Jay: Try this: Location Contents Opcode Comment 001000 012700 mov #1,r0 Load 1 into R0 001002 000001 001004 006100 rol r0 Rotate R0 left 001006 000005 reset Initialise bus (70ms) 001010 000775 br .-4 Loop back to 'rol r0' from: http://www.psych.usyd.edu.au/pdp-11/11_40.html#chaser Not sure if it works on all models. De