During the last years, the growing application complexity, design, and mask costs have compelled embedded system designers to increasingly consider partially reconfigurable application-specific instruction set processors (rASIPs) which combine a programmable base processor with a reconfigurable fabric. Although such processors promise to deliver excellent balance between performance and flexibility, their design remains a challenging task. The key to the successful design of a rASIP is combined architecture exploration of all the three major components: the programmable core, the reconfigurable fabric, and the interfaces between these two. This work presents a design flow that supports fast architecture exploration for rASIPs. The design flow is centered around a unified description of an entire rASIP in an architecture description language (ADL). This ADL description facilitates consistent modeling and exploration of all three components of a rASIP through automatic generation of the software tools (compiler tool chain and instruction set simulator) and the RTL hardware model. The generated software tools and the RTL model can be used either for final implementation of the rASIP or can serve as a preoptimized starting point for implementation that can be hand optimized afterward. The design flow is further enhanced by a number of automatic application analysis tools, including a fine-grained application profiler, an instruction set extension (ISE) generator, and a data path mapper for coarse grained reconfigurable architectures (CGRAs). We present some case studies on embedded benchmarks to show how the design space exploration process helps to efficiently design an application domain specific rASIP.