YOTOS

32bit x86 OS built from scratch in C and Assembly.

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Features

• Protected mode — full 32-bit x86 protected mode with GDT (ring 0/3 segments)
• Memory management — physical memory manager (bitmap allocator), paging (VMM), kernel heap
• Interrupts — IDT: 32 ISR stubs, IRQ remapping via PIC 8259, syscalls via int 0x80
• Processes — ELF binary loader, user-mode execution (ring 3), TSS
• Drivers — VGA text mode (80×25), PS/2 keyboard, PIT timer, ATA/IDE disk, UART serial
• Filesystem — VFS abstraction layer with a full FAT16 implementation
• Shell — interactive shell with built-in commands
• User programs — hello world and a playable Snake game
• klibc — minimal C library (printf, string utilities)
• UART terminal — yotos_terminal.py, a pygame GUI client that connects to the UART serial port over TCP and lets you interact with the OS remotely

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Project structure

.
├── boot/                Multiboot header (NASM)
├── kernel/              GDT, IDT, PMM, VMM, heap, processes, ELF loader, syscalls
├── drivers/             VGA, keyboard, timer, ATA, UART
├── fs/                  VFS layer and FAT16 implementation
├── shell/               Shell and built-in commands
├── klibc/               Minimal libc (stdio, string)
├── user/                User-mode programs (hello, snake)
├── include/             Header files
├── linker.ld            Kernel linker script (loads at 2 MB)
├── grub.cfg             GRUB bootloader config
├── yotos_terminal.py    A client for a remote connection to the terminal of YOTOS
└── makefile

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Quick start (no build required - So Windows users can run the project too)

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Download yotos.iso and disk.img from the latest release, then run:

"C:\Program Files\qemu\qemu-system-i386.exe" -boot d -cdrom yotos.iso -drive file=disk.img,format=raw,index=1,media=disk -serial tcp::4444,server,nowait

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Adjust the QEMU path to match your installation. Download QEMU for Windows at https://www.qemu.org/download/#windows

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Building

You need a Linux environment (native or WSL).

Dependencies

sudo apt install nasm make qemu-system-x86 grub-pc-bin xorriso mtools dosfstools
for the python remote client: pip install pygame

You also need an i686-elf cross-compiler. Follow the OSDev GCC Cross-Compiler guide to build it, then add it to your PATH.

Build commands

make          # Build the kernel ELF
make iso      # Build a bootable ISO
make snake    # Build the Snake user program
make disk-snake  # Copy Snake to the disk image
make hello    # Build the hello world user program
make disk-hello  # Copy hello to the disk image

Output goes to build/:

File	Description
build/yotos.elf	Kernel ELF
build/yotos.iso	Bootable ISO
build/disk.img	64 MB FAT16 disk image
build/snake.elf	Snake user program
build/hello.elf	Hello world user program

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Running

Linux

make run

This boots the ISO in QEMU with the FAT16 disk attached. A serial port is available on TCP port 4444 for a telnet like protocol.

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Shell commands

ls                          List files
cat <file>                  Print file contents
cd <dir>                    Change directory
mkdir <dir>                 Create a directory
touch <file>                Create an empty file
echo <text>                 Print text
echo <text> '>' <files>     Create a files with text in it
exec <file>                 Execute a user ELF binary

Running Snake

exec SNAKE.ELF

Use wasd keys to move. Press q to quit.

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Syscalls (int 0x80)

Number	Name	Description
0	sys_exit	Terminate the current process
1	sys_write	Write a string to the VGA console
2	sys_readkey	Read a keypress

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Memory layout

Address	Contents
0x00000000 – 0x000FFFFF	Low memory / BIOS
0x00200000	Kernel
0x00400000	User programs
0xBFFFF000	User stack (4 pages)

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Clean

make clean