EitanOS

A 64-bit hobby operating system kernel for x86-64, built from scratch in C and NASM assembly. Boots via the Limine bootloader (UEFI/BIOS) and runs in QEMU.

Note: Some subsystems are still being migrated from the original 32-bit version and may not be fully functional.

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Features

Memory Management

• Bitmap-based Physical Memory Manager (PMM) backed by the Limine memory map
• 4-level paging Virtual Memory Manager (VMM) with vmm_map_page, vmm_alloc, vmm_free, vmm_virt_to_phys, and per-CPU page table loading
• Kernel mapped at 0xffffffff80000000, HHDM at 0xffff800000000000
• Heap allocator built on top of the VMM

Interrupts and Syscalls

• Custom 64-bit IDT with handlers for all 32 CPU exceptions and 16 hardware IRQs
• Syscall interface via int 0x80, dispatched inside the IRQ/exception handler
• Supported syscalls: exit, run, kill, malloc, free, read_keyboard, print, clear_screen, read_file, write_file

Process Scheduler

• Preemptive round-robin scheduler driven by the PIT timer (IRQ0)
• Per-process virtual address spaces, each with its own PML4 cloned from the kernel
• Unix-style signal definitions (SIG_KILL, SIG_TERM, SIG_SEGV, etc.)

Program Loader

• ELF32 loader: parses the ELF header and program headers, maps loadable segments into the process address space, and hands off to the scheduler
• Programs are stored on disk and launched at runtime via the run syscall

Filesystem

• Custom filesystem with sector-level disk I/O
• Supports: read_file, write_file, delete_file, list_files, list_dirs

Shell

• Userspace shell compiled separately and loaded as an ELF32 process
• Supports: ls, cd, cat, touch, write, rm, echo, clear, man
• Can launch other ELF programs from disk using the run syscall
• Uses its own heap (via malloc/free syscalls) and reads keyboard input scancode by scancode

Display

• Limine framebuffer-based screen driver with scrollable output
• PSF font rendering (Zap font, pre-compiled into the kernel image)

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Project Structure

src/
├── kernel.c                  # Kernel entry point, init sequence
├── main_asm.S                # ISR/IRQ stubs, GDT helpers, SSE setup
├── screen.{c,h}              # Framebuffer display + scroll
├── VGA_screen.{c,h}          # (Legacy) VGA text mode driver
├── gdt.{c,h}                 # GDT + TSS setup
├── filesystem.{c,h}          # Custom disk filesystem
├── memory/
│   ├── pmm.{c,h}             # Physical Memory Manager
│   ├── vmm.{c,h}             # Virtual Memory Manager (4-level paging)
│   └── allocator.{c,h}       # Heap allocator
├── process/
│   ├── interrupts.{c,h}      # IDT init, exception/IRQ dispatch, syscall handler
│   ├── process_scheduler.{c,h}
│   └── program_loader.{c,h}  # ELF32 loader
├── util/
│   ├── io.{c,h}              # Port I/O, keyboard
│   ├── string.{c,h}          # String utilities
│   ├── panic.{c,h}           # Kernel panic
│   ├── limine.h              # Limine protocol types
│   ├── stdint.h              # Type definitions
│   └── util.{c,h}            # Misc helpers
└── compiled_programs/        # Pre-compiled userspace programs (shell, test)

programs/
├── shell/                    # Shell source (compiled separately, loaded as ELF32)
└── test/                     # Test program source

fonts/
└── zap.psf                   # PSF bitmap font (compiled into the kernel)

linker.ld                     # Kernel linker script
build_run.sh                  # One-shot build + QEMU launch script

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Building and Running

Dependencies

• gcc (x86-64)
• nasm
• cmake
• ld (binutils)
• xorriso
• qemu-system-x86_64
• OVMF firmware (/usr/share/ovmf/OVMF.fd)

Build and run

./build_run.sh

This will compile userspace programs and fonts, build all kernel sources via CMake, link the kernel ELF, assemble a bootable ISO with Limine, and launch QEMU with UEFI firmware.

QEMU is started with -S -s (paused, GDB server on port 1234). To attach GDB:

gdb -ex "target remote :1234" build/kernel.elf

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Memory Layout

Region	Address
Kernel ELF	0xffffffff80000000
HHDM (physical memory view)	0xffff800000000000
User processes	Lower half (per-process PML4)
Process stack	0x00007fffffffffff (grows down)

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License

Apache 2.0 — see LICENSE.