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

From: Philip.Belben_at_pgen.com <(Philip.Belben_at_pgen.com)>
Date: Mon Oct 26 08:49:57 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.
Actually the figure is 29 bits (it was 512M not 512K) but I agree with you
100% in principle.


The way I look at it is this: Memory sizes are growing exponentially. So
address bus widths can be expected to grow linearly. It has taken roughly
30 years between 64K words or bytes getting tight (early '70s, DEC up the
Unibus from 16 bit addressing to 18 bit, but there's probably an example
from the late '60s) and now, when the 4 GWord or GByte address space of 32
bit addressing looks like it might get tight. (Note this is on small
computers. On large computers, I'd estimate 10 years earlier in each
case.)



I therefore see address buses growing at 16 bits every 30 years. That's
just over a bit every 2 years - slower than I expected but not much.
Someone (I forget who) said that memory chips double in capacity every 18
months. This would give 16 bits in 24 years.



I claim that the assertion that we'll see even 64-bit address spaces being
used anything like up by 2003 is unfounded. According to that growth rate
above, we will start hitting the limit of 48-bit addressing - 256 TWord -
in the '20s, and the limit of 64-bit addressing, 16 exawords (or exabytes,
possibly), in the '50s (or '40s at 1 bit per 18 months). Many of us will
probably still be alive then (I shall be celebrating my 83rd birthday in
March of 2050) - and I for one would like to see what sort of technology
will be used to store 16 exabytes in a space smaller than a mountain!

Philip.
Received on Mon Oct 26 1998 - 08:49:57 GMT