PDP-11 Overlays

Recently, there have been a number of references to using overlays on the PDP-11. There have also been strong suggestions that overlays were structured differently under the 3 operating systems: RSTS/E, RSX-11 and RT-11. Obviously, I understand how RT-11 overlays were set up, but for those readers who don't: ROOT - contains overlay code subroutines and data tables - data used by more than one overlay FIRST Overlay Region - size of the largest overlay in the region - one or more overlays and the data used by just that overlay SECOND Overlay Region - size of the largest overlay in the region - one or more overlays and the data used by just that overlay THIRD Overlay Region - size of the largest overlay in the region - one or more overlays and the data used by just that overlay FREE MEMORY Any overlay could be called from any location. About the only requirement was that the calling instruction code (specifically the code which followed the calling instruction) had to be in memory when the code returned from the overlay. In practice, the usual protocol was that an overlay in a higher region was only called I understand that RSTS/E and RSX-11 were a bit more complex. Can anyone briefly summarize and also provide a link to the details in the appropriate manual if it is available on the internet? Jerome Fine