Back in high school I implemented a version of exec.library that ran in real mode on my 286 and was built using Microsoft C 5.1 for DOS. Got it booting off a floppy and taking complete control over the system complete with preemptive multitasking. After that I started implementing a couple of the device drivers and ended up getting a simple console working with serial.device to work as a terminal emulator. Eventually I got drawn into Linux once I had a 386 and could run a real 32 bit OS. It was a great learning experience to disassemble the KickStart ROM on an Amiga 500 and figure out how things worked. The modular system used in KickStart to discover the modules was elegantly simple and worked so well. KickStart was tight and simple code. I'd like to thank the folks who did that work for teaching me so many important concepts about software during my teenage years!