[Simh] Compatibility you can use Was: VAX/VMS

[Dave Wade]

> >
> > You can't seriously mean that you think that a 32-bit application and
> > a 64-bit application would be expected to be compatible with each other?
> > I would expect the 32-bit code to work in 32-bit mode, but having it
> > work if you are in 64-bit mode is a ridiculous expectation.
> 
> Really? It works fine on IBM's z/OS.
> 
> It seems ridiculous to me that you think it shouldn't. This is what I have been
> saying. IBM moved from 24 bit to 31 bit to 64 bit and everything still works.
> No expanded footprint, no duplicate libraries, no problem.
> 

That’s not (quite) true. As I said before problem state code works fine. Anything that uses supervisor mode will likely fail. 

It took IBM a lot of work, microcode (SIE instruction) and a new Hypervisor to get from MVS/SP to MVS/XA.
I am told VM/XA only exits because they needed it to debug MVS/XA.

There is much more I info about IBM and especially VM's history on Melinda Varian's home page:-

http://www.leeandmelindavarian.com/Melinda/

and as for backwards compatibility the book "The soul of a new machine"

http://www.amazon.co.uk/The-Soul-Machine-Modern-Library/dp/0679602615 

has some interesting observations about building a new machine...

Dave
G4UGM