Central to Sonoma is the 'Alviso' chipset, which will initially be made available in two forms, the discrete 915PM and the integrated 915GM. Alviso began shipping in December 2004. A key feature is support for a 533MHz FSB, up from the original Centrino's 400MHz bus, so it was only a matter of time before boxed 533MHz Pentium Ms appeared.
Features include the 'no execute' bit, which when used with Windows XP SP2 can prevent some viruses running from 'data only' memory, and of course 2MB of L2 cache. The new chips are clocked at 1.60, 1.73, 1.86 and 2.00GHz.