Core and I/O clock design for the Pentium(R) 4 microprocessor is described. Two phase-locked loops generate core and I/O clocks supporting concurrent multiple frequencies. A clock distribution network with skew optimization and jitter reduction is designed to achieve low clock inaccuracies for processors at frequencies /spl ges/2 GHz for the core and /spl ges/4 GHz for the rapid execution engine. A global medium clock frequency is distributed. Local clock drivers generate pulsed or regular (nonpulsed) clocks at fast, medium, and slow frequencies. A 3.2-GB/s system bus is achieved using a dedicated I/O phase-locked loop with glitch protection and detection. Silicon speed path tools and clock debug features are designed to enable a short debug cycle.