Yeah, I'd read that - I was hoping for some actual technical info on the KS11, though. (I'm assuming he has given the name there correctly, or if his memory has dropped a bit - a thing which human memories do! :-) - since I've never been able to find a single mention of it, including in the Spare Module Handbook, which covers other Special Systems products). The best description is in the DECSystem-10 Hardware Reference Manual (mine is EK-10/20-HR-001, but alas that version appears not to be online - I'll scan my copy and put it online when I get a chance.) This version: http://pdp10.nocrew.org/docs/ad-h391a-t1.pdf does appear to cover it: pp. 5-38 through 5-40 (pp. 352-354 of the PDF) for the KA10, and pp. 5-15 to 5-30 (pp. 329-344) for the KI10. The KA10 provided one (optionally) two base/bounds register pairs, where the base register contains the location in real memory. With two pairs (the second one applied to high user address space), the high part could be write-protected, for sharing pure code. The KI10 provided something similar to this, but more complicated; it included paging, but also something called User 'Concealed', which allowed proprietary subroutine packages to be used, while hidden from the rest of the user's code. Almost certainly the KA10. Hard to say. Kernel mode might or might not have mapping, and User mode might have provided one, or two, segments; the KA10 did have an option for single-segment. Sorry, what's a KL-11? The only 'KL11' I know of is the serial line board (M780) which was the predecessor to the DL11. Noel