Re: basic questions on MPS

[Helmut Eller <eller.helmut@gmail.com>]

On Sun, Apr 28 2024, Richard Stallman wrote:

> Do the CPUs we use nowadays have hardware features for that?
> What aout the CPUs in the newest non=backdoored machines, such
> as the Thinkpad t800 and x200s?

Yes, of course.  An MMU is needed to implement virtual memory.  Without
MMUs, operating systems like Unix would not be possible. 

>
>                                                  Software barriers are a
>   > data structure that must be updated on every read (or write) operation.
>   > This requires compiler assistance or disciplined programmers.
>
> That sounds rather slow.  But maybe you have practical experience to
> measure it.  Can GCC insert this cod eautomatically?  Or does Emacs do
> it manually?  Either way, how much slowdown does it actually cause?

There are clever ways to do it.  Like putting part of the data structure
in the lower bits of pointers or if an object is accessed in a loop, the
update only needs to be done once.  It's more practical for higher level
languages, like Java or Haskell, that don't expose addresses directly to
programmers.  For GCC and Emacs (with MPS) a hardware based solution is
probably better.

I'm no expert on this matter.  That's why I asked the question in the
first place.

Helmut