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I've just loaded Ubuntu 12.04 onto a brand box with an AMD 645 quad CPU with 8GB RAM. In the BIOS it says, the CPU is running at 3100MHz. However when i boot into Ubuntu 12.04 it says 800MHz?

I've checked using: cat /proc/cpuinfo command from the terminal and running System profiler and benchmark application. Both say 800Mhz at each core?

Any ideas or input would be much appreciated.

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It's power saving. Read this screenshot in detail: Should I be suing intel? :P – izx Jun 17 '12 at 15:13
@izx You may want to turn this into an answer... – Eliah Kagan Jun 17 '12 at 20:53

In fact, this happens to many users using latest AMD CPUs and Ubuntu. Modern CPUs try to consume less power when the full performance capability is not required. Ubuntu ships some drivers which can handle this ability and rise the performance if necessary.

Have a look at this forum thread if you want to know some details:

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I have checked into it and i concur, that seems the most likely answer to the question. Thanks for the clarification. – Ken Jun 23 '12 at 4:50
Thanks. I'd appreciate it if you could mark my reply as the correct one! – Valentin Jun 23 '12 at 19:00

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