CoreOS

CoreOS is an experimental AMD64 monolithic operating system written in Rust with a small C-based UEFI bootloader.

The project focuses on low-level OS architecture: bootstrapping, paging, memory allocation, process scheduling, syscall handling, and a small ring 3 userspace.

Status

Active development.

The current tree boots through UEFI into a Rust no_std kernel, runs an embedded syscall test suite in ring 3, and launches a userspace shell from an in-memory filesystem.

Features

Boot

• UEFI bootloader written in C
• Custom boot protocol passing framebuffer and memory map
• Loads kernel, font, and initial userspace ELF
• Hands off into a higher-half Rust kernel

Kernel

• Rust no_std kernel
• Physical memory manager (bitmap)
• 4-level paging with direct-map and higher-half kernel mappings
• Slab heap allocator
• Preemptive round-robin scheduler
• Task and process infrastructure
• ELF loader for userspace programs
• Syscall entry path with file, process, memory, and time primitives

Hardware Support

• PIT timer
• PS/2 keyboard driver
• RTC clock
• Serial debugging (COM1)

Filesystem

• In-memory RAM filesystem
• VFS-style facade over RamFS
• Basic file operations (open, read, write, create, remove, stat, lseek)
• Pipe-backed file descriptors

Graphics

• Linear framebuffer rendering
• PSF font support
• Kernel header bar and userspace text console

Userspace

• Ring 3 execution
• ELF program loading
• Interactive shell
• Minimal libc shim
• Syscall regression test program

Requirements

• Rust nightly
• clang + lld
• QEMU
• OVMF (UEFI firmware)
• mtools (mcopy, mmd, mkfs.fat)

Build & Run

make
make run

make builds the user programs, kernel, UEFI bootloader, and a bootable FAT disk image at build/coreos.img.

Validation

The normal serial boot path currently does the following:

• boots through OVMF/UEFI
• initializes memory, paging, interrupts, syscalls, and tasking
• runs the embedded syscall_test userspace binary
• starts the userspace shell

The syscall test covers basic process, FD, pipe, memory, timing, and filesystem operations.

Project Layout

bootloader/   UEFI loader and boot protocol definitions
kernel/       Rust kernel source
user/         Userspace programs, startup asm, and libc shim
assets/       Fonts and static resources
build/        Generated EFI image and disk image artifacts
Makefile      Top-level build and run targets

Internal Layout

Notable kernel areas:

• kernel/src/main.rs: boot orchestration and subsystem bring-up
• kernel/src/arch/amd64/: GDT, IDT, paging, and CPU helpers
• kernel/src/mem/: PMM and slab allocator
• kernel/src/proc/: ELF loading, tasking, process state, FDs, and VM metadata
• kernel/src/syscall/: syscall entry and domain-specific syscall handlers
• kernel/src/drivers/ and kernel/src/hw/: framebuffer, serial, PIC, PIT, keyboard, RTC
• kernel/src/vfs.rs: VFS-facing wrapper around the in-memory filesystem