PDP-11 Overlays

Jerome H. Fine jhfinedp3k at compsys.to
Mon Sep 21 19:09:11 CDT 2015

 >Paul Koning wrote:

>>On Sep 21, 2015, at 5:16 PM, Jerome H. Fine <jhfinedp3k at compsys.to> wrote:
>>Another important aspect is that RT-11 has a few extra instructions
>>in the overlay handler which determines if the overlay is already in
>>memory. ...
>>My question is if TKB overlays also have these extra instructions?
>Yes, it also tracks if something is already resident.  I haven't looked into the details.  (The only overlay machinery I studied to any significant extent is the RT-11 one.)
Is there any documentation as to the actual details?

As I mentioned, For the RT-11 overlay handler integrated with
the LINK program (both are essential), when those aspects
are combined with overlay regions defined by the maximum
size of the overlay in a given region, the first word of every
region is devoted to the overlay (*6) number.  Since that
location is not available to the overlay and is unique to the
overlay, the overlay handler can check to determine which
overlay is resident in the region.

With overlay trees in TKB, that method can't be used unless
the line up the tree is intact.  I am attempting to visualize the
code which could determine if that is true, but at the moment
it escapes me.  Can anyone provide any details?

Otherwise, I can see where the first overlay in any tree can
be determined, but unless a given tree is enforced, there does
not seem to be any way to check if overlays further up the tree
are also resident?

Is there a manual available on overlays using TKB that is
on the internet?

Jerome Fine

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