Microprocessor cores are a major challenge in the test arena: not only is their complexity always increasing, but also their specific characteristics intensify all difficulties. A microprocessor embedded inside a SOC is even harder to test since its input might be harder to control and its behavior may be harder to observe. Functional testing is an effective solution which consists in forcing the microprocessor to execute a suitable test program. This paper presents a new approach to automatic test program generation exploiting an evolutionary paradigm. It overcomes the main limitations of previous methodologies and provides significantly better results. Human intervention is limited to the enumeration of all assembly instructions. Also internal parameters of the optimizer are auto-adapted. Experimental results show the effectiveness of the approach.