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

From: Allison J Parent <allisonp_at_world.std.com>
Date: Wed Oct 28 07:41:55 1998

< So, while a 256 or 512 bit data bus is usefull (and
< already in use in main frames - only called data path),
< even a 128 Bit address bus is just nonsense.

There is some truth to this. DEC went to 64bits as they knew 32bits
were not enough for some applications. They were also trying to design
for the next 20years to repeat the life of the VAX (32bits).

Now, having a large address space isn't completely about filling it.
It's about having addressing modes and arithmetic adaquate to deal with
large data objects. In the 32bit model 4gb is the limit without
thrashing. Those limits was exceeded by disk farms with tens of
Gigabytes of databases and solving large mathmatical models. The
current example would be doing windows graphics programming
(1280x1024x16m) using only 16bit pointers, you do a lot of thrashing
to get the the next 64k chunk and there will be a lot of them! That
was the problem that spawned the VAX!

Brief history lesson: The PDP11 was reachhing the point where a fully
filled machine (4mb of ram) could not process in one lump the large
problems like weather models or MRI data. It was NOT a speed issue it
was 16 bits made for too small a data pointer. Also doing 32/64bit math
required more steps (or FP hardware) as the registers were all 16bit.
So to add two 24bit numbers on a PDP-11 required multiple steps and that
was a speed impact. Going to 32bit registers made that a single
instruction and paid back as speed without processors running at faster
clocks.

If it were speed the 11/74 would have been an ongoing event rather than
the 4-6 than did get made. The birth of the VAX (and DGs Eclipse) grew
from this need.

So computer designers bumped the size of things to assure they could
comfortably address the likely largest object they can resasonably expect
to see for some time to come. Not so they could have 2^64bytes of ram
but because they already have 2^40bytes of disk farm!

All this was no surprize to computer designers. They foresaw it in
the 60s knowing that larger memories and disks were needed to solve the
problems that were growing.

The datapath issue is strictly about speed as the VAX could have an
8bit data bus (in a trade for speed) like the 68008. While it has a
big effect on speed it has no interaction with addressability. It's
addressing the system problem of multiple devices competing for the
common resource (MEMORY). By making the datapath wide you can grab
several bytes at the same time and while the cpu is dealing with
that the ram is available to run a IO cycle to a disk system or
other IO. How big wide enough is, that is determined by how much
silicon you have.

Allison
Received on Wed Oct 28 1998 - 07:41:55 GMT