PDP-11 Overlays

Johnny Billquist bqt at update.uu.se
Mon Sep 21 21:16:11 CDT 2015

On 2015-09-22 02:09, Jerome H. Fine wrote:
>  >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?


Chapter 3 and 4 give the basics. Chapter 5 through 9 give details in 
relations to what the respective chapter is about. Figures on pages 3-5 
and 3-6 might give you a goo starting idea of what you can do.

> 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?

There are no checks that the tree is intact at runtime. This is checked 
during the task build. You cannot refer to symbols that are outside you 
call tree if you look upwards.

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

Gave it above.


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

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