So I lied: it turns out that some of the later tests (in the extended tests - JSR, memory) do in fact halt on error. I realize it's not needed immediately now (Josh having solved his problem), but I've got the low part (diagnostics and TA11 boot) done, working on the top part now. Here's what I have so far: http://ana-3.lcs.mit.edu/~jnc/tech/pdp11/M9301-YA.mac I haven't densely commented the CPU tests as they seemed, at first glance, pretty non-complicated. The memory diagnostics are pretty inscrutable, I don't really understand them yet (may not bother). The top part (console and othr boots) - hoo boy, talk about 'sphagetti code'!! I think there's a picture of this in the dictionary when you look that term up. Very, very ugly code. That part may take a while... Noel