If you restrict yourself to all variants of x86 and ARM, the number of high bits for which I could not find conflicting uses is 6 bits (bits 57-62). The other high bits are reserved in some hardware contexts and therefore may create conflicts.
Using 16 bits may be risky on recent x86. For example, IIRC Linux enables 5-level page tables on microarchitectures that support it, which can put valid address data in bits 48-56.
There is no guarantee that those 6 bits are safe either. They are just the only bits for which I could not find existing or roadmap usage across x86 and ARM sources when I last did a search.
> Using 16 bits may be risky on recent x86. For example, IIRC Linux enables 5-level page tables on microarchitectures that support it, which can put valid address data in bits 48-56.
Linux will not allocate past the 47-bit range, even with 5-level paging enabled, unless specifically requested, by providing a pointer hint to `mmap` with a higher address.
Using 16 bits may be risky on recent x86. For example, IIRC Linux enables 5-level page tables on microarchitectures that support it, which can put valid address data in bits 48-56.
There is no guarantee that those 6 bits are safe either. They are just the only bits for which I could not find existing or roadmap usage across x86 and ARM sources when I last did a search.