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From: ted@loft.tnolan.com (Ted Nolan <tednolan>)
Newsgroups: comp.os.linux.misc,alt.folklore.computers
Subject: Re: Unix on x86, Hmmm ... Downloaded Xenix - But It's *41* Floppies Worth
Date: 30 Aug 2025 17:39:49 GMT
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In article <108uhef$26t$1@news.misty.com>,
Johnny Billquist  <bqt@softjar.se> wrote:
>On 2025-08-29 22:52, Ted Nolan <tednolan> wrote:
>> In article <108su32$3e8$1@news.misty.com>,
>> Johnny Billquist  <bqt@softjar.se> wrote:
>>> But even more important, on the PDP-11, there is support for overlaid
>>> programs, which makes heavy use of the MMU. Basically, programs can be
>>> way larger than 64K code. You can place functions in different overlays,
>>> and call between them, and you can run up to many hundred of K of code
>>> very easy and straight forward on the PDP-11, and it's all because the
>>> MMU helps you out with it, moving the pages mapping around as needed.
>>>
>> 
>> My memory is that at leat for BSD Unix, overlays were not supported until
>> um, 2.9BSD I think, and that using them was not at all straight-forward.
>> It may have been easier for official DEC OSes...
>
>The timeline is the bit I'm not entirely sure about. It uses the 
>capabilities that were in the PDP-11 hardware all the time, though. So 
>it's an interesting thing to remember/compare with Unix on an 8086.
>
>As for ease of use, you got it backward. While overlays in DEC OSes 
>actually are way more advanced, and capable that overlays in Unix on the 
>PDP-11, using them on the Unix side is basically a no brainer. You don't 
>need to do anything at all. You just put modules wherever you want to, 
>and it works.
>
>With the DEC OSes, you have to create an overlay description in a weird 
>language, and you can't call cross overlay trees, and you need to be 
>careful if you call upstream, which might change mapping, and all that.
>None of those restrictions apply for Unix overlays. The only thing you 
>need to keep an eye out for is just that the size is kept within some rules.
>
>   Johnny
>

Since you've done it, I defer.  I just recall that when we got 2.9BSD, I
considered trying to port some big Vax program to the 11 and from reading
the man pages I got the impression I would have to get intimately familiar
with said program's call graph (which I definitely was not) to partition
out the overlays and ended up moving on to something else.
-- 
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What's not in Columbia anymore..