I don't have any experience making operating systems, but you could look at FreeDOS? It's open source, written in C, and most importantly 16-bit like you want. Most people likely go straight for 32-bit nowadays.
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I am looking to do an OS with a GUI, not one that is purely text. I want to do something user friendly. I am also thinking about having it run on it's own Kernel cause there is hardly any executables for Linux flavors. I do 16 bit cause that's where Super Mario and Mario Kart, Sonic, DOOM and some very famous games looked good. The SNES classic is also the next console to come out, and it's 16 bit. That's kinda why I am doing 16 bit right now other than it's simplicity. I will include 32 bit support later on, but kinda sticking with this for now.