Protected mode is a lot wierder.  You access memory, the operating system
can redirect your access to wherever it feels like directing it, or gain
control so that it can read the requested memory off of the hard disk
(allowing virtual memory).  If a program tries to access memory that
doesn't belong to it, the access fails.  An instruction like MOV AX,[1234]
if the memory pointed to by DS:1234 had been swapped out, the operating
system would get control and read that memory in to a place in real
memory.  In other words, the operating system can play games with memory,
moving it around and whatever else it feels like doing.  Interrupts are
done a little bit differently, and there are some interrupts that the
processor generates, so you have to test to see it it was the processor
that generated the interrupt or if it was a programmer executed INT 21 or
whatever.  Lastly, the operating system can write virtual device drivers,
VDD's, so that if a program tries to reprogram a device with IN/OUT
instructions, the VDD can pretend that the command was carried out,
respond with what the real device would respond with, etc.  So then a
program could reprogram the VGA card, it would think everything was fine,
but it was actually running in a window, so the operating system just
changed the way the window looked.

Anyway, if you have the tools and the need, protected mode gives a lot of
nifty advantages.  Otherwise, all of this protection can slow down your
computer for no reason.

Protected mode allows multitasking and rearranging of memory.

--
Ben Scott
bsc...@hopper.unh.edu