@dosnostalgic VAX/VMS circa 1978 used to boot with PDP-11 RSX-11M compatibility mode available and a fair chunk of the apps in the early VAX/VMS versions were RSX-11M apps running in compatibility mode.
The VAX-11 boxes supported PDP-11 instructions in hardware.
You could run your existing PDP-11 RSX-11M apps directly, too.
That all ended at VAX/VMS V4.0 (~1984), and with then-new VAX models after VAX 8600.
VAX 8600 was originally to be named VAX-11/790, but marketing marketed and dropped the -11 with the “architecture for the ‘80s”.
PDP-11 RSX-11M compatibility mode became a separate product, and the PDP-11 instructions were emulated, and the -11 was dropped from VAX.
Technically, an LSI-11 console processor booted RT-11 from the 8” console floppy which then booted the VAX-11/780 (organizationally within he hardware, the VAX was an enormous LSI-11 peripheral) which ran VAX and PDP-11 instructions and which could run simh emulator to emulate PDP-11 running RT-11. If the LSI-11 failed—as happened on a couple of occasions—the VAX could continue to run. Just not reboot.
The approach Apple used for migrations with Rosetta and Rosetta 2 was far smoother.
Yeah. Fun times. When it all worked.
There are shenanigans in newer boxes too, but they’re usually somewhat better hidden.