The CY29FCT818T contains a high-speed 8-bit general-purpose data pipeline register and a high-speed 8-bit shadow register. The general-purpose register can be used in an 8-bit-wide data path for a normal system application. The shadow register is designed for applications such as diagnostics in sequential circuits, where it is desirable to load known data at a specific location in the circuit and to read the data at that location. The shadow register can load data from the output of the device, 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. Data can be loaded simultaneously from the shadow register to the pipeline register, and from the pipeline register to the shadow register, provided setup-time and hold-time requirements are satisfied, with respect to the two independent clock inputs. In a typical application, the general-purpose register in this device 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 that 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 then is compared with the expected value to diagnose faulty operation of the sequential circuit. This device is fully specified for partial-power-down applications using Ioff. The Ioff circuitry disables the outputs, preventing damaging current backflow through the device when it is powered down.