PDP-11 Overlays

[Jerome H. Fine]

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
from a lower region or the root.

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