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From: Michael S <already5chosen@yahoo.com>
Newsgroups: comp.arch
Subject: Re: 80286 protected mode
Date: Tue, 15 Oct 2024 11:59:27 +0300
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On Tue, 8 Oct 2024 21:03:40 -0000 (UTC)
John Levine <johnl@taugh.com> wrote:

> According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:
> >>If you look at the 8086 manuals, that's clearly what they had in
> >>mind.
> >>
> >>What I don't get is that the 286's segment stuff was so slow.  
> >
> >It had to load the whole segment descriptor from RAM and possibly
> >perform some additional setup.  
> 
> Right, and they appeared not to care or realize it was a performance
> problem.
> 
> They didn't even do obvious things like see if you're reloading the
> same value into the segment register and skip the rest of the setup.
> Sure, you could put checks in your code and skip the segment load but
> that would make your code a lot bigger and uglier.
> 

The question is how slowness of 80286 segments compares to
contemporaries that used segment-based protected memory.
Wikipedia lists following machines as examples of segmentation:
- Burroughs B5000 and following Burroughs Large Systems
- GE 645 -> Honeywell 6080
- Prime 400 and successors
- IBM System/38
They also mention S/370, but to me segmentation in S/370 looks very
different and probably not intended for fine-grained protection.

Of those Burroughs B5900 looks to me as the most comparable to 80286.