On Tue, Nov 12, 2002 at 01:08:59PM -0600, Jeremy Derr wrote:
:
: On Tuesday, November 12, 2002, at 12:30 PM, Eugene Lee wrote:
:
: > On Tue, Nov 12, 2002 at 11:10:53AM -0600, Jeremy Derr wrote:
: > :
: > : All Unixes have Virtual Memory (although, not the same kind of VM
: > : that's in OS 9). The VM in OS X serves to provide the Protected
: > : Memory that makes OS X more stable than OS 9. OS X, like all Unix
: > : OS's, does not function without this (or at least, does not
: > : function well).
: >
: > Actually, VM does not provide any sort of protected memory. Protected
: > memory is often implemented directly by the OS (and with some help of
: > the local CPU). Virtual Memory is just a form a secondary storage for
: > pieces of primary memory to get stored to.
:
: let me rephrase my statement, then.
:
: in most UNIX OS's, protected memory does not function without some sort
: of VM.
Conceptually, protected memory has *nothing* to do with virtual memory
--- except for the word "memory". They serve different needs.
: in UNIX, it's actually called 'swap',
[...]
If it goes to disk.
: In a UNIX, you usually have exactly as much Swap as Physical.
Actually, swap should be 2x to 4x physical memory. Depends on the box.
: It's the 'mapping' function of swap space that allows UNIX's to have
: Protected Memory.
Nope. You can have protected memory without swap (diskless computers).
And you can have swap without protected memory (old Mac OS, old Windoze).
Protected memory means that processes can only hose their own memory
space, and not hose other processes' memory spaces.
Okay, repeat:
Virtual memory: when you run out of real memory
Protected memory: your app can't corrupt the memory
space of other apps
They don't really need each other. But the fact that pages are the same
size makes the implementation lots easier.
--
Eugene Lee
[EMAIL PROTECTED]
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