“Something’s not right here. I calculate that 16 TB can be reached with 44 bits. A 64 bit address space spans approximately 18 million TB.”
The 16 TB limit is per process, and since it’s approximately 8,000 times larger than the memory in a decently equipped computer today it provides some serious headroom.
At current prices 16 TB of RAM runs approximately $280,000...also providing some headroom. ;-)
The 16 TB limit is per process,“Something’s not right here. I calculate that 16 TB can be reached with 44 bits. A 64 bit address space spans approximately 18 million TB.”
. . . but that still seems arbitrary and not obviously necessary.and since it’s approximately 8,000 times larger than the memory in a decently equipped computer today it provides some serious headroom.
At current prices 16 TB of RAM runs approximately $280,000...also providing some headroom. ;-)
Since RAM cost has been declining according to Moore's Law, I look at "headroom" on a log scale. It took 16 bits to address the 64K memories readily available in the early 1980s, and it takes 32 bits to address the 4Gig of RAM readily available now, one human generation later. Were Moore's Law to continue in effect for another human generation, that would suggest the need for about 48 bits to address the memory that would then be in common currency. And it would take another generation again to exhaust the full 64 bit address space.Recall the big issue over the Y2K transition, and it does give one pause over assuming that Moore's Law will break down before reaching 44 bits of address space. Of course I have to admit that it would be no trick at all to go to 128 bit cores long before that - were there a reason - but still. What bang do they get for that buck? What it means is that your son could very easily see the time when an unnecessary software limitation creates a crisis in the operating system. Which, looked at in that way, is pretty optimistic after all. Why would OS X necessarily last two human generations?