There was very little OS needed. Most of the keyboard and joystick reads were single memory locations, there was no ROM support for graphics or sound, and most games bootstrapped in a fast loader anyway (later for copy protection as well). If necessary, a program could flip the ROM back into memory space to perform I/O, then flip it back out to use the RAM underneath.
digi_owl|8 years ago
May be of interest for those that want more details.