Week 3 - Test Drive Development in XP

http://c2.com/cgi/wiki?TestDrivenDevelopment

Test Driven Development(TDD)
TDD is the inner core of the outer shell of the Extreme Programming(XP). In this web-page, the author write some basic rule that we should notice before undergo the TDD. WE should think what we want to do, should think about how to test and write a small test, write enough code that just pass the tests.

I would like to ask him if the TDD is a good way to design the project, can it be applied to develop the project that as big as developing an Operating System, or a huge database management system? Obviously this methodology cannot do that since it focus on dividing the big problems into small piece and just work it, solve it but test-code-debug process. In a small part of the system, it may not be testable before combining with other components that developed by developer in the same team or even in other teams. So what about if the program cannot be tested? The whole methodology crashed and not workable at all.