Parameterizable configurations are regular FPGA configurations in which some of the configuration bits are expressed as a Boolean function of a set of parameters. These configurations can rapidly be transformed to a specialized configuration by evaluating the Boolean functions for a specific set of parameter values and are therefore ideal for use in run-time reconfigurable systems. In this paper, the concept of Parameterizable Bitstream (PBS) is introduced to accommodate the use of parameterizable configurations in commercial FPGAs. A stack machine system is implemented that can efficiently transform a PBS to a specialized configuration.