docs about building kernel

README.md

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+Build yourself a Linux
+======================
+
+% TODO explain Makefile
+
+Abstract
+--------
+
+*This started out as a personal project to build a very small Linux based
+operating system that has very few moving parts but is still very complete and
+useful. Along the way of figuring out how to get the damn thing to boot and
+making it do something useful I learned quite alot. Too much time has been
+spent reading very old and hard to find documentation, or when there was none
+the source code of how other people were doing it. So I thought, why not share
+what I have learned.*
+
+The Linux Kernel
+----------------
+
+The core component of our operating system, the kernel, that which manages the
+processes and talks to the hardware on our behalf. You can retrieve a copy of
+the source code easily from [kernel.org](https://www.kernel.org/). There are
+multiple versions to choose from, choosing one is usually a tradeoff between
+stability and wanting newer features. If you look at the
+[Releases](https://www.kernel.org/category/releases.html) tab, you can see how
+long each version will be supported and keeps receiving updates. So you can
+usually just apply the update or use the updated version without changing
+anything else or having something break.
+
+So just pick a version and download the tar.xz file and extract it with ``tar
+-xf linux-version.tar.xz``. To build the kernel we obviously need a compiler and
+some build tools. Installing ``build-essential`` on Ubuntu (or ``base-devel`` on
+Arch Linux) will almost give you everything you need. You'll also need to
+install ``bc`` for some reason.
+
+The next step is configuring you build, inside the untarred directory you do
+``make defconfig``. This will generate a default config for your currect
+architecture and place it in ``.config``. You can edit it directly with a text
+editor but it's much better to do it with an interface by doing ``make nconfig``
+(this needs ``libncurses5-dev`` on Ubuntu). Here you can enable/disable features
+and device drivers with the spacebar. ``*`` means that it will be compiled in
+your kernel image. ``M`` means it will be compiled inside a seprate kernel
+module. Which is a part of the kernel that will be put in a seperate file and
+can be loaded in dynamically in the kernel when they are required. The default
+config will do just fine for basic stuff like running in a virtual machine. But
+in our case, we don't really want to deal with kernel modules so we'll just do
+this: ``sed "s/=m/=y/" -i .config``. Building the kernel is now just running
+``make``. Don't forget to add ``-jN`` with `N` the number of cores of this might
+take a while.
+
+Other useful/interesting ways to configure the kernel are:
+
+  * ``make localmodconfig`` will look at the modules that are currently
+      loaded in the running kernel and change the config so that only those are
+      enabled as module. Useful for when you only want to build the things you
+      need without having to figure out what that is. % TODO LSMOD and caveat
+
+  * ``make localyesconfig``,the same as above but everything gets compiled in
+      the kernel instead as a kernel module.
+
+  * ``make allmodconfig`` generates a new config where all options are enabled
+      and as much as possible as module.
+
+  * ``make allyesconfig``, same as above but with everything compiled in the
+      kernel.
+
+  * ``make randconfig`` generates a random config...