... The doc says that RT and Unix do it differently (no WAIT). I haven't seen the Unix code but I did see the one for RT (F/B version), and indeed, no WAIT instruction there. I'm not sure why not. RT11 S/J seems to just be full of spin loops, no central idle of any kind that I can see. paul Paul, I think that depends on whether or not the idle loop pattern (for the console light display) is being used. The lights pattern most certainly makes use of the WAIT instruction. The SJ monitor is a completely different beast than FB and friends - no question about it! -Steve I was referring to RT V2, which is what I have at hand. But it sounds like it's still the same in later versions (except for having sysgen options -- no sysgen in V2). The thing to remember is that there are multiple ways to tweak the lights, at least on later PDP11s -- WAIT displays R0 (if the Data display switch, if there is one, is set to "Data Paths"), while writing to 177570 sets the Display register (which can also be displayed by setting the switch to Display Register). But some older machines don't have a display register and don't have that switch. The 11/20, if I remember right, always displays R0 at a WAIT and ignores writes to 177570. I forgot what the 11/05 and 11/10 do on a WAIT (since they only have a single display, not a separate address and data display). paul