Something wonderful has been done by the people over at www.visual6502.org. Something that several years back would have been thought impossible or at least implausible. Looking at it makes me realize how far computers have come.
The microprocessors used in the earliest home computers (we are talking late 70s, early 80s), were wonderful bits of engineering, but by todays standards very simple. It was still possible for hobbyists to understand the CPUs fully and to program them in assembler. Actually the whole architecture of the home computers themselves was simple and could be understood. The chips used for input and output, the operating system and its calls, all could be described in a single book.