I agree with Rick. The most likely suspect seems to be a corrupted RK05 image. But not to worry. Do you have any important files on that OS/8 SYS: / DSK: RK05 disk image? If so, you'll want to set that image aside first. Assuming you know your way around Raspbian OS
1) Go to the directory:
2) Rename the file
to something else, like
If there's nothing important on that disk image, you can skip steps 1) and 2).
3) Go to the directory:
4) Copy the file:
into the directory:
That will reset you back to the original OS/8 SYS: / DSK: RK05 image that was first installed with the PiDP-8/I software.
The other thing to pay attention to is what SIMH tells you when you do the CTRL/E after it hangs:
Simulation stopped, PC: faaaa (iiiii)
What are faaaa and iiiii? faaaa is the memory address and iiiii is SIMH telling you (one of) the instruction(s) that it's hanging up on. It's possible to use that to figure out where in the boot process it's hanging up (to some extent, anyway).