Not sure if it's the one albertcardona was referring to; but this sounds like Thomas Ray's Tierra simulator. Details at http://www.nis.atr.jp/~ray/pubs/tierra/tierrahtml.html

From his abstract: "Digital organisms have been synthesized based on a computer metaphor of organic life in which CPU time is the 'energy' resource and memory is the 'material' resource. Memory is organized into informational 'genetic' patterns that exploit CPU time for self-replication. Mutation generates new forms, and evolution proceeds by natural selection as different 'genotypes' compete for CPU time and memory space. In addition, new genotypes appear which exploit other 'creatures' for informational or energetic resources."