Sebastian Br?ckner wrote, in part: Do you have Volume 2 of the maintenance manual, or generally speaking a copy of the schematics? If so, and you know the nomenclature for the basic gates (AND, OR, NAND, etc.) and flip-flops, you can have a lot of fun tracing the various signals through the processor and gradually learn how it processes the various commands in hardware. (Appendix A of Volume 2 gives a foothold for getting started.) When you're new to it, tracing how the logic works can make your head hurt at first <g!>; but as you get more accustomed to it it'll put you on a much more solid footing for troubleshooting. (It also can become positively enjoyable. At least, _I_ think so. <g>) That said... I'd try other commands that affect the AC (TAD would be a good candidate) to see if they did anything out of the ordinary. Hope- fully they won't <g>, and that would narrow down the search. If the problem is confining itself to rotates, it would be worth trying each of the four rotates (RTL, RAL, RAR, RTR) to see if all of them are malfunctioning or just a subset of them. The rotate signals are all generated on the same card - the M617 at E31, if the 8/L and 8/I share the same backplane layout - so that might be a good starting- point to look for trouble. (Look for the page "Shift & Carry Gate Control" in the schematics. The rotate signals go from there to the "Major Register Gating" page, where they choose the rotate-data to be gated into the AC. It's really quite simple and elegant how they take the information from the major-register adders and just select the version that provides the rotation desired... Okay, okay, I'm waxing poetic. My apologies.) Hope this helps! -O.-