, 11 Jul 2007 02:00:22 _0800
  8. charge-n-go

    charge-n-go Newbie

    This is not my view, but specification says so. C1 does not mentioned anything about FID/VID, but C1E does mentioned.
    C1: execute HLT instruction and put the core into halt state.
    C1E: execute HLT instruction, put the core into halt state and transition the core into lowest P-state.

    No, P-state is a subset of C-state power management. It is an active power management, where C1 state is a processor sleep state power management. The main difference is, active state executes instructions while sleep states don't execute. Besides this, EIST is an Operating System PM decision.
    If u read the article carefully, I've included it into the diagram in conclusion page:
    C0 - active state, CPU is not sleeping. P-state PM resides here.
    C1 - sleep state (halt)
    C2 - sleep state (stop grant)
    C3 - sleep state (deep sleep)
    C4 - sleep state (deeper sleep) ......

    Yes, A processor can have C1E without EIST, or EIST without C1E. In C1E, only a portion of EIST logic is enabled - it will go to maximum and minimum P-state only. Let's say a CPU has P0, P1, P2 and P3, it will only transition between P0 and P3 if there is C1E but no EIST supported, and this only occur when the CPU is in C1 state.

    You are giving an example of single core processor. At 2.0GHz, the multiplier is 10x and during P-state, it only works at 2.0GHz even though the workload is very light, as it doesnt support EIST. However, when the CPU is doing nothing at all, HLT instruction will be executed automatically and go into C1E state, where the clock speed will go down to 1.2GHz once the C1 state is entered.
    Last edited: Jul 11, 2007
  9. The_YongGrand

    The_YongGrand Just Started

    Cool guide! Sorry, couldn't resist the pun... ;)

    Hey, which kind of state do the C'n'Q and EIST are in? Also, how am I going to enable the SpeedStep in Vista with a computer equipped with Pentium Dual-Core? :)
  10. charge-n-go

    charge-n-go Newbie

    Thanks dude.

    Alrite, here is your answer

    EIST -
    manages processor P-state in the C0 state only.

    C1E -
    manages processor P-state in C1 state, using the same logic circuit in EIST.

    Cool and Quiet -
    manages all the processor power management states, including P-states and C-states. So far it has up to C1E for desktop variant. I don't know how far the Turion's CnQ goes, but i guess it would be at least C2 and above.

    I am not sure about Vista, you can try the Windows XP method - change the power scheme to max battery saving.
  11. The_YongGrand

    The_YongGrand Just Started

    Hey charge-and-go,

    I happened to have the new Celeron 420 - the heatsink is really small, and it's just as cool as cucumber. The heat output - is absolutely minimal!! I touched the heatsink and it's just a little warm.

    I don't have any measuring devices for wattage levels of CPU, but I'm pretty sure that it's really 35W... :haha:
  12. Dashken

    Dashken Administrator!

    PC Power Management Guide Rev. 2.0

    Today, we revisit the PC Power Management Guide with some inside information about the AMD Quad Core (Barcelona) Power Management and also some detailed explaination on EIST and C1E.

    Have you ever wondered what all those C-state, P-state, S-state terms mean? Sure, you know they are related to power management in your PC, but what exactly do they do?

    The PC Power Management Guide is here to answer all your power management questions. If you want to know what those terms mean, this is the guide for you.

    Quote from the guide:-

    Link : PC Power Management Guide Rev. 2.0
  13. charge-n-go

    charge-n-go Newbie

    It is actually around 25 - 30W when full load and averaging about 15-20W :thumb:
    I am still waiting for my single core to arrive. Time unknown yet :p
  14. The_YongGrand

    The_YongGrand Just Started

    You are buying a low-wattage single core too? The Celeron 420 is the best one, for those budget mini-ITX replacements. :twisted:

    I happen to purchase another new Asus P5PE-VM and that will be paired with my Celeron 420. :haha:

    And I don't have to hunt around for 35W Semprons. I now have a new replacement.

    The another Intel board with onboard processor (don't know what model already, something ends with GLY) only drinks up 29W for the processor. :mrgreen:
  15. charge-n-go

    charge-n-go Newbie

    Yea, Sempron is really cheap, but the performance isn't good. From Tomshardware CPU Power chart, Sempron is godlike in terms of power consumption. Although AMD published 35W, but it will be way lesser, bcoz 35W is its super max load power which we cannot reach in real life.

    Yeap, I will be getting a 2.8GHz Celeron (Core 2 Solo?) 1MB version in a few weeks time. Most probably will pair up with Biostar T-force and a low power graphic card. Going to give this to my daddy to replace his old K7 Sempron :mrgreen:
  16. Chai

    Chai Administrator Staff Member

    Are you sure about EIST?

    From my experience, EIST manages the same way as C1E. Both will drop voltage and multiplier at 0% load. The only difference that I see is this:
    1. EIST requires OS/driver support unlike C1E.
    2. EIST drops the voltage of my processor to stock voltage, while C1E drops even more, down to 1.1V, causing stability issue, on my overclocked C2D that is.
    Last edited: Jul 31, 2007
  17. charge-n-go

    charge-n-go Newbie

    Yes I am absolutely sure about EIST.

    1. EIST (a.k.a. P-state) requires OS support, I mentioned in the guide.
    2. C1E uses EIST circuit to lower down the Vcore to 6x multiplier / 1.1V, because Intel defines C1E must use 6x multi with 1.1V (for certain stepping it is even lower). EIST is OS (Windows) controlled, I have no knowledge on how WinXP implements P-state.
  18. Chai

    Chai Administrator Staff Member

    I think you need to read what I've just said.

    This is from my personal experience, I can show screenshot as proof. C1E was disabled in the BIOS, only EIST. No power saving was active in Windows until I set the power scheme to Laptop. And both voltage and multiplier dropped. And EIST was only active at C1 state.
  19. charge-n-go

    charge-n-go Newbie

    If you are really interested to know how EIST work, I can start from the basic. First of all, EIST is controlled by writing some value to IA32_PERF_CTL register (MSR 0x199) as stated in Intel Manual Volume 3B Appendix B-55. Those values consist of clock ratio and VID that the CPU must run at. So what Windows XP does is to write this register when laptop power scheme is enabled. Microsoft just doesn't like the idea of having desktop PC to use EIST feature by default !!

    C1E is a HALT state with lowest P-state enabled. This is a hardware mechanism where the CPU will automatically write the IA32_PERF_CTL register with the lowest possible clock ratio and VID (6x and 1.1V) to force the CPU to the lowest possible clock speed. There is a dedicated piece of hardware (circuit) to read the value in IA32_PERF_CTL register, translate them into the correct voltage and PLL signals and transition the CPU to the specific state. That is why I said C1E and EIST are using the same circuit, and the circuit name is commonly referred as EIST (P-state) circuit because its main function is to control the clock ratio and VID.

    It's up to you to believe or not. I have real life of experience in the CPU power management hardware, I m very sure I don't provide misleading info here. :)
    Last edited: Aug 2, 2007
  20. Chai

    Chai Administrator Staff Member

    Actually, I agree with most of your info except this.

    "EIST -
    manages processor P-state in the C0 state only."

    Are you sure it's C0 state, not C1 state? Because everything you said is right, except that, from my experience anyway. I mean, I see no purpose in having EIST when it's in C0 state. While would you want to throttle the CPU when there's CPU usage?

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