The Open FPGA project develops an open-source virtual FPGA that can be layered on top of a proprietary FPGA to provide an open programming interface. The user is free to use the development tools provided by this project, or write their own tools. Unlike proprietary FPGAs (Altera, Xilinx, etc.), the bitfile format of the virtual FPGA is completely open. Once the virtual FPGA is installed in the nonvolatile memory of the FPGA, the proprietary tools from the physical FPGA vendor are no longer needed.