It could be either. One possible cause of the symptoms you're seeing is that address line A04 on the bus is being held to one value (high or low) by something on the bus, so that whether the CPU tries to set it to 0 or 1, it has no effect. Also, of course, there could be some fault in the CPU, so that when it tries to do something with address 0, it gets 020 (or vice versa). And similarly for the memory. I'd try to write a small (two instruction) loop that sets that address line high/low (e.g.: 5037 CLR @#1020 1020 775 BR .-4 and look and see if that address bit is flickering on/off on the UA11 (it will be on, but dully; constant assertion is bright on, constant de-assertion is full off). If so, the problem is almost certainly in the memory; if not, it could be either. Sorry, didn't follow that? Did you mean that if you store 02000 in location 0, all other locations now report the 02000 bit set? Noel