The shadow registers can load data from the output of the FCT818T, and can be used as a right-shift register with bit-serial input SDI and output SDO, using DCLK. The data register input is multiplexed to enable loading from the shadow register or from the data input pins using PCLK. Note that data can be loaded simultaneously from the shadow register to the pipeline register, and from the pipeline register to the shadow register provided set-up and hold time requirements are satisfied with respect to the two independent clock inputs. In a typical application, the general-purpose register in the FCT818T replaces an 8-bit data register in the normal data path of a system. The shadow register is placed in an auxiliary bit-serial loop which is used for diagnostics. During diagnostic operation, data is shifted serially into the shadow register, then transferred to the general purpose register to load a known value into the data path. To read the contents at that point in the data path, the data is transferred from the data register into the shadow register, then shifted serially in the auxiliary diagnostic loop to make it accessible to the diagnostics controller. This data is then compared with the expected value to diagnose faulty operation of the sequential circuit.