How is \"single-user\" done?

[Ron Natalie]

> It means that it cannot process several jobs at once.  Granted, the
> IBM PC has the ability to run several *processes* belonging to the same user,
> the capability of context switching with regard to several users is a bit
> trickier.

Wrong, the difficult in doing context switching is the same under UNIX
irregardless of the uid's of the processes being run.

>  The CPU does not have the capability to block illegal memory
>  accesses (PAGING/VM), nor the speed required for multi-user.

PAGING/VM is not required, just simple memory protection.  The lack of
memory protection alone is not the reason for the PC being a single user
machine.  Protecting one users task from trashing the memory of another
of his, or writing on the operating system's memory is just as important
as keeping him from stomping on other users.

The real reason the IBM-PC is a single user machine is that no one in
his right mind would consider it otherwise.  First, as you stated before,
it is slow.  Second, the first user gets to use the CPU display and keyboard,
everyone else is a second class citizen when they use the serial ports.
IBM-PC software developers end of developing all their software to run
assuming your using the console, since it is the easy way out and is the
only terminal on the system in 99.9% of the cases.