The overall veriJication approach used in the design and development of the fill custom 64 bit UltraSPARC microprocessor will be described. A balanced hierarchical approach was critical in validating a design with this level of complexity. The tools, developed internally and externally, which aided the verijication effort will also be described. The environment was flexible enough to support various revisions of major tools. The method developed could easily be applied to derivative and next generation microprocessors.