What is a JTAG test?

While designed for printed circuit boards, it is nowadays primarily used for testing sub-blocks of integrated circuits, and is also useful as a mechanism for debugging embedded systems, providing a convenient "back door" into the system. When used as a debugging tool, an in-circuit emulator which in turn uses JTAG as the transport mechanism enables a programmer to access an on-chip debug module which is integrated into the CPU via JTAG. The debug module enables the programmer to debug the software of an embedded system.