PAE mode, Physical memory beyond 4GB

One of our developers was asking me how our Virtual Server 2005 machines were capable of addressing more than 4GB of memory on a 32-bit platform. This is actually a pretty interesting question that I’ve been hit with more than occasionally, so I thought I’d post a quick entry explaining this, and linking some relevant resources.

Memory is a key component in what I would term, “recently overlooked hardware components”. I say that because processing costs have come down so much, and OS platforms have improved so dramatically, that even among IT professionals, people tend forget about architecture limitations.

That being said, physical address extension (PAE) mode is memory extension provided by Intel to enable support for greater than 4GB of physical memory in 32-bit Intel architectures. Essentially, PAE maps up to 36-bits of memory to a 32-bit virtual address space, allowing for up to 64GB of physical memory.

Windows 2000 Advanced Server, Windows 2003 Enterprise edition, and Windows 2003 Datacenter edition, are all capable of accessing various amounts of physical memory beyond 4GB via PAE mode (check this link for a detailed table with memory limitations ).

Probably the key take away for IT pros is that if you’re configuring a server with more than 4GB of RAM, make sure you have the right OS to take advantage of the additional RAM.

Additional Resources:
http://www.microsoft.com/whdc/system/platform/server/PAE/pae_os.mspx
http://www.microsoft.com/whdc/system/platform/server/PAE/PAEdrv.mspx

Update: 12/7/05
I ran across another link that provides some more depth to this...
http://www.microsoft.com/technet/prodtechnol/windowsserver2003/library/TechRef/26eccf33-2454-4222-841a-c6d5aa1fc54c.mspx