On 2025-07-25 at 20:27, Frederick Virchanza Gotham via Std-Proposals wrote:
>
>
> On Friday, July 25, 2025, Oliver Hunt wrote:
>
>
> > Not really. There isn't a computer in existence today -- I don't
> think
> > -- that uses more than 49 bits for a memory address. 64-Bit ARM uses
> > 48 bits but it can be extended by 1 bit to 49 bits.
> >
> > So you can mark a pointer as 'bad' by manipulating the top 15
> bits. Or
> > even just set the top bit high.
>
> This is nonsense.
>
> High bits are 100% valid on numerous platforms.
>
> Numerous platforms make use of the high bits: CHERI, ARMv8.3 with
> PAC extensions, MTE, etc
>
> In addition to that many OS’s use high bits in kernel addresses. e.g
> 0x111….. is kernel space, 0x0000…. is user space.
>
>
>
>
> Gonna make an attempt at deductive reasoning here.
>
> Computers nowadays have 32-Bit or 64-Bit pointers. Some microcontrollers
> have 8-Bit or 16-Bit pointers.
>
> Talking about 64-Bit pointers . . . if each individual increment is one
> 8-Bit byte, then a 64-Bit pointer can address 18 million terabytes.
>
> But nobody has that much memory.
Where have we heard that before? 640k, 16M, 4G, 18 Gazillion. Never
happens. Ever!

Except when it did.