Near the end of the 1970s, DEC PDP-11 customers were reaching the limit of what the PDP-11 architecture could support: PDP-11 is a 16-bit architecture: the firs

Near the end of the 1970s, DEC PDP-11 customers were reaching the limit of what the PDP-11 architecture could support: PDP-11 is a 16-bit architecture: the first generation could address a whopping 56kb of memory (the top 8kb of the memory map was for I/O). Over time, extensions were added to extends that out to 18 bit addressing, and finally 22 bit, for a total of 4Mb. But programs could still only directly address 16-bit chunks at a time, and would have to twiddle the memory management hardware to access other chunks. VAX was the "Virtual Address eXtension" to PDP-11, which was released right around 1979/1980, was a 32-bit architecture, and additional processor instructions. VAX ended up being a much different beast, but you could still see that it was extended from the PDP-11 platform. DEC made a lot of tooling to make the transition from PDP-11 to VAX as smooth as possible for developers- they provided compilers that were source-compatible, etc., for FORTRAN, COBOL, C, BASIC, etc.