5. CPU Performance: Sisoft Sandra 2002 SP1

The Arithmetic Benchmark from Sisoft Sandra 2002 underlines the results of PC Mark 2002:

Again, the CPU only delivers 50 percent of its performance capacity in battery mode. Obviously, the CPU only operates internally at half-speed when running on the battery.

SmartStep's Low-Energy Setting: Clock Throttling

In principle, there are two methods of reducing a CPU's energy consumption. Either you can reduce the operating voltage or you can lower clock speed. Reducing the operating voltage is a whole lot more effective, because a CPU's power consumption is proportional to the square of its voltage. The clock speed is merely a linear value in its power consumption. Both methods are combined to save energy, for example, in the Enhanced Speedstep mechanism of the mobile Pentium 4 M processor.

The desktop variant of the P4 doesn't "recognize" Speedstep. Thankfully, the Advanced Power and Configuration Interface (ACPI) has another legal method of saving electricity up its sleeve. Within the system state "System working" (G0), there are altogether four processor modes (C0-C4), according to ACPI specs. In C0 status (normal operation), the processor executes commands. But within this status the processor can switch between two states, "Full Speed" and "Throttling." In "Full Speed" mode, the processor runs at full speed. In "Throttle" mode, the CPU maintains clock speed for a limited time only. The rest of the time, clock supply is prevented via the "Stopclock" signal. To lower clock speed by half using this method, for example, the chipset would send a "Stopclock" signal via the corresponding pin on the processor with a mark space ratio of 50 percent. That means that the processor only receives "its" speed half of the time. This behavior is controlled via the chipset. The disadvantage of this power-saving mechanism is that the user cannot influence it. In other words: in SmartStep, the processor runs only at half-speed in battery operation . Although that needn't be an issue when running simple office applications, anyone who uses the CPU primarily to run intensive applications - and wants to do this on the road - will be bitterly disappointed by the device's performance.