Once upon a time, in an era long ago, embedded Linux was not really a thing. I know that sounds a bit crazy, but it's true! If you worked with a microcontroller, you were given (from the vendor) a specification, a design sheet, a manual of all its registers and nothing more. Translation: you had to write your own operating system (kernel included) from scratch. Although this guide assumes the standard generic 32-bit x86 architecture, a lot of it reflects what had to be done back in the day.