AlmeidaOS

AlmeidaOS

Simple, self-contained, clean code, well-documented, hobbyist 64-bit operating system for Intel and AMD processors. It’s written in Assembly (Intel syntax) and C (GNU99 Standard) in which I strive for adding comments to the source code as much as possible so it could be used as a reference for future OS developers.

It contains the following components written from scratch:

BIOS

Component	Description	Source code
MBR	First stage bootloader	code
Loader	Second stage bootloader	code

Kernel

Component	Description	Source code
PML4	Paging Structure	code
Buddy	Memory allocator System	code
PrintK	printf-like string format parsing utility	code
Serial Driver	send printk msgs via RS232 to help debugging	code
Core Dump	Dump CPU registers for debugging purposes	code
Syscall/Sysret	method chosen to jump to Ring 3 and back	code
PIT	Programmable Interval Timer	code
PIC	Programmable Interrupt Controller	code
(x)delay	Based on tightloops given that I’m using PIT	code
CMOS RTC	Real-time clock	code
Scheduler	Simple Round-Robin scheduler	code

libc

functions are being added on-demand: code

More to come! Star/Watch this repo to stay tuned!

Build

In order to build this app, you are expected to have make and docker installed in your machine. All of the other building dependencies should be taken care of automatically by the docker image created during this step.

To build it you have to run:

make build

Run

In order to run this app, you are expected to have make and qemu-system-x86_64 installed in your machine.

make test

Wishlist

To make sure I won’t lose focus on what I want this OS to be able to do, I decided to write a list of features that I want to implement in the short to medium term.

• Serial port driver
• Early printk (contingent on serial port driver) -> will help debugging in real computers
• limited libc -> (It will always be “Work in progress” as I add functions as I need them)
• Userspace
• Rudimentary Bash-like terminal

Stretch goals

Things that would be fantastic to have but I am not sure if I have what it takes to do within my lifetime.

• networking capabilities (I would be damn happy with UDP already…I wouldn’t dare trying to implement TCP)
• Filesystem
• Scheduler / Time-sharing OS

References

These are all the references that helped me a lot during the development of AlmeidaOS

Forums:

• https://www.osdev.org/
• https://www.reddit.com/r/osdev/

Books:

• https://www.amazon.com/Understanding-Linux-Kernel-Third-Daniel/dp/0596005652
• https://www.amazon.com/Linux-Device-Drivers-Jonathan-Corbet/dp/0596005903/
• https://www.amazon.com/Hackers-Delight-2nd-Henry-Warren/dp/0321842685 - (thanks @igorlg)
• https://www.amazon.com/Understanding-Using-Pointers-Techniques-Management/dp/1449344186

Existing OSes:

• https://github.com/beevik/MonkOS
• https://elixir.bootlin.com/linux/latest/source

Courses:

• https://www.udemy.com/course/writing-your-own-operating-system-from-scratch/