> and apply a mask before using it for memory accesses

So it never goes onto the address bus. So it never causes an addressing issue.

The 24-bit issue of the 68000 is fundamentally different. Those tagged 32-bit pointers were used directly, but truncated by the width of the actual physical interface so they never touched the outside world.

...until Motorola released CPUs with full 32-bit address pins on the package. Oops.

malloc usually gets more aligned with time, not less. The tagging we're talking about goes in the high bits of a pointer.

x86-64 actually requires that every memory access occur with a "canonical" address, specifically to ensure that everything will be "64-bit clean". That is, even though virtual address space in some cases may be limited to 48 or 56 bits the actual address has to be sign-extended to 64 bits, so everything is "clean".

"Abusing" unused address bits is then just a matter of ensuring that any virtual addresses you use are restricted to the appropriate range. "x32" binaries do this in order to ensure that any pointers will fit within 32 bits even on x86-64, for example.