Hello,

I am checking about memory issue document in the following link.

http://kerneltrap.org/node/2450

When they say 4 GB virtual address space, does it mean swap space? Please let me know.

thanks in advance

no, it's not any specific form of memory, RAM chips, swap on a disk etc... it's an abstraction of whatever physical memory resources you have, 64gb of physical memory or 4gb of memory and a whopping great swap disk... That article actually gives a very very good overview of what you're asking in the first place.

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 play-pen.

The reality, known only 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!) "reality."

Never mind 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.