Duncan Christie wrote in a message to Mike Bilow:

 DC> Ok...but can't a protected mode program only address 4GB of
 DC> memory (using  memory paging).  Then if 8 programs are
 DC> running (each using all of their .5GB  memory space) that
 DC> would take up all of the space.

No, it doesn't work that way.  Each program sees its local 0.5 GB of memory
mapped by the Local Descriptor Table.  Since each program has a unique LDT,
each program sees a different set of pages comprising that 0.5 GB.  You can
have as many programs each seeing its own 0.5 GB as you can have LDTs in
the system.

 DC> Then were does the 3.5GB for each program come from?

The memory seen by all processes in common is mapped by the Global
Descriptor Table; there is only one GDT in the system.  This allows the
operating system to switch tasks easily without swapping itself.

 DC> Also, do you know where are could get more info  on the 
 DC> subject?  

You might start with Robert Hummel's excellent book, "The Processor
and Co-Processor."  Another good book is Deitel and Kogan's "The
Design of OS/2."  There is the Intel 386 Programmers Reference Manual,
but it is hard reading.
 
-- Mike


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