kfs

Bootable x86 kernel built from scratch — KFS-1 and KFS-2 with a freestanding libc.

Overview

A toy operating-system kernel built incrementally from the bare bones up. KFS-1 boots from a multiboot-compliant entry and prints to a VGA text terminal; KFS-2 adds GDT + IDT setup, ISR/IRQ handlers, PS/2 keyboard input, an interactive shell, and a freestanding libc with stdio, stdlib, and string implementations. The kernel runs under QEMU using a custom i686-elf cross-compilation toolchain and a reproducible ISO build pipeline.

This project was built as part of the 42 school cybersecurity / kernel track with a partner (Jungmoo Cheon) · KFS-1 score: 116/100, KFS-2 score: 118/100.

Tech Stack

Layer	Technologies
Language	C, x86 assembly
Toolchain	i686-elf-gcc cross-compiler, GNU binutils
Boot	Multiboot, GRUB
Build	Bash scripts, linker scripts, Makefiles
Run	QEMU (x86)

Key Features

• Bootable kernel: multiboot-compliant entry, custom linker script, runs under QEMU and as a GRUB-bootable ISO
• CPU bring-up: GDT setup and IDT with ISR / IRQ handlers (isrs.S, irqs.S)
• VGA text mode: terminal driver with cursor handling, scrolling, and color attributes
• PS/2 keyboard: scancode translation and basic keymap
• Interactive shell: simple in-kernel REPL built on the keyboard + terminal stack
• Freestanding libc: hand-written stdio, stdlib, and string implementations with no external dependencies
• Reproducible build pipeline: cross-compile, link, ISO generation, QEMU runner — all script-driven

Architecture

kfs/
├── kfs/                       # current stage (KFS-2)
│   ├── kernel/
│   │   ├── arch/i386/         # arch-specific bring-up
│   │   │   ├── boot.S         # multiboot entry
│   │   │   ├── gdt.c          # GDT setup
│   │   │   ├── idt.c          # IDT setup
│   │   │   ├── isr.c          # ISR dispatch
│   │   │   ├── irq.c          # IRQ handlers
│   │   │   ├── tty.c          # VGA terminal
│   │   │   ├── keyboard.c     # PS/2 keyboard
│   │   │   └── io.c           # port I/O
│   │   ├── include/
│   │   ├── kernel/
│   │   │   ├── kernel.c       # kernel main
│   │   │   └── shell.c        # interactive shell
│   │   └── Makefile
│   ├── libc/
│   │   ├── stdio/             # printf, putchar, ...
│   │   ├── stdlib/            # malloc, abort, ...
│   │   ├── string/            # memcpy, strlen, ...
│   │   ├── arch/
│   │   ├── include/
│   │   └── Makefile
│   ├── build.sh               # full build
│   ├── iso.sh                 # ISO generation
│   ├── qemu.sh                # run under QEMU
│   └── ...                    # supporting scripts
├── kfs-1-barebones-backup/    # KFS-1 snapshot
└── kvm/                       # KVM-related setup

Getting Started

Prerequisites

A POSIX environment with:

i686-elf-gcc          # cross-compiler
grub-mkrescue         # ISO generation
xorriso               # ISO toolkit
qemu-system-i386      # x86 emulator

Build & run

git clone https://github.com/sungyongcho/kfs.git
cd kfs/kfs
./build.sh            # cross-compile kernel + libc
./iso.sh              # produce a bootable ISO
./qemu.sh             # run under QEMU

What This Demonstrates

• Bare-metal programming: booting an x86 kernel via multiboot, setting up CPU descriptor tables (GDT/IDT), and handling interrupts without an OS underneath.
• Toolchain construction: configuring an i686-elf cross-compiler and a reproducible build pipeline from scratch.
• Freestanding C: implementing the parts of libc (stdio, stdlib, string) that the kernel needs, with no external dependencies.

License

This project was built as part of the 42 school curriculum.

---

Part of sungyongcho's project portfolio.