Re: 80286 protected mode

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.