Kernel booting process. Part 1.

[Nytro]

Kernel booting process. Part 1.

If you have read my previous blog posts, you can see that some time ago I started to get involved with low-level programming. I wrote some posts about x86_64 assembly programming for Linux. At the same time, I started to dive into the Linux source code. It is very interesting for me to understand how low-level things work, how programs run on my computer, how they are located in memory, how the kernel manages processes and memory, how the network stack works on low-level and many many other things.

I decided to write yet another series of posts about the Linux kernel for x86_64.

Note that I'm not a professional kernel hacker, and I don't write code for the kernel at work. It's just a hobby. I just like low-level stuff, and it is interesting for me to see how these things work. So if you notice anything confusing, or if you have any questions/remarks, ping me on twitter 0xAX, drop me an email or just create an issue. I appreciate it. All posts will also be accessible atlinux-internals and if you find something wrong with my English or post content, feel free to send pull request.

Note that it isn't official documentation, just learning and knowledge sharing.

Required knowledge

• Understanding C code
  
• Understanding assembly code (AT&T syntax)
  

Anyway, if you just started to learn some tools, I will try to explain some parts during this and following posts. Ok, little introduction finished and now we can start to dive into kernel and low-level stuff.

All code is actual for kernel - 3.18, if there will be changes, I will update posts.

Magic power button, what's next?

Despite that this is a series of posts about linux kernel, we will not start from kernel code (at least in this paragraph). Ok, you pressed magic power button on your laptop or desktop computer and it started to work. After the mother board sends a signal to the power supply, the power supply provides the computer with the proper amount of electricity. Once motherboard receives the power good signal, it tries to run the CPU. The CPU resets all leftover data in its registers and sets up predefined values for every register.

80386 and later CPUs defines the following predefined data in CPU registers after the computer resets:

IP 0xfff0

CS selector 0xf000

CS base 0xffff0000

The processor works in real mode now and we need to make a little retreat for understanding memory segmentation in this mode. Real mode is supported in all x86 compatible processors, from 8086 to modern Intel 64bit CPUs. The 8086 processor had a 20 bit address bus, which means that it could work with 0-2^20 bytes address space (1 megabyte). But it only had 16 bit registers, and with 16 bit registers the maximum address is 2^16 or 0xffff (640 kilobytes). Memory segmentation was used to make use of all of the address space. All memory was divided into small, fixed-size segments of 65535 bytes, or 64 KB.

Articol complet: https://github.com/0xAX/linux-insides/blob/master/linux-bootstrap-1.md