TyOS is a really (really!) simple operating system prototype demonstrating the two stage loading.
First stage, the boot loader, fits entirely within the 512-byte master boot record of a USB stick. It is meant to be loaded through legacy BIOS boot method and execute in real mode on any x86 platform.
Upon loading, stage1 loads the second stage right after the boot loader. The second stage does nothig, except by printing a welcome message. Everyting runs in x86 real-mode (we're not going 32bit yet).