BIT, which uses hardware and software designed into the board itself, is a powerful technique for functional test. You gain access to test results and diagnostic information through a serial port. The drawback of BIT is that it requires a significant investment in hardware and software engineering during a PCB's design. The monitor program acts as an interface between the test program, written in a high-level language, and the emulator, which passes commands to the UUT microprocessor. The result is that the test engineer can control the microprocessor on the UUT through a test program written in a high-level language.
Emulation is conceptually simple.
The emulator is basically a passive device that simply waits for the UUT's microprocessor to fetch commands. Emulation tests a UUT at full speed. UUT synchronization is automatic because the UUT microprocessor fetches commands from the emulator under the microprocessor's own timing control. Note that ROM emulators designed for testing differ from the traditional ROM emulators used for software development. Traditional emulators also provide breakpoints and other tools to assist engineers in debugging code.