Date   : Sun, 05 Jun 1983 05:40:00 EDT
From   : Roger L. Long <BYTE@mit-mc>
Subject: UNIX on the 68000

Put simply, you know not of what you speak!

You should really take a look at the specs for the 68451 MMU before you
try to tell the rest of us how it doesn't work.  I think I can speak
with a bit more authority since I just finished porting UNIX to my
companies 68000 product which uses two 68451 MMUs.  First of all, the
page size may be any power of two, so the typical process might use
4 or 5 segments.  You can get along easily with only one 68451 - our
company chose to use two in order to reduce swapping of descriptors
in systems with LOTS of main memory.  Running in single-user mode,
you will typically not use more descriptors than will fit on one MMU
unless you have a lot going on in background.

In comparing the 68000 with the National 16032, I think a main
consideration to keep in mind is not the raw power of the CPU.  If
you look at benchmarks comparing the 68000 to a VAX in terms of raw
CPU power, I believe the 68000 running at 8MHz is .8 the power of a
VAX 780.  However, you are not going to see a 68000 UNIX system
outperform a VAX UNIX system.  The bottleneck is the disk!  UNIX is
such a disk-intensive OS and most implementations, including ours,
use winchesters, which are fast compared to floppies, but not
compared to the disks that hang off a VAX.

What started this all off was a query about putting UNIX on an S100
box in what I would assume would be a single-user environment.  
Whether it be a 68000 or a 16032, I think you're going to have more
than enough CPU to keep the average hacker happy in that kind of
environment.

       -roger