I understand that Linux modules may be loaded at boot time via a script (although methods to do this vary), and can be loaded a run-time too with modprobe or insmod, for example.

However I was shocked today when I accidentally rmmod'ed the wrong module. In fact I rmmod'ed the ochi-ocd module - that is my USB 1.1 Host Controller driver. lol - note I lost control of my keyboard and mouse and had to reboot.

The point is, I thought that only modules could be loaded/unloaded and that kernel built-in functionality was there to stay?

You are correct. During runtime, statically build kernel code cannot be rmmoded or 'modprobe -r'ed, only modules can be removed by these means.

If you have your system running and open a console and type 'lsmod' it will show you what-all modules are currently plugged in (and thus removable).

Sasha

To clarify, just because something seems basic doesn't mean it's going to be compiled in. Do take a look at the command Sasha mentioned; you'll see a lot of things you might have suspected weren't modules. As for why, the best I can give you is that the Linux kernel runs on an obscene variety of platforms and functionality that seems critical for your desktop may not be for all or even most of them. Note that whether something is compiled into the kernel or as a module is a compile-time option and you can make your own choices about this if you ever compile your own kernel.