4 ms·

The address space of AMD64 was designed so that both the upper and lower spaces of the address are used when using less than 64 bus lines specifically to addres

• saagarjha 7y ago
  Right, but a lot of high-performance code will tag the high bits of pointers anyways and apply a mask before using it for memory accesses.
  • that_jojo 7y ago
    > 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.
    • saagarjha 7y ago
      It's still going to break software, though. But if you're looking for a platform that lets you do this, look no further than ARMv8's TBI.
• jdsully 7y ago
  That only ensures the programmer fixes the pointer before dereference. Tagged pointers that don’t make it to the bus are still a compatibility problem. It means malloc() can’t return an address overlapping the tagged bits.
  • saagarjha 7y ago
    malloc usually gets more aligned with time, not less. The tagging we're talking about goes in the high bits of a pointer.
    • jdsully 7y ago
      Yes and as memory increases more of the high bits are used as actual address bits. X64 started with 40 physical bits. We’re now at 48.