Re: 80286 protected mode

On 18/10/2024 18:38, Vir Campestris wrote:

On 16/10/2024 08:21, David Brown wrote:

>
I don't see an advantage in being able to implement them in standard C. I /do/ see an advantage in being able to do so well in non-standard, implementation-specific C.
>
The reason why you might want your own special memmove, or your own special malloc, is that you are doing niche and specialised software. For example, you might be making real-time software and require specific time constraints on these functions.  In such cases, you are not interested in writing fully portable software - it will already contain many implementation-specific features or use compiler extensions.
>

I have a vague feeling that once upon a time I wrote a malloc for an embedded system. Having only one process it had access to the entire memory range, and didn't need to talk to the OS. Entirely C is quite feasible there.
 

Sure - but you are not writing portable standard C.  You are relying on implementation details, or writing code that is only suitable for a particular implementation (or set of implementations).  It is normal to write this kind of thing in C, but it is non-portable C.  (Or at least, not fully portable C.)

But memmove? On an 80286 it will be using rep movsw, rather than a software loop, to copy the memory contents to the new location.
 _That_ does require assembler, or compiler extensions, not standard C.
 

It would normally be written in C, and the compiler will generate the "rep" assembly.  The bit you can't write in fully portable standard C is the comparison of the pointers so you know which direction to do the copying.