#: Train-OS A training session to build an operating system. Starting from the ground up, only using resources from AMD and Intel.

STILL IN ALPHA STAGE

This repository is not polished yet and still has to be regarded in alpha stage. The code should compile and work, but the texting is just the first draft I noted down besides while I was coding and researching.

What is different to other tutorials?

There are a lot of tutorials and a large part of the material I present is covered also there. So why bother? I especially like and got inspired by these:

• phil TODO: link
• intermezzOS: TODO: link

What is different in this version? When I learn new things, I often have the feeling that I don't know/learn how I could do what I just did completely on my own the next time. I don't know if it is the way I learn or just a typical thing that is easily omitted when writing tutorials. Here are some points I am wondering about regularily and that I want to address (I will probably miss out on other essential stuff to make up for it)

• Provide some more details on what points simply are conventions that were agreed on, what is a standard, what is architecture specific. I still remember when I first started to learn programming as a teenager I was like "uuhh... how does the computer know that the function main has to be called first", and none of my friends or teachers new. It was just "magic". These things bother me, I want to uderstand where things come from...
• Point to the standard and work from there. I won't present a finished solution that you just copy. We take the standard, and actually look things up. Just as we would have to to if we would like to run the code on let's say an ARM Cortex instead of your x86_64 machine. Or, framed differently, what if we would be the first ones to write a tutorial, how would we get that bootloader going, the interrupts configured or the cpu into 64-bit mode in the first place?
• Do things twice. I'll first go barebones, this helps to understand what's happening and then, we use tooling to avoid doing that ground work every time.
• Intermediate steps and "checkpoints"
• give an outline first -> to see the bigger picture
• give examples into real code -> e.g. that the first kernel bin is actually exactly how linux is built
• present alternatives/choices for each dependency we start using. To me it's bummer if I want to follow a tutorial and a lot of external libraries do the magic that I actually wanted to learn.

Outline

A rough outline of what I want to do. Hopefully, in a more or less chronological order. The point is that there is so much we could possibly do that it actually is quite hard to choose a path.

First, I would like to get a better understanding of the boot process. What happens before the bootloader jumps in. Actually rolling your own bootloader (link to osdev) is a huge task in by itself.

My plan is to explore the early phase in the boot process a little bit. The goal of that is to understand and appreciate what a bootloader is doing for us. At the end, I would like to have a rough understanding of the different processor modes, what they offer and how to switch to the 32-bit protected mode. The concrete goal is to be able to switch over to the protected mode and print Hello World! to the screen. Everything implemented in assembly and without the use of a boot loader.

After that, I plan to switch over and use GRUB as a bootloader. And continue from there.

TL;DR

1. Write a minimal bootable image.
2. Bootloader that prints Hello World in 16-bit real mode.
3. Manually switch to 32-bin protected mode and print Hello World.
4. Manually switch to 64-bin long mode
5. Use GRUB as bootloader and print Hello World.
6. Switch to long mode (again).
7. Jump into C, print Hello World.
8. Stack
9. Implement VGA video driver.
10. Interrupts (Part 1) - Tell CPU where to find handlers, in asm
11. Interrupts (Part 2) - Move code from 10. over to c
12. Implement better handlers
13. Debugging
14. Handle interrupts/exceptions
15. Implement memory paging.
16. Remap Kernel

unspecified:

• refactoring?
• utilities (memset?)

Later on...

• filesystem
• processes
• scheduling
• system calls
• explore the stack, provoke stack overflows and exceptions
• connect with GDB via UART (for debugging and embedded preview)
• cross-compile some (or many) parts to an ARM processor. (I have an STM and Infineon board lying around unused)

Prerequesits?

None. Seriously, don't get scared. The whole point of this tutorial is to develop all required knowledge from the ground up. There is only one condition I assume, and that is that you can program decently. By decent I mean that you at least have heard about the stack and heap and can be productive in some language. I guess that should be enough. After all, we are not trying to develop the next gen mainstream os here, we are tinkering on a hobby level.

And one things I can assure you: Even if you will never touch this code again and will never write an OS. The experience will change your understanding of the systems you are working on deeply. The knowledge gained from developing at kernel level spreads like seeds throughout your career and the insights will prove useful in the most unexpected moments.

Scratchpad (unorganized ideas from here on)

Ideas

• what does the cpu do if you put instructions after the first 512 bytes? Are they executed? (CPU is in 16-bit real mode)

Outline

• Bootload -> two choices roll your own or use existing, e.g. GRUB.
• 16bit mode
• entering 32bit protected mode
• entering 64bit long mode
• enter c
• write memory managemen module

Cycle 0 - Environment & Cross-Compilation Toolchain

• you may skip some of the steps. We won't need the cross compilation toolchaine before before we start writing C code.
• start building your toolchain somewhere under $HOME
• make sure the scope of environment variables is correct
• errors like "don't have permission to create directory /usr/lib/i686-elf" indicates that $PREFIX is not correctly set.
• We're not getting the "real" barebones CPU, there's still the BIOS before us. It initializes the CPU and hands over to us with the CPU in real mode.

Cycle 0 - Bootloader

Resources

1. NASM Documentation https://www.nasm.us/xdoc/2.14.02/html/nasmdoc3.html
2. Intel Manuals