CassiOS

A microkernel operating system targeting i386 (32-bit x86), written in C++ and assembly. Boots via GRUB using the Multiboot specification. Built from scratch with a custom modular standard library -- every component from the GDT to the IPC layer is hand-written.

Features

• Microkernel architecture -- kernel provides IPC, scheduling, memory management, and interrupt routing; everything else runs as userspace services
• Synchronous IPC -- message passing (send/receive/reply) and fire-and-forget notify between processes
• Userspace drivers -- PS/2 keyboard, PS/2 mouse, VGA terminal, ATA PIO disk, all as separate services
• Userspace services -- nameserver for service discovery, FAT32 filesystem with long filename support, interactive shell with 16 commands
• Higher-half kernel mapped at 0xC0000000+ with per-process address spaces
• Preemptive round-robin scheduler driven by PIT timer (~1000 Hz)
• ELF32 loader for statically linked userspace binaries via GRUB multiboot modules
• Syscall interface via int 0x80 (IPC, memory, process control, system info)
• Physical memory manager, kernel heap with new/delete, paging
• Persistent FAT32 filesystem -- 32 MiB disk image, on-demand FAT via LRU sector cache, directories, LFN, case-sensitive
• Modular standard library -- 17 modules under libs/ (std:: namespace), from core types/mem/str to service clients (std::vga, std::vfs, std::kbd, etc.)
• Two-tier test framework -- 247 kernel unit tests + 97 userspace integration tests, run headlessly via QEMU

Prerequisites

• g++, as, ld (GNU toolchain with 32-bit support)
• qemu-system-i386 (for running)
• grub-mkrescue and xorriso (for building the bootable ISO)
• dosfstools and mtools (for FAT32 disk image creation)

Build

make kernel    # compile and link the kernel binary
make run       # build and launch in QEMU with all services
make test      # build and run all tests (kernel + userspace)
make iso       # build a bootable ISO
make clean     # remove build artifacts