Microprocessor development emphasizes hardware and software co-design. Hw/Sw co-design is a modern technique aimed at shortening the time-to-market in designing the real-time and embedded systems. Key feature of this approach is simultaneous development of the program tools and the target processor to match software application. An effective co-design flow must therefore support automatic software toolkits generation, without loss of optimizing efficiency. This has resulted in a paradigm shift towards a language-based design methodology for microprocessor optimization and exploration. This paper proposes a formal grammar, UNI-SPEC, which supports the automatic generation of assemblers, to describe the translation rules from assembly to binary. Based on UNI-SPEC, it implements two typical applications, i.e., automatically generating the assembler and the test suites.