When any program runs, in any virtual-memory-based operating system (such as Unix or Linux or OS/X or Windows or IBM MVS or .. .. ..) it has a perspective of "main memory" that never
matches the physical reality.
Each program .. every
program .. "sees" no other program other than itself. "All of memory, as far as it can see," is its own private
to the operating system, is that "memory is, in fact, an intensely-shared
resource." But the only part of the system which can actually know or appreciate that fact is
"the operating system," because only the operating system is ever able to know how physical
memory is actually being allocated.
Every user-land program is (entirely without its knowledge or consent...) obliged to view "memory" through a hall-of-mirrors.
And what is
this "view, as seen by a user-land program, through this 'hall of mirrors' that you speak of?" It is that "'memory' is no larger than 4 gigabytes, and it contains exactly what you
think it does, and nothing more." Period. That
is what your textbook is referring to.
Yes, indeed... that illusion
, artfully pretending to be (and, from the perspective of the user-land program, indeed, being!)
what "the physical
reality" is! It does not matter, after all, because a user-land program will never knows anything about what "the physical reality" might be: nor does it care.