Although the engineering costs of a chip are extreme some obsolete chips have actually had their architectures published many years after the fact. It is important to realize that open source hardware is a recent concept, and does not yet reach down to the chip level. The closest we come is the logical description, which describes how the circuit works in terms of logical signals. This is, however pretty well what you would need for a first level of utility. However there are always some gotcha's and errata where circuits don't quite work as described sometimes this gives us new features, and sometimes it just gives us errors in our designs.

http://www.visual6502.org/ has a complete physical model of a simple processor that runs in the browser.

As far as I know, ARM sells processor designs. I can understand that no processor manufacturer gives out their designs for free and that some prefer to keep them under wraps. The processor design is the most important advantage over other manufacturers. Intel repurposes some of their old designs for new processors: The cores of the Xeon Phi are based on a Pentium from around 1995! It would be stupid to give that to their competitors.

Why does not then then,Intel,say discloses the internal circuitry of Intel 4004 which is now obsolete , I guess.

If one could get that circuit one can more logically understand the functioning of the processor instead of cramming some hard facts such microprocessor sends acknowledgement signals ( it may appear to happen by itself to ourselves because of no knowledge of internal logic circuit).

But then what you are asking for presents a quandary!

I mean on one hand, if indeed the Intel 4004 is entirely obsolete, what you learn from it would be limited and possibly even irrelevant.

On the other hand, the company may need/keep the designs for future designs like the Phi as Dieter says, which by giving up detailed workings would be bad business, potentially destroying their competitive advantage.

Things will become slowly more open source as time progresses, but if you wish to work in detail on the structure and design of the workings of complex microprocessors, rather then fundamental contributions such as deposition techniques, you really need to work for a bluechip company, as they are the ones which apply wider research findings to the product..

Can I just ask, why would you want to know that? Do you want to build your own and need a place to start? If so, would it not be easier to go and work for intel or microchip - to get the info from the source? Anything published often contain errors because the version published is old and the competitive advantage is to great to risk as mentioned earlier.

From reading quite a few papers and trying replicate results I find that there are quite a few errors and I imagine in any published hardware design it probably not any better.

Maybe I am asking a silly question here, but I am just curious.

Simply talking, everything has to do with intelectual property (IP) of each microprocessor design. It's obvious that a serious company will never provide such in-depth information, most of the times copyrighted into associated patents. In case that someone would like to know more details on how a microprocessor architecture works, then all public available information would be more than enough in order to achieve this. The rest is knowledge necessary by VLSI designers or similar.

the really question is > What's the importance of knowing the interior design !, it is very complex and can't be easily absorbed ?

I agree that there is little to be gained by looking at a schematic, it would be too complex to comprehend as such, except perhaps for relatively simple processors. The functional architecture is the more important. I'd be curious to know whether for example the circuit diagrams of the the Atmega-328 are available, but in any case the functional architecture is the more important...