Re: Segments

On Mon, 6 Jan 2025 22:02:30 +0000, Thomas Koenig wrote:

Hmm... a beginning of an idea (for which I am ready to be shot
down, this is comp.arch :-)
>
This would work best for languages which explicitly pass
array bounds or sizes (like Fortran's assumed size arrays,
or, if I read this correctly, Rust's slices).
>
Assume a class of load and store instructions containing
>
- One source or destination register
- One base register
- One index register
- One ubound register
>
Memory access is to base + index, with one additional point:
If index > ubound, then the instruction raises an exception.

Now, you are only checking the ubound and not the lbound; so,
you only stumble over ½ the bound errors.
Where you should START is with a data structure that defines
the memory region::
     First Byte accessible Possibly lbound
     Last  Byte accessible Possibly ubound
     other stuff as needed
Then figure out how to efficiently perform the checks in ISA
of choice (or add to ISA).

This works less well with C's pointers, for which you would have
to pass some sort of fat pointer.  Compilers would have to make
sure that the address of the base object is passed.

I blame the programmers for not using FAT pointers (and then
teaching the compilers how to get rid of most of the checks.)
Nothing is preventing C programmers from using FAT pointers,
and thereby avoid all those buffer overruns.

Comments?