Desktop Vs. Mobile CPU

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This one is hard to explain, because there are so many different mobile CPUs.

First, the Tjunction of the mobile CPUs is important. The Intel Mobile CPUs have a Tjunction of 100°C. When you compare it to the Tjunction of the Desktop CPU you might think, hey it's only a 20°C difference! But remember that the heat spreader under your keyboard and the fan has to get rid off that additional 20°C. This will make a hot notebook or at least a very loud one, because the fan will be running most of the time. You might have saved some bucks with the purchase, but now you've got the loudest notebook around.

Speedstep

The mobile Pentium III introduced 100MHz front side bus and Speedstep technology to the notebook market.

Speedstep technology automatically detects whether the mobile PC is using AC power or battery. When the notebook is using AC Power, the CPU will run at full speed.

In battery mode the CPU will reduce its clock speed and core voltage. By doing this the CPU will consume less power.

Speedstep
AC-Mode Battery-Mode Vcccore/V AC-Mode Vcccore/V Battery-Mode TDPmax/W AC-Mode TDPmax/W Battery-Mode
600 MHz 500 MHz 1,60 1,35 20,0 12,2
650 MHz 500 MHz 1,60 1,35 21,5 12,2
700 MHz 550 MHz 1,60 1,35 23,0 13,2
750 MHz 600 MHz 1,60 1,35 24,6 14,4
800 MHz 650 MHz 1,60 1,35 25,9 15,1
850 MHz 700 MHz 1,60 1,35 27,5 16,1

How does it work? Changing the voltage and the clock-multiplier usually requires a reboot with a regular CPU. The clock multiplier register is only being set during start-up of the processor. A Speedstep Controller at the mobile module or the Notebook Motherboard can detect the power mode change. The Operating System (Windows 98/ME/2000) defines the new state to the controller and puts the CPU into sleep. The Speedstep Controller adjusts the voltage regulator and changes the CPU multiplier. After those changes the Controller forces the CPU out of sleep. All of this takes less then 1ms.


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