Date   : Tue, 11 Oct 1988 02:00:30 GMT
From   : elbereth.rutgers.edu!ron.rutgers.edu!ron@rutgers.edu (Ron Natalie)
Subject: a very naive Question???

The article is inaccurate.  System V has virtual memory of almost
any definition.  Originally System V had problems with virtual memory.
There are two differing definitions of Virtual Memory.

The more liberal definition says that Virtual Memory provides for swap
extension of real memory and some sort of address translation.  UNIX has
always done this.

The more restrictive definition is that some mechanism such as paging or
segmented swapping exists as well.

The difference is summarized in the following.  Suppose you have 1 M
of real memory and you want to run a 2 M process.  Prior to recent system
V versions, you were out of luck.  It always expected to load a whole
process as a unit.  On paging systems, it only needed to bring in the
parts needed for the execution of the current instruction.

The virtual memory argument was best summarized by Bruce Crabill of
the University of Maryland.  "It ain't virtual unless it's not all there."
This means that the liberal definition is kind of weak if you have to
have real memory to back your virtual memory for any give process.

-Ron

Of course, they may have meant VM as Virtual Machine.  It is doubtful
that anything other than AIX/370 is going to support that.


End of INFO-CPM Digest
******************************