Re: 80286 protected mode

mitchalsup@aol.com (MitchAlsup1) writes:

On Wed, 9 Oct 2024 20:22:16 +0000, David Brown wrote:
>

On 09/10/2024 20:10, Thomas Koenig wrote:
>

David Brown <david.brown@hesbynett.no> schrieb:
>

When would you ever /need/ to compare pointers to different
objects?  For almost all C programmers, the answer is "never".

>
Sometimes, it is handy to encode certain conditions in pointers,
rather than having only a valid pointer or NULL.  A compiler,
for example, might want to store the fact that an error occurred
while parsing a subexpression as a special pointer constant.
>
Compilers often have the unfair advantage, though, that they can
rely on what application programmers cannot, their implementation
details.  (Some do not, such as f2c).

>
Standard library authors have the same superpowers, so that they
can implement an efficient memmove() even though a pure standard C
programmer cannot (other than by simply calling the standard
library memmove() function!).

>
This is more a symptom of bad ISA design/evolution than of libc
writers needing superpowers.

Throughout this long thread you keep missing the point.  Having
different instructions available doesn't change the definition
of the C language.  It is possible to write code in standard C
(which means, C that does NOT depend on any internal details of
any implementation) to copy bytes from one place to another with
semantics matching those of memmove(), BUT that code is clunky.
To get a decent implementation of memmove() semantics requires
knowledge of some internal implementation details that are not
part of standard C.  Whether those details are part of the
compiler or part of the runtime environment (the library) is
irrelevant - they still aren't part of standard C.  Adding new
instructions to the ISA, no matter what those new instructions
are, cannot change that.