Re: 80286 protected mode

On 14/10/2024 21:02, MitchAlsup1 wrote:

On Mon, 14 Oct 2024 15:04:28 +0000, David Brown wrote:
 

On 13/10/2024 17:45, Anton Ertl wrote:

 

I do think it would be convenient if there were a fully standard way to
compare independent pointers (other than just for equality).  Rarely
needing something does not mean /never/ needing it.

 OK, take a segmented memory model with 16-bit pointers and a 24-bit
virtual address space. How do you actually compare to segmented
pointers ??

void * p = ...
void * q = ...
uintptr_t pu = (uintptr_t) p;
uintptr_t qu = (uintptr_t) q;
if (pu > qu) {
...
} else if (pu < qu) {
...
} else {
...
}
If your comparison needs to actually match up with the real virtual addresses, then this will not work.  But does that actually matter?
Think about using this comparison for memmove().
Consider where these pointers come from.  Maybe they are pointers to statically allocated data.  Then you would expect the segment to be the same in each case, and the uintptr_t comparison will be fine for memmove().  Maybe they come from malloc() and are in different segments.   Then the comparison here might not give the same result as a full virtual address comparison - but that does not matter.  If the pointers came from different mallocs, they could not overlap and memmove() can run either direction.
The same applies to other uses, such as indexing in a binary search tree or a hash map - the comparison above will be correct when it matters.