the host interface provides access to the diagnostic registers in most of the complex logic on the chip except for the host interface itself this advantageously permits the host cpu to obtain direct access to register contents in the specialized chip application_number US 8765402 A description the numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment by way of example and not of limitation standard scan chain testing during chip production test all full scan or some partial scan of the internal registers are put into a test mode so that they can be driven by the chip tester as a shift register this allows test patterns to be shifted into and out of the chip under control of the tester and this is how most chips are tested during manufacture the area overhead associated with this method stems from the need to connect the registers together into a scan chain only during test mode this re configuration is done using multiplexers at the input to every register in the design in test mode the multiplexers are switched to route the output from each register directly to the input of the next register in the chain these multiplexers are therefore only used during test mode and are redundant once the manufacturing process is complete in circuit testing using scan chains it is often necessary to build into a chip some extra circuitry to allow parts of the design to either be observed or controlled directly during operation this otherwise redundant logic can significantly improve debugging.