Old computers appear to be much simpler than the computers we have nowadays. I am thinking of learning to use and program an older computing platform to better understand low level programming in a constrained environment. I would probably appreciate a computer more if I understood nearly every single part of it (similar to the enjoyment of understanding how to implement the Scheme programming language). What computing platforms would you recommend? I am currently leaning towards the Commodore 64 and/or MS-DOS because of their large enthusiast communities and the abundance of old software produced for these platforms.