The Pentium M and its support chipset run at 100Mhz front side bus speed; however, it’s “quad pumped” much like the Pentium 4 platform and thus the marketing calls it 400Mhz. The next Alviso + Sonoma platforms bump the front side bus speed up to 133Mhz and thus a quad-pumped 533Mhz bus.
Here’s a good explanation of FSB .