On Fri, 16 Feb 2018 14:51:27 -0500 Clem Cole <cl...@ccc.com> wrote: > curmudgeon warning below..... > > On Fri, Feb 16, 2018 at 11:06 AM, Ethan Dicks <ethan.di...@gmail.com> > wrote: > > > > > I started on a VAX with 2MB of physical memory in a 16MB physical > > address space but with 4GB virtual addresses. Switching over to the > > PDP-11 was odd from that. > > > > ?Sigh... I fear that is a fault of your education. > > If you ask many (younger) programmers what VM was designed to solve > (particularly those that never memory constrained systems such as you > get in 8, 12, 16 or 18 bit processors), they will tell you 'So can > have more addressable memory.'? The problem is said programmers > never experienced or learned about overlays. Conceptually, a PDP-11 > can allow a lot more than the 64Ks physical limit by 'swapping out' > and 'overlaying parts' and calling subroutines through > 'thunks' [which to quote my old friend Paul Hilfinger from page 427 > of his book: *"an onomatopoetic reference to the sound made by a > pointer as it moves instantaneously up and down the stack"*]. A > process has be allowed to be larger that 64K, but only 64K (128K on > seperate I/D systems) in the set up memory maps at a time. If you > need to call a subroutine to (optionally) bring in the routines and > its data into memory if it is not already there, and then set up the > map to point the routine in question. > > BTW: If you play with BSD 2.11 or the like, it uses overlays to allow > programs to grow in size. This was needed as people started to try > to move features from 4BSD and later back to the PDP-11. At this > point, I believe you must have what was sometimes referred too as > '17th address bit > - i.e. I/D space which gives you 128K bytes of mapped in memory at a > time. But you can (with care) let you programs grow. > > The point is that VM is a mechanism* to automatically manage > overlays*. The implementation of this management gets easier if there > are more address bits than physical address bit, but that is the key > item that is happening. > > Sadly, since people stopped learning about overlay management, the > context of what it was doing under the covers was lost.
When I teach virtual memory, I start by talking about the 'old days' and overlays. I remember Overlay Description Language on the PDP-11! _______________________________________________ Simh mailing list Simh@trailing-edge.com http://mailman.trailing-edge.com/mailman/listinfo/simh