ScaraOS is a 32-bit, multiboot-compliant, monolithic OS kernel. It has the beginnings of a paged VM system and VFS. It supports PCI, DMA, AT floppies (read only), EXT2, and can do all the low-level stuff expected of an OS kernel (program the PIC, handle interrupts, control the timer, etc.). It was written primarily to learn OS fundamentals. It boots using any multiboot bootloader, and it has been tested with grub on qemu and KVM. Bootable floppy images are available.
