Point in case: Ben Eater recently uploaded a video where his "BIOS" consists of two I/O functions and compiles into less than 250 bytes. Double that if you count his port of WOZMON. We can argue later if that constitutes an operating system - I mean, it's not POSIX compliant but it does operate.

https://www.youtube.com/watch?v=0q6Ujn_zNH8

Edit: watch that space. The venerable MS BASIC is on the way after this.