64bit data 128Bit address...Re: Building a Z-80 (64bit!!!)

From: Eric Smith <eric_at_brouhaha.com>
Date: Sat Oct 24 17:30:18 1998

"Max Eskin" <maxeskin_at_hotmail.com> wrote:
> No application _requires_ any number of bits > 1. It's a question of
> performance. After all, a Z80 could have 512M RAM, just not
> contiguously (and would probably require a lot of hardware to access
> it).

OK, then the Z80 system will require 19 bits of address. Sure, some of those
bits aren't coming directly out of the CPU, but they're coming from
somewhere.

So how does your example show that no application requires more than one
bit of addressing? In two bytes you can't even write a virtual memory
system to page the rest of the memory in, let alone provide a place to
page it.

> There are plenty of tasks that can use up much more than 4GB ram,
> most notably databases and various graphics.

Certainly. That's why high-end machines have gone from 32-bit (4G)
addressing to 64-bit addressing. But many people seem to have the
misconception that doubling the size of the address bus just doubles
the addressable memory (or otherwise increases it by a relatively small
amount). In actuality, *each* added address bit doubles the addressable
memory, so going from 32 to 64 bits of address ups the space from
4.3x10^9 bytes to 1.8x10^19 bytes.

> But, what's the amount of RAM
> a 128-bit processor can access?

3.4x10^38 bytes.

I previously posted an back-of-the-envelope calculation showing that
current DRAM technology would require more silicon than exists in the
solar system in order to produce this much memory. Even allowing for
technological improvements over the next five years (Moore's law), it
would still take most if not all of the silicon in the solar system.

It's possible that some breakthrough in storage density will occur in
the next five years, but even an improvement by a factor of 1000 is
not going to be enough to make 128-bit addressing obsolete. In practical
terms, it won't even make 64-bit addressing obsolete.

Video is commonly referenced as an application requiring phenomenal
amounts of storage. If every person currently on this planet had their
entire life (say 75 years) recorded on NTSC video with no compression,
that would only require 3.7x10^26 bytes of memory. 128-bits is enough
to address a trillion times this much memory.

I'm not going to go out on a limb and say that no one will ever need this
much addressable memory. But I will say that within my lifetime, no one will.

Hmmm... maybe I shouldn't post this until *after* I've taken a few $1000
bets from naive people. :-)

Eric
Received on Sat Oct 24 1998 - 17:30:18 BST