The present invention relates to computer viruses and more particularly to a method and system, for use in a virus-free certificate cache, of caching one or multiple virus-free certificates, each virus-free certificate certifying that a file is virus-free. The method includes allocating a surface within a physical memory and mapping the surface to a plurality of virtual memory addresses within a CPU page table. Methods are also disclosed for producing suitable metrics for use with the new butterfly unit, and for optimizing these metrics by dynamic scaling and adaptation for additive noise.