How are operating systems debugged?

How are operating systems typically debugged? They cannot be step开发者_Python百科ped through with a debugger like simple console programs, and the build times are too large to repeatedly make small changes and recompile the whole thing.

They aren't debugged as a multi-gigabyte programs! :)

If you mean the individual user-mode components, they can mainly be debugged just like normal programs and libraries (because they are normal programs/libraries!).

For kernel-mode components, though, each OS has its own mechanism; here is some information regarding the way that we do kernel debugging in Windows. It can be done using the help of another machine connected to the machine you're debugging, via a serial port or something. I'm not familiar with the process itself, but that's the gist of how they work. (You need to set some boot loader options so that the system is ready for the debugger to be connected as early as possible.)

It depends on which part of the operating system you're talking about. When I worked at MSFT, I worked on the IE team. We debugged IE and the shell (Windows Explorer) in Visual Studio and stepped through them line by line all day long. Though, sometimes, it's easier to debug using a command line tool such as NTSD.

If, however, you want to debug anything in Kernel land such as the OS kernel or device drivers, which I suspect is really what you're asking, then you must use the Kernel debugger. For Windows that is a command line tool called kd, and generally you run the debugger on one machine and remotely debug the target.

There are a whole set of techniques throughout history from flashing lights on the console, to the use of hardware devices like an ICE, to more modern techniques utilizing fairly standard debuggers. One technique that is more common among OS developers then application developers is the analysis of a core dump. Look at something like mdb on solaris for ideas about how Solaris kernel developers do some of their debugging. Also tracing technologies are used. Anywhere from fairly straightforward logging packages to more modern techniques like dtrace.

Also note that the techniques used depend on the layer of software. Initial boot tends to be a fairly hard place to get your fingers into. But after that the environment of modern operation systems looks more and more like the application setting you are use to. In the end, it is all code :)