Hello,
Under the IBM PC x86 hardware industry standard architecture, all
of the expansion devices in a computer (PCI, AGP and PCIe busses,
devices like serial and parallel ports and even keyboards) communicate
by mapping (associating) a memory address (or range of memory addresses)
to a particular device in order to communicate with it. The operating
system and the device drivers provide an abstracted (higher-level)
interface to the device, so you can print something by sending it to
the LPT1: device instead of having to write a tiny assembly language
program to send data to 0x378-x037F in order for it get printed.
As you install various devices in a computer, such as a video card in a
desktop system, the computer has to allocate (reserve) some of the memory
address space for communicating with the device. For example, a video
card with 256MB of dedicated video memory would require 256MB of address
space from somewhere in the 4GB of memory address space available to
(addressable by) a 32-bit CPU. This also explains why a 32-bit computer
with 4GB of RAM in it typically reports that less than 4GB of RAM is
installed in it: The "missing" memory's address space is just being used
to communicate with the computer's hardware and any actual DRAM memory at
those addresses mapped out and inaccessible. Please note this is not the
same as the "on board" video cards integrated into the motherboard's
chipset or commonly used in notebooks. Those allocate a block of the
computer's on-board DRAM for framebuffer. When you see a computer that
reports it has 8MB, 64MB or some-other-recognizable-as-a-power-of-two
amount of memory missing from the total amount installed that is typically
where it disappeared to.
That said, there is a "trick" to workaround this limitation, sort of:
Some Intel and AMD CPUs support a technology called Physical Address
Extensions (PAE) which allows a 32-bit ("x86") CPU to access 64GB of RAM,
however, since the programs themselves running on the computer are still
32-bit, it just gives them separate 32-bit address spaces.
Desktop versions of Microsoft Windows don't support PAE; it's only available
in server versions of Microsoft Windows, and those tend to be quite a bit
more expensive than their desktop counterparts. However, if you have a
license for a Windows Server OS that you can install, I suppose you could
try it on a ThinkPad and see what the results were. I have never done this
myself, but one caveat I have system administrators who have tried this is
that it does not work as well as using a native 64-bit operating system; in
particular, you need to verify that device drivers work with PAE.
Under the x86-64 architecture (often referred to as x64, AMD64 or EMT64),
memory address space has been increased from 4GB to 1TB of RAM, which
eliminates the issue of memory-mapped address space blocking access to memory
when running a 64-bit operating system on an x64 CPU. Keep in mind, though,
that there are other limitations which come into play, such as the operating
system's ability to support such amounts of RAM. Depending upon which 64-bit
edition of Microsoft Windows 7 you install, it may be limited to 8, 16 or 192
GB of RAM. There is also the limitation of how much memory can physically be
installed in the computer's system board. I have seen desktop computer
motherboards with six DIMM sockets, with a maximum of 4GB per DIMM, that means
24GB or RAM can be installed. A ThinkPad W510 has four SO-DIMM sockets, with
a maximum of 4GB per DIMM, for a total of 16GB.
For more information:
PAE (Wikipedia) - http://en.wikipedia.org/wiki/Physical_Address_Extension
Windows Memory Limits (MSDN) -
http://msdn.microsoft.com/en-us/library/aa366778%28VS.85%29.aspx
"The 4GB Windows Memory Limit: What does it really mean?" (blog) -
http://www.brianmadden.com/blogs/brianmadden/archive/2004/02/19/the-4gb-windows-memory-limit-what-does-it-really-mean.aspx
W510 (ThinkWiki) - http://www.thinkwiki.org/wiki/Category:W510
Regards,
Aryeh Goretsky
At 08:01 PM 3/25/2011, you wrote:
Message: 3
Date: Fri, 25 Mar 2011 16:45:47 -0400
From: Stefan Monnier <[email protected]>
Subject: Re: [Thinkpad] [T23][X100e] Re: T23 battery
To: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain
> from NewEgg). You will, of course, need a 64-bit operating system to
> take advantage of more than 4GB of RAM. The X100e uses DDR2, and the
I don't understand the "of course" above. Don't know about Windows, but
Linux kernel compiled for ia32 can use more than 4GB, assuming you
compiled it with the appropriate config options.
That doesn't mean it's the recommended way to use such a machine, just
that you don't *need* a 64bit OS to make use of more than 4GB.
Stefan
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