As a fellow employee at Dual Systems (but later than 1981), let me add that
Dual was *not* the first non-MMU Unix system.

Some people would say that MUNIX, which ran on an LSI-11/03 and put the
kernel (if I vaguely recall) from 0-32KB and user programs from 32KB
to 64KB, was the first non-MMU UNIX system. This would have been about
1977.

However, there were other variants, one of which was described in
the article  "Unix on a Microprocessor", H. Lycklama, Bell System
Technical Journal, Vol 57, No. 6, Part 2 (July-August 1978)*, which
could run in 20KW on an LSI-11.

Further, the original paper on Unix (same BSTJ volume) describes
running Unix on the PDP-7 and PDP-9 as well as "unprotected" on
the PDP-11/20, which meant that Unix ran w/o the benefit of an MMU
back in 1969.

Further, one could split hairs and say that the segmentation registers
on the PDP-11 (as well as those on the 68451 that Erik referred to)
aren't *really* an MMU unit... but that would be really stretching things
a bit....

At any rate, the take-home lesson here is probably that with judicious
modifications of your language tools, it is quite possible to make
a usable (but not bulletproof from malicious intent) U*ix system.

-matt

*: A 'must have' volume for anyone interested in Unix. The elegance and
clarity of the writing and thinking (especially Ritchie's) establish
a standard which we should all aspire to.