This paper describes μGP, an evolutionary approach for generating assembly programs tuned for a specific microprocessor. The approach is based on three clearly separated blocks: an evolutionary core, an instruction library and an external evaluator. The evolutionary core conducts adaptive population-based search. The instruction library is used to map individuals to valid assembly language programs. The external evaluator simulates the assembly program, providing the necessary feedback to the evolutionary core. μGP has some distinctive features that allow its use in specific contexts. This paper focuses on one such context: test program generation for design validation of microprocessors. Reported results show μGP being used to validate a complex 5-stage pipelined microprocessor. Its induced test programs outperform an exhaustive functional test and an instruction randomizer, showing that engineers are able to automatically obtain high-quality test programs.