For fun, I tried building the kernel under V1 and booting it and it looks like it simply works. I did the following, outside the simulator: tools/assemv2 mkdir fs/usr/kernel cp -pi build/u?.s fs/usr/kernel tools/imgbuild boot/installboot tools/pdp11 boot/simh.cfg Then, I logged in as root and did: chdir /usr/kernel as u?.s chdir ../boot sh run msys2 u ../kernel/a.out More realistically, the kernel should be built in /usr/sys. James Markevitch