Bare Minimum OS

A 32-bit x86 operating system written from scratch in C and x86 Assembly.

Currently, the OS runs entirely in kernel mode (Ring 0). User mode and system calls are the next major development milestones.

Features

Boot & CPU

• Custom BIOS bootloader with disk loading
• 32-bit x86 protected mode
• Global Descriptor Table (GDT)
• Interrupt Descriptor Table (IDT) and interrupt service routines
• 8259 PIC remapping and hardware IRQ handling

Memory Management

• BIOS E820 memory map detection
• Bitmap-based physical memory manager
• Next-fit physical frame allocation
• Two-level x86 paging
• Recursive page directory mapping
• Dynamic kernel heap (kmalloc / kfree)
• Heap growth, block splitting and coalescing

Multitasking

• Preemptive scheduler
• Kernel threads
• Task control blocks
• Task states and lifecycle management
• Task sleeping and waking
• Context switching
• Task termination and cleanup

Drivers

• VGA text-mode driver with back buffering
• PS/2 keyboard driver with circular input buffer
• Programmable Interval Timer (100 Hz)

Kernel Utilities

• Interactive kernel shell
• Custom freestanding C library
• Kernel test suite

Roadmap

Userspace

• Kernel-managed GDT
• Ring 3 code/data segments
• Task State Segment (TSS)
• System call interface
• Per-process address spaces
• ELF executable loader

Storage

• ATA driver
• Virtual File System
• FAT32

Longer Term

• User-space shell
• UEFI boot support
• Graphics / GUI

Screenshots / GIFs

Boot

Kernel Shell

Kernel Test Suite

Building & Running

Requirements

• i686-elf-gcc cross-compiler and binutils
• NASM
• QEMU (or another x86 emulator)

See Toolchain Setup for instructions on building the cross-compiler.

This project uses a standard Makefile for building and running the OS.

make all        # Build the kernel and create boot.img
make run        # Run the OS in QEMU
make clean      # Delete build files

📂 Folder Structure

osdev/
|----- build/       # Compiled binaries and final boot.img
|----- cross/       # i686-elf-gcc toolchain location
|----- docs/        # Detailed project documentation and images
|----- src/
|       |----- bootloader.asm  # Custom BIOS bootloader
|       |----- kernel_entry.asm
|       |----- kernel.c        # Kernel entry point
|       |----- apps/           # Kernel shell and test suite
|       |----- interrupts/     # IDT, ISR and PIC
|       |----- drivers/        # Keyboard, VGA and PIT drivers
|       |----- libc/           # Custom freestanding C library
|       |----- mm/             # PMM, paging and kernel heap
|       |----- task/           # Multitasking and scheduling
|
|----- linker.ld    # Linker script
|----- Makefile     # Build commands
|----- README.md    # This file
|----- LICENSE

References

• OSDev Wiki

This project is open-source and licensed under the terms of the LICENSE file.