Hi Gary,

It is even worse than that, most systems will only see 3GB or 3.5GB max
under XP.

I have found that many 32 bit programs run fine under 64 bit Windows. 64 bit
Windows has a subsystem call WoW64 which is used to run 32 bit programs on
the 64 bit OS. It has a layer (called a thunking layer) to translate the 32
bit addressing to the 64 bit addressing when necessary to communicate to the
kernel.

The biggest problem, especially for older systems, is the availability (or
not) of 64 bit drivers for the various hardware pieces. Unlike usermode
programs, kernel mode drivers MUST be native 64 bit.

Another benefit to running 64 bit Windows is for very large programs. Being
a developer that has used Windows extensively, we came across a situation
where we had a process (embedded system) that would grow to over 2GB of
memory usage. By default, Windows 2K allowed only 2GB address space per
process. Windows XP addressed this somewhat by allowing you to specify a
boot option to have the kernel grant >2GB address space to a process as long
as the executable was marked LARGE_ADDRESS_AWARE. However, due to the fact
that 4GB is the limit for a 32 bit processor, and the kernel needs some
space, as well as the physical devices themselves (video, network, disk,
etc), realistically, 2.5GB to 3GB is about all you can get under any
circumstance. Server versions of 32 bit windoes can use a feature in most
Intel processors (sorry AMD processor users :-() called PAE that allowed up
to 64GB of memory. The nice thing about 64 bit Windows is that the kernel
and physical devices can be located above the 4GB barrier, leaving almost a
full 4GB address space for user mode programs.

Russ Poffenberger
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-----Original Message-----
From: Personal Computer Hardware discussion List
[mailto:[log in to unmask]] On Behalf Of gary tennesen
Sent: Friday, February 06, 2009 9:16 PM
To: [log in to unmask]
Subject: Re: [PCBUILD] Subject: Could more RAM help?


Sorry if I missed the post if someone already has said it:

XP and VISTA can only "see" up to 4gb RAM.

The exception to the rule is the 64-bit version of these OS; however, there 
is a trade off in compatibility issues should you decide to go with a 64-bit

operating system any time soon.

I just wanted to put it out there so someone doesn't make the same mistake 
as I did when I bumped my computer from 4gb to 8gb RAM and found myself 
scratching my head asking, "where arrrrrre you, $140 worth of new RAM????" 
<grin>

[log in to unmask]


 

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