> I recall a long time ago reading something about potential uses of a 128-bit CPU architecture with 128-bit addressing.

Go back to the IBM System/38 and the AS/400 and that is exactly what you have. Programs are actually 128 bit oriented bytecode, so the actual CPU architecture isn't visible, doesn't matter, and has been upgraded many times over the years.

They also did virtual memory differently. Everything is in the same address space, which means everything can (after access control) communicate and cooperate. Task switches are also very fast. They also use tagged pointers, but it is a single bit outside of what the apps can see, and marks a pointer as modified. Consequently pointers can be given out, and the kernel knowns if the app has tampered with them.

I highly recommend reading the architects book about how all this stuff worked, mixed in with how business machines should behave. Many things are very different than conventional Unix/Linux/Windows NT kernels, and a good way of broadening your mind. The second edition book is good - later editions may have removed earlier interesting detail. http://www.amazon.com/Inside-AS-400-Second-Edition/dp/188241...

> 2 bits of tags, and they're thinking general purposes (e.g. GC) as well as security. That's big, something we haven't seen in hardware since Lisp Machines, to my knowledge.

The IBM AS/400 or whatever it is called this week uses tags too for security. (A bit is flipped if a pointer is written too thereby preventing changing pointers.) The PowerPC chip has optional support for tags because of this.

Source: http://www.amazon.com/Inside-AS-400-Second-Edition/dp/188241...

The apps and most of the OS are distributed as the equivalent of high level byte code using 128 bit pointers. Once run it gets translated into the local machine code.

For anyone unfamiliar with the AS/400 and predecessors I highly recommend the expensive book by Frank Soltis who is largely the architect of it. Many things were done very differently than other operating systems and applications. http://www.amazon.com/Inside-AS-400-Second-Edition/dp/188241...

Indeed, OS/400 is just wonderfully advanced under the hood. Get one of the cheap used copies of this book for a lot more: http://www.amazon.com/Inside-AS-400-Frank-Soltis/dp/18824196...