Reconfigurable Systems-an-Chip (RSoCs) on the market consist of full -fledged processors and large Field -Programmable Gate Arrays (FPGAs). The latter can be used to implement the system glue logic, various peripherals, and application - specific coprocessors. Using FPGAs for application -specific coprocessors has certain speedup potentials, but it is less present in practice beca use of the complexity of interfacing the software application with the coprocessor. In the present application, we present a virtualisation layer consisting of an operating - system extension and a hardware component. It lowers the complexity of interfacing and increases portability potentials, while it also allows the coprocessor to access the user virtual memory through a virtual memory window. The burden of moving data between processor and coprocessor is shifted from the programmer to the operating system.