It sounds like the CPU may be too broken to run diagnostics. Why don't you try writing some small programs to test various bits and pieces of the CPU (e.g. add instructions, etc), and toggle them in, and try and find what's busted that way? (Although for 'add', if the thing's that broken, I'm amazed it even boots.) Here's an example: http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/tests/print.s It's probably too long to toggle in, but you could steal pieces from it and try them. I'd probably try out the shift and multiply instruction to start with; trying to figure out what's probably not used during booting, but could be used during number printing, those are two good bets. Anyway, there's no point to running diagnostics until you get that first problem fixed.