Sort of. Stop-A drops you into the firmware prompt (OpenBoot), suspending the OS in the process. OpenBoot, of course implements a forth interpreter shell.

Ahh, the good old days... I once implemented a nice little boot device selecting utility in for for some of my Suns. All written in Fourth, and executed by the firmware on every boot.

Actually, I did a lot of RPN development on Suns. Not in Forth but in PostScript within NeWS/OpenWindows developing front ends for large scale Lisp applications.

Yes, I believe that's true. Stop-A pops you to a prompt (which I believe is all running Forth) where you can do all kinds of cool stuff. It completely suspends the OS. I remember I was once able to TFTP in a custom boot logo at that gets loaded right into the Bios.