QNX 4 was mostly written in C and the demo disk fit on a single floppy and included the whole system including a TCP/IP stack, drivers for a few common network

QNX 4 was mostly written in C and the demo disk fit on a single floppy and included the whole system including a TCP/IP stack, drivers for a few common network adapters, a GUI, a web browser (with javascript), and various demo applications. Granted, I think it's probably easier to limit size (and more importantly, complexity) by using assembly language, it's by no means necessary. Lately I've been exploring minimizing complexity by using less-capable text editors like ed and notepad.