Cool\'n\'Quiet and Memory Frequencies for Athlon64 Explained
This article is an unofficial FAQ for some features of Athlon64 family: the Cool\'n\'Quiet and memory frequencies. I hope this article can help you to understand more about the architecture and the information about these two features.
Last updated: 3rd Jan 2008
1. What is Cool\'n\'Quiet (CnQ)?
According to AMD,
引用:
AMD Cool\'n\'Quiet technology controls your system\'s level of processor performance automatically, dynamically adjusting the operating frequency and voltage up to 30 times per second, according to the task at hand.
In short, CnQ is used to save energy by reducing frequency and voltage when the CPU load is low.
2. How can I enable CnQ?
The following 4 steps are ALL NEEDED to enable CnQ:
a. Get a CnQ-compatible motherboard (I can\'t see one without CnQ support now)
b. Get an Athlon64 / Sempron series CPU with stock frequency of 1.8GHz or above (Any of the CPUs sold by AMD now)
c. Enable the CnQ in the BIOS (pretty easy as most motherboards enable it by default)
d. Select "Minimal Power Management" scheme in the Power Options of the Control Panel (You need to do it yourself
3. What if I choose other schemes in the Power Options?
In my experience, for any scheme other than "Home / Office Desk" scheme, the CPU frequency is reduced at idle. But to place safe, it is recommended to choose "Minimal Power Management" scheme.
4. How does CnQ work?
AMD defines the "P-state" (Processor Performance State) as "valid combinations of processor voltage and
frequency". All the P-states are stored in the processor.
There are three types of P-states:
1. Minimum P-state (sometimes we call it idle frequency)
2. Intermediate P-state
3. Maximum P-state (the same as the stock frequency)
Since the idle frequency for Socket 939 and Socket AM2 CPUs is 1.0GHz, the intermediate frequencies of the CPUs start from 1.8GHz, and then 2.0GHz, until the stock frequency of the CPU. For revision G processors with half multiplier, the last transition is of only 100MHz.
Graphical representation: 1.0GHz <=> 1.8GHz <=> 2.0GHz ... <=> Stock frequency
The P-states of all the Athlon64x2 CPUs are listed in Table 1. And a screenshot of a CPU\'s P-states is captured as Screenshot 2.
In the other words, CnQ only works with CPUs with stock frequency of 1.8GHz or above.
5. How is the memory frequency changed with the processor frequency?
Since Athlon64 series CPUs have integrated memory controller, the memory frequency is determined by the CPU core speed. The memory frequency is determined by the "memory divisor", which is the minimum divisor of CPU core speed to let the memory to run at or below the rated speed.
For example, A64x2 3800+ (2.0GHz)
To run at DDR2-533 (266MHz),
The memory divisor is 8 (2GHz / 8 = 250MHz, DDR2-500),
but not 7 (2GHz / 7 = 285.7MHz, DDR2-571)
Another limitation of the memory divisor is the minimum value of the divisor is 5, meaning that for Socket AM2 CPUs, the memory will run at DDR2-400 (200MHz) when the processor is idle (1GHz).
You may wonder that there are "sweet spots" of CPU frequencies which the memory will run at its full speed.
Table 3 shows the memory frequencies vs CPU frequencies for socket AM2 CPUs.
For more advanced users who overclock, I have converted the table into Table 4, which shows the CPU multiplier vs memory divisors.
6. Should I disable CnQ when I overclock?
It is recommended to disable CnQ for overclocking. As the HyperTransport frequency is increased, the CPU frequencies for the idle and intermediate states are also increased, increasing the chance of instability of the CPU due to insufficient CPU voltage.
Pictures:
Table 1: P-states of Athlon64x2 CPUs
Screenshot 2: A CPU\'s P-states
Table 3: Memory frequency vs CPU frequency (Socket AM2 CPU)
Table 4: CPU multiplier vs memory divisor (Socket AM2 CPU)
Reference:
http://www.amd.com/us-en/assets/ ... allation_Guide3.pdf
http://www.amd.com/us-en/assets/ ... tech_docs/32559.pdf
http://www.amd.com/us-en/assets/ ... tech_docs/33954.pdf
