Outside of the instruction set of the 6502 CPU there was zero overlap between the C64, the Atari 400/800 and the Apple II. There were no common tools to speak o

Outside of the instruction set of the 6502 CPU there was zero overlap between the C64, the Atari 400/800 and the Apple II. There were no common tools to speak of, no common data formats, no compilers (compiler? pshaw! sub-optimal code!), no common memory map, no common audio hardware, and between the Commodore/Apple/Atari of the time, there wasn't any commonality in how the screen memory could be read or written. The BBC Micro (also 6502) and the Apple II did share a common video chip in the form of the MC6845, which was a particularly strange beast to program for, but again, the overlap between those two systems was minimal at best. I wrote code for published games and applications for all of those systems, and more besides like the TI99/4a, including a couple of C compilers.