Re: 80286 protected mode

On Thu, 10 Oct 2024 6:31:52 +0000, David Brown wrote:

On 09/10/2024 23:37, MitchAlsup1 wrote:

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.

>
No, it is not.  It has absolutely /nothing/ to do with the ISA.

For example, if ISA contains an MM instruction which is the
embodiment of memmove() then absolutely no heroics are needed
of desired in the libc call.
Thus, it IS a symptom of ISA evolution that one has to rewrite
memmove() every time wider SIMD registers are available.