This is one of those deep nerdy windows internals posts that will interest some and make other say "wow,  Tom needs to spend more time away from his computer".  There is actually a good reason why I've dug into this topic and it stems from my recent hardware allocation in my new job.  When I started I received a new Lenovo laptop with 4 GB of memory and my base image came with Vista 32 bit.  I didn't think too much of it and loaded everything up.  This was about 6 months ago so there was no Vista SP 1 at the time. 

To my dismay when I opened my computer properties I saw that I only had 3070 MB of available physical memory.  So somehow I lost an entire gig of RAM.  I spoke with some of my other colleagues that were also new hires and they didn't have the same issue.  Many of them had HP or Dell notebooks and they had more like 3.5 GB of available physical memory on their machines.

It was right around this time that I got my first deep internals class for my new job.  In this class we learned all of the tips and tricks for doing advanced perfmon analysis of memory leaks, disk performance issues, CPU bottlenecks, the list goes on and on.  Well the instructor was just incredible so when there was a break in the class I asked him about this issue.  He very quickly explained that the reason you see different amounts of available ram was because of the chipset and reserved space for device drivers (video cards typically the biggest hogs).

Ok,  no big deal then right.  Lesson learned,  when you look to purchase a laptop that can support up to 4 GB of physical memory make sure you find out how much it will make available to windows.  Then,  Vista SP1 came out and I couldn't help but notice when I went to the computer properties the window now said I had 4 GB of ram available.  Well isn't that interesting,  someone must have lied to me!!!!  Actually it turns out that with SP1 we decided to show the installed RAM instead of the available RAM.  This is very confusing as it's never been that way for any previous OS version.  If you want to learn some more about it you can take a look at this kb article http://support.microsoft.com/kb/946003/. 

Now my story here is one of confusion and ignorance.  If you're the type that wants to really understand this stuff then you have to go the best windows internals guy on the planet.  Of course I'm talking about Mark Russinovich,  he wrote a long blog post just this week (which is what reminded me I wanted to blog about this) that gives a complete explanation of this issue along with some other really cool stuff about RAM limits on other OS versions.  He also shows a screenshot from the largest test server in the world used to define the limits for memory in 64 bit windows (currently 2 TB because there was nothing any higher available that we could test with).  This is a must read http://blogs.technet.com/markrussinovich/archive/2008/07/21/3092070.aspx.  If you just want the technical detail behind the 4GB issue I talked about above then scroll down to the "32-bit Client Effective Memory Limits" section.

So the main lessons here are:

     1) If you are still planning to run 32-bit windows clients and you plan to install more than 3 GB of RAM then not all hardware is created equal.  Make sure you ask how much physical RAM is made available to windows.  This becomes even more important when you start doing a lot of Virtual Lab work as I often do.  It sure would be nice to get that third Win 2K3 server running with a full GB of RAM.  Why didn't I install the 64 bit version of Vista Ultimate again?

     2) The 64-bit client version of windows is really the only way to ensure you can start to have enough RAM for doing a lot of virtualization work.  It's also the only way to get in and see how many gaps your hardware has left in the device driver ranges (see Mark's review of this).  64-bit is the future ... embrace it ... love it ... live it.