I can see in ads that the pentium 4 processors for notebook , some end with the code -M, some without it. Those with -M is more expensive. Why? What's the difference?

They have the P4-M to be better suited for mobile needs. It is able to lower its mhz speed to decrease heat and lower battery requirements, etc. It's more about energy saving than anything else. Of course you can find more detail from Intel's site. ;)

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