I named the project VAX-11[4] or virtual address extension to the “11” to keep us on track. It was going to be an evolution on the “11”. The way we dealt with compatibility was to put a PDP-11 in the instruction set to run all the RSX-11 software. This gave us a tremendous head start on software as well as a base. VAX ran a lot of PDP software for a long time, including many compilers. This allowed us to get all kinds of software done in another environment and then simply moved over rather than having to do it all from scratch.
This story was repeated at Microsoft when Dave Cutler, a member of VAX A, went to Microsoft to invent Microsoft’s NT. He made that system also compatible with the PC hardware and all the apps. In that case, it was nearly impossible because of the lack of discipline and definition of the PC and the various interfaces because of the way the PC evolved in a chaotic, free market. Microsoft was left to make it all these loosely compatible components work! I claim nobody but Dave could have done this.