The X300’s L7100 processor
Let’s start with something interesting: the X300’s processor. Lenovo used a Core 2 Duo L7100 processor. This device, which could not be located on Intel’s Website, is something out of the normal fray.
A Core 2 Duo SFF
Like MacBook Air’s processor, the L7100 uses a SFF package (Small Form Factor). The advantage is simple: the processor takes up less space on the motherboard. It is soldered in place and can’t be removed or changed. Its frequency is low (1.2 GHz) and its listed TDP is just 12 W.
The Clock Speed Issue
The L7100 has a 1.2 GHz clock speed, with an 800 MHz front-side bus (FSB). However, according the results of our first tests, CPU-Z sent us back a different reading for the clock speed: 1.6 GHz (8x 200 MHz). After doing some research, and with France Delattre’s help, we uncovered the reason for this frequency change: Intel’s Dynamic FSB feature.
In reality, the processor runs at 1.2 GHz (6x200 MHz) when plugged in, but once it goes into power economy mode on a battery charge, the Dynamic FSB technology kicks in. Here lies the source of the problem: to reduce power consumption, Intel designed the processor to function like an 800 MHz processor by adding idle cycles in the FSB and raising the coefficient multiplier to x8 instead of x6. This way the FSB runs at 200 MHz (and the processor at 1.6 GHz) but an idle cycle is inserted between every processor cycle, so the Core 2 Duo functions like a device with a 100 MHz FSB instead (and thus, an 800 MHz CPU).
Why Include Idle Cycles?
Intel uses this technique because it is easy to implement, but more so because it can be activated quickly. Really modifying the FSB’s frequency would clearly be a more elegant solution but it would also mean changing all of the FSB’s synchronized clocks (memory, PCI-Express, PCI etc.). Plus, even when possible, changing the FSB on the fly requires the insertion of more than just idle cycles.
In the end, even if CPU-Z and other benchmarks indicate that the actual frequency is 1.6 GHz, you have to keep in mind that the processor is really acting like an 800 MHz model. Let us take note that special versions of CPU-Z allow you to label the frequency as “relative,” as you can see in the images.
