🚀 go-pugleaf

<munch>

Re: What is virtual memory?:

One of the big programmer benefits of modern operating systems is that
they give each user his own "virtual machine" to run his programs on,
that is, to some extent, each program thinks it has the machine all to
itself, and doesn't have to worry about what other processes are
doing. Virtual memory is basicaly the "memory" of this virtual
machine. For example, in the PDP-11 example -- each process thought it
was running on a 64K machine. While on this Vax, each program thinks
it is running on a 4 gigabyte machine. While on the 80286, the
machine's virtual memory consists of the fact that it makes it look to
each process as if it can access 64,000 64k segments. Note that paging
is not the only way to do things -- the 80286 has no paging and yet
its memory management scheme certainly qualifies as "virtual memory",
while the PDP-11 also swapped out its 64K segments when something else
needed to be run. But, basically, the point is:

a) Each process is given its own virtual machine, which has a constant
amount of memory, so that

b) programs can be designed for that virtual amount of memory, and do
not have to contain a bunch of special clauses for when the actual
physical amount of memory changes sometime in the future.

The grossest abuse of course is VM, where not only do they make it
look like the process is running on its own processor, but has its own
directly addressible i/o devices also. Talk about taking things to
extremes!
 
  Eric
-- 

      Eric Green {akgua,ut-sally}!usl!elg
        (Snail Mail P.O. Box 92191, Lafayette, LA 70509)

  -- Tengo lo mismo que doy y solo sirve al presente.