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.
In later days people started to emulate the chip's microcode in software on more powerful, modern computers, allowing us to play the old games and generally be nostalgic (e.g. xformer or atari800). Still these emulators were just that; they emulated the behaviour of the chips, like a computer program emulating a psychiatrist.
Over at visual6502 they have reconstructed the MOS 6502 chip by photographing the actual chip. All its connectors, gates, wiring. Then they created a simulation of the chip as it operates. You can actually see the logic states changing as the simulation runs. Registers, counters, all can be studied intimately. Mesmerizing to follow.
Their simulator actually runs inside the browser, using JavaScript and the new canvas features of HTML 5. So it is 2011 and we are simulating the earlier chips completely in the browser.