JTAG logic in chips

Examples of built-in resources in chips accessible through the JTAG interface are the boundary-scan register and the microcontroller debug logic.
A boundary-scan register gives direct access to device pins without using the device functionality (the device core). Connections between different devices can easily be accessed for testing or programming. The boundary-scan register is at the heart of boundary-scan test and programming. Microcontroller debug registers/logic gives direct access to the microprocessor busses without using the microprocessor functionality.

The microprocessor debug logic is at the heart of emulative test and programming. Both internal and external memory peripherals connected to the processor busses can easily be accessed for testing or programming without embedded software.