The earliest Eee PC models used a 900 MHz Intel Celeron M processor underclocked to 630 MHz. Later models shipped with this processor at normal speed. Newer models use a 1.6GHz Intel Atom processor. The Atom is a simple, scalar, in-order execution microprocessor that is significantly slower per-clock than the Celeron M due to its microarchitecture. Still, 1.6 GHz Atom-equipped Eee PCs tend to perform similarly to their Celeron-equipped counterparts, due to the higher clock speeds and Hyperthreading. (*)