This work presents a dynamically reconfigurable platform called EXPRESS-1, which uses the commercially available embedded processor FPGA. The system makes the most of a fully reconfigurable logic part to explore the area of fine-grained reconfigurable computing. A dynamic reconfiguration mechanism is implemented utilizing a real-time operating system, so device reconfiguration in response to application demand works without suspending other services. EXPRESS-1 features a transparent execution mechanism. Whether a function is executed by the hardware or software, the mechanism frees users from awareness of its execution manner. Furthermore, there is no need to explicitly specify reconfiguration commands into a program because the system determines if reconfiguration is needed based on current conditions. The development of EXPRESS-1 and the runtime reconfiguration mechanism of the fully reconfigurable logic are described. System capabilities are also reported through fundamental evaluations with some practical applications such as JavaVM, encryption processing, and image processing.