Gooijen> RE: PDP 11/45 light chaser Gooijen> The drawback of the light chaser program that uses the RESET Gooijen> instruction is that the heads load and unload of the RX02 Gooijen> drive in my PDP-11/40. That clunck-clunck sound is not Gooijen> pretty for a longer while ... Gooijen> Hmm, I thought that the RESET instruction provides the ~100 Gooijen> ms delay so that the lights are visible (on the 11/35 they Gooijen> are LEDs anyway) but this instruction also asserts the Gooijen> RESET/ (BUS INIT-L ?) signal. That is what makes the RX Gooijen> drive clunk - at least that's what I thought. Exactly right. That's why I suggested WAIT -- it will pause until the next interrupt, and leave the INIT line alone. WAIT, of course, is what you'd find in the idle loop in a real OS: NULJOB: MOV R2,R1 ; Reset delay counter 10$: WAIT ; Now wait... SOB R1,10$ ; for specified delay ROL R0 ; Shift the lights BR NULJOB ; and continue paul