It's a Real Mode (16-bit) 'OS', very similar to DOS. When you run in Real Mode, the BIOS's provides a few hooks for doing some basic 'syscall' type stuff like c

It's a Real Mode (16-bit) 'OS', very similar to DOS. When you run in Real Mode, the BIOS's provides a few hooks for doing some basic 'syscall' type stuff like changing the video mode, so you can get away with doing less in the actual OS code by doing it via the BIOS. That said, this code he has will run on any hardware that'll run DOS (for the most part). If he did this via 32-bit assembly he'd have a fair amount more code to get things done (Though he still uses very little hardware anyway so he could get away with not setting up most stuff).