Hi, Rich. Thanks for broaching an interesting topic of which I'm rather fond... :-) On 2010-10-27 00:16, Rich Alderson<RichA at vulcan.com> wrote: Sorry, but I'll have to totally disagree with you, and so does all the processor handbooks for the PDP-11. :-) Virtual addressing have nothing to do with demand paging, or even paging (even though I know Wikipedia uses that very broken definition (or atleast used to)). A virtual address is... Well... Virtual. It does not match a physical address. Instead it is an address into an imagined memory space, virtual memory. This virtual memory space would appear as just your own in a system. Another process in the same system have another virtual address space, using the same virtual addresses you are, but you are not actually referring to the same memory. It's virtual. :-) A virtual address gets translated into a physical address by an MMU. The "Virtual Adress eXtension" (aka. VAX) was a virtual address extension to the PDP-11, nothing more. The PDP-11 already have virtual addressing. Pick up any PDP-11 processor handbook and read about it yourself. Or check http://www.google.ch/url?sa=t&source=web&cd=2&ved=0CB4QFjAB&… , for the (arguably) yucky PDP-11/40 online. Look at chapter 6 - Memory Management. On a PDP-11, an address can either be physical, in case the MMU is turned off, or else virtual. The virtual address gets translated to a physical address by the MMU, by using the page table and some simple adders. In case you run on a PDP-11 with the MMU turned off, the 16-bit address just gets extended to 18 (or 22) bits by using zeroes for the high bits of the physical address. if the MMU is turned on, the virtual 16-bit address is translated into a 18 or 22-bit physical address. VMS added demand paged virtual memory. There is nothing preventing you there is nothing preventing you from writing an OS for the PDP-11 which would do demand paging (assuming you have a PDP-11 model with the neccesary features implemented in the MMU, which not all have). No additional comments. And you know the PDP-10 much better than I do. :-) Sorry, just as with you, I don't know of anyone who did that. But I can tell you how you should do it, in case you'd be interested in actually doing it. There is nothing in the hardware that prevents you. The biggest reasons why noone did it are that: 1) The physical memory is much larger than the virtual memory on a PDP-11, meaning that it's normally not a big problem to actually have all virtual memory always paged in when a process is running. You will never see working sets that are larger than what can be held in memory at one time, thus one big reason for demand paged memory is lost. 2) The page size of the PDP-11 is 8K, which at the time was pretty big. With that size, you actually only have eight pages for a process. Now, why do demand paging, when you can at the most get 8 page misses before everything is paged in? You might as well preload them, and skip the whole demand paging mechanism. 3) Not all models of the PDP-11 MMUs support all the fancy stuff you need. People weren't that interested in writing software that only worked on some models (that did eventually happen in other ways anyway, but only by slower evolution at a later stage). So, no, I cannot point you to any system that implemented demand paging. Maybe someone did it without us knowing about it. The fact still stands, though. The PDP-11 allows you to do it. Is it then relevant if someone actually did it or not? Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt at softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol