In synchronous circuits, even when

In synchronous circuits, even when the output computed by a block of combinatorial logic is not required, the block keeps computing its output from observed input every clock cycle. In order to save power, entire execution units comprising of combinatorial logic and their state registers can be put in a stand-by mode by disabling the clock and/or powering down the unit. Special circuitry is required to detect and power-down unused units and power them up again when they need to be used.