sr (nobody.delete@this.nowhere.com) on September 21, 2021 10:48 am wrote:
>
> 80386äs MMU was great achievement, it combined both segmentation and paging beautifully together
> already at 1985. The real problem was that it was not used because it offer possibility to
> combine all segments to one linear address space. In hindsight was that a mistake?

No. The reason segments weren't used is that nobody wants to use them. They are a horrible model, and the whole "artificially partition the virtual memory" is just bad, bad, bad.

And yes, people are still trying to do it. They tend to call it other things ("capabilities" or other things like that - because "segments" has a bad name), but I think that's a desperate "we didn't learn our lesson, and maybe if we change things a bit it will work now".

And yes, the x86 segments were bad. They could have done more interesting things, and having segments be truly independent address spaces (ie give them separate page tables) could have made them much more powerful and useful. They had other issues too. But the fact that nobody else has ever done the concept well - whether you call it capability pointers or segments or something else - should give people pause.

Linus