almostOS

Something almost but not quite entirely unlike OS. Implementing it for my own amusement and fun (some would say I'm a very boring person). There are no big upfront plans for architecture; everything is done ad-hoc and improved iteratively later.

Current state: It's at a very early stage. At the moment, it looks more lika a bootable shell-like program than an OS, with a very basic custom filesystem on a RAM disk, supporting simple versions of 'ls' and 'cat' commands. It also has a command "dump memory", similar to the hexdump, but dumping the content of memory instead of a file.

What it has or can do:

• A rudimentary scheduler
• A simple FAT-like filesystem
  • at the moment it has only the most basic functionality, but it is mature enough to be mounted on Linux (and on AlmostOS)
• Can output text to VGA or COM port
• Can read input from keyboard or COM port
• Support for "write" and "exit" system calls
  • System calls are Linux compatible, meaning, it can now execute (artificialy) simple Linux 32-bit x86 programs
• rudimentary shell
  • The shell is built into the OS, as currently there are still not enough system calls available to implement a shell in userspace

I'm not sure in which direction I will go with this project, but I will definitely at least implement an ATA driver to have a filesystem on the actual disk instead of an in-memory one. Had only tested it on QEMU and Bosch x86 PC emulator, but in theory, it should also boot on VirtualBox/VMWare.

Currently, I'm implementing my version of "virtual filesystem switch" and adding more system calls

It is ugly, but not ugly enough to be rejected by GRUB

How to run it

Prerequisites

apt install nasm build-essential genisoimage qemu-system

Running the OS

Run:

make qemu

This command will compile everything, create an ISO image containing the kernel and Grub, and then run QEMU with this image loaded

Available commands in the build-in shell

help

ls (list all files)

cat filename (outputs the content of the file)

ps (list all running processes)

kill PID (kill a process)

run num (run a test process)

dump address (dumps the 128 bytes of the memory starting from the specified memory address)

(Yep, that's everything)

Entry point

The kernel entry point is in the file "kernel_startup.asm", which sets the Multiboot signature and defines the function start_kernel(). This function is defined in the linker script "kernel.ld" as the entry point for the compiled artifact. The start_kernel() just outputs the string "hello_msg", sets the stack pointer, and then jumps to the first C code function: kernel_c_main().

Statistics

Lines of code:      4679
Number of files:      60
OS market share:       1 (user)