How is \"single-user\" done?

[Greg Davidson]

Before we get too excited about explaining the impossibility of running
UNIX in multiuser mode on the IBM PC, I would like to point out that
XENIX and VENIX are frequently run multi-user on the PC, and have
reasonable multi-user performance when doing, for example, general
office work (mostly editing and light database operations).

The Santa Cruz Operation (XENIX) people explained how they handle the
memory protection aspect:  User programs are limited to 16-bit data
pointers, and user tasks are always given a whole 64K segment to play
with.

It is simply not economical to use a PC with a hard disk single user.
Whether you run UNIX or DOS) the cost/user is uncompetitive.  Of
course, you can do even better if you have enough users to timeshare
a super micro.

BTW, speaking of other known impossibilities, I played with VENIX on a
DG/1 at the trade show in Dallas.  It didn't have a hard disk, yet
ran fairly nicely (noticably more responsive than with HP/UX on the
HP Integral).  I ran both vi & csh (the only big programs on the disk)
and they loaded fast and ran fine.  I've seen VENIX running on floppy
based LSI-11 systems before, so this impossibility is also fairly common.

Still, I fully believe that these feats would be impossible for a bulky
system like PC/IX, let alone my favorite UNIX, 4.2BSD.

_Greg