PDP-11/45 RSTS/E boot problem

I'm going to respectfully semi-disagree! I think that at this point there's a good chance we can localize to within a gate or two before we start applying test instuments. My thinking starts with two pieces of data; i) your discovery that when the MM trap happens, the end of the pure text segment contains a fragment of code memory where that _should_ have been is full of zeros - i.e. never been written into. The latter is, I think, due to the fact that Unix clears all of main memory on startup; I think it's just chance that that memory hasn't been used yet for something else, and is still 0's. (Unix does clear main memory in a few places during regular operation - e.g. when expanding the stack, the newly added area is 0'd - but in general, e.g. when swapping in a pure text, it doesn't seem to bother, which makes sense since it's all about to be over-written anyway.) Anyway, those two, together with my previous analysis that this was unlikely to have happened when the text was first being read in from the file, block by block, lead me to believe that the likely cause is that the BAR on the RK11 skipped up a whole bunch (setting the 04000 bit at some point) when it was reading the pure text back in from the swap, and skipped writing into that zero-filled block of main memory, putting the stuff that should have gone there up 04000, instead. (Why it's swapping the text back in is too complicated to be worth explaining here; anyone who _really_ wants to know should look here: http://gunkies.org/wiki/Unix_V6_internals in the last section, "exec() and pure-text images".) It's easy to confirm all these suppositions/deductions fairly easily, without having to connect up, configure, etc the LA: we can just stop the machine after the text is first read in (in xalloc()) from the file-system, and confirm that the text looks good there; if so, either the swap-out (albeit unlikely, since that doesn't account for the 0's) or subsequent swap-in had an issue. The latter would be easy to confirm: just halt the machine after the text is swapped in, and see what the RK registers contain. At that point, as I said, we'll know to within a few gates where the issue is, and then it'll be time to bring out the LA. Actually, a plain oscilloscope would do; it's interesting to recollect that these machines were designed and maintained without benefit of a LA, purely with an oscilloscope! We're so spoiled now! :-) Noel