How do local kernel debuggers work?

[Hakman]

Some kernel debuggers are capable of working on the same machine where target OS is being debugged. There is no need to set up a virtual machine or connect two computers via a cable in such case, everything is done locally. For instance, I've used Syser debugger and noticed that it adds message during Windows XP booting screen. During kernel debugging everything is unresponsive, except debugger window. And if you install it on Windows 7 it makes system unbootable. So it definitely has something to do with the way everything boots.

How do they achieve that?

Do they alter the way computer boots?

Do they supply their own boot loader which loads their code, then OS?

If that's true, how do they prevent OS from accidentally overwriting that code in memory?

 

Edited February 14, 2016 by Hakman