A register is the most basic storage element in an FPGA. It has an input (

in

), an output (

out

), and a clock signal (

clk

). It is synchronous, that is, it synchronizes output changes to a clock. In an ALM, a register may store the output of the LUT.

The following figure illustrates a register:

The following figure illustrates the waveform of register signals:

The input data propagates to the output on every clock cycle. The output remains unchanged between clock cycles.