 Thus, an LFSR is most often a shift register whose input bit is driven by the XOR of some bits of the overall shift register value.The initial value of the LFSR is called the seed, and because the operation of the register is deterministic, the stream of values produced by the register is completely determined by its current (or previous) state.Likewise, because the register has a finite number of possible states, it must eventually enter a repeating cycle.