Reconfigurable computing is maturing rapidly as FPGAs combining hard core processors and high density logic block arrays become widely available at low cost. Application developers have been developing algorithms that cross the hardware software divide for some years but will in addition want to express the dynamic reconfiguration of FPGAs made available via an operating system for reconfigurable computing. Whilst there are many behavioural languages available for expressing reconfigurable computing applications very few of them are comprehensive enough to address simultaneously these two requirements. In this paper we present an experimental language based on Java which aims to achieve the twin goals of a transparent hardware software interface and an integrated expression of dynamic reconfiguration. Hardware Join Java (HJJ) uses a common threading abstraction and synchronization based on the Join calculus to unify the semantics and interface between hardware and software. The language extends the dynamic class instantiation mechanism of Java (supported by the services of an operating system for reconfigurable computing) to express user initiated dynamic reconfiguration of the FPGA. In this paper we present basic syntax and semantics of HJJ and give our initial experience with the prototype compiler.
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