Implementing OpenCL with an existing operating FPGA

I am new to OpenCL. I am wondering how could I use OpenCL to call a function that will carry out some work on an already preconfigured FPGA. In essence I want to know how to make openCL talk to the FPGA. How do I implement the memory mappings etc, that I need to make an OpenCL function carry out a task on the FPGA? Thanks for you help.