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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.arch
Subject: Re: 80286 protected mode
Date: Tue, 15 Oct 2024 12:38:40 +0200
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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.