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Before using Memory Remapping to sort out the MMIO issue, make sure your MB/OS supports it properly, so don’t turn it on when using 32bit Windows XP or Vista.

Since MMIO takes such a huge space(over 1GB if you’re running SLI), the lost is unacceptable. Luckily enough, chipset manufacturers have takes some efforts to solve it, and the technology is called Memory Remapping.

How Memory Remapping Works

Memory remapping is used to recover addressable memory space lost to MMIO space, here is a system with 5GB physical RAM installed. 

(5GB of Physical Memory, with 1GB allocated to Memory Mapped IO), here TOLM means Top of Low Memory, and TOM stands for Top of Memory

In this case, the amount of memory remapped is the range between TOLM and 4GB, which is 1GB. It will be mapped to the physical address range between 5GB and 6GB. All the hassle is taken cared by chipset which is programmed by the BIOS and totally transparent to applications and Windows.

Memory remapping was first introduced in Intel server platforms, and more and more modem desktop chipsets feature it nowadays.

Memory Remapping in Real World

There are various implications on memory remapping. Some motherboards do it in a smarter way: when 4GB or more is plugged, the feature is on automatically. Most MBs, however, have a BIOS setting to control it.

(Memory Remapping Settings in an ASUS MB)

Chipsets Support Memory Remapping

INTEL	955x 975x P965/G965 P35/G33…and perhaps all future chipsets Various server platforms
AMD	All the chipsets support Athlon64 939 or newer*
Nvidia	650i/680i SLI(for Intel)

(*All chipsets support AMD64 should be capable of memory remapping, include those from Nvidia, Via, ATi and AMD etc. However, the actual implication is depends on manufacturer.)

Please notice that though Intel Chipset 915/925/945 and before claims “4GB Memory support”, we can’t get full 4GB, regardless the operation system. Its also quite common that in low-end board, the memory remapping might not present in BIOS though the chipset supports it.

So when memory remapping is enabled, the system memory address space is actually more than 4GB. In order to get FULL 4GB, the OS should be capable of addressing over than 4GB.

Windows Systems capable of addressing over 4GB

 

Operation System	Maximum Memory
Windows Vista Starter Edition	1GB
Winders Server 2003  Web Edition	2GB
Windows 2000 Professional Windows 2000 Server Windows XP(Home, Professional, MCE) Windows Server 2003 Standard Edition Windows Vista Home Basic Windows Vista Home Premium Windows Vista Enterprise Windows Vista Ultimate	4GB
Windows 2000 Advanced Server Windows Vista Home Basic 64bit Edition	8GB
Windows Vista Home Premium 64bit Edition	16GB
Windows 2000 Datacenter Server Windows XP 64bit Edition Windows Server 2003 Enterprise Edition	32GB
Windows Server 2003 Datacenter Edition Windows Server 2003 Enterprise 64bit Edition	64GB
Windows Vista Business 64bit Windows Vista Enterprise 64bit Windows Vista Ultimate 64bit	128GB
Windows Server 2003 Datacenter 64bit Edition	512GB

No doubt that all versions with 8GB or greater memory support would handle 4GB RAM properly when Memory Remapping is on, as shown in the table above.  There are two exceptions, however. The Windows 2000 Server and Windows Server 2003 Standard Edition, have no problem in supporting 4GB RAM though Microsoft claims a maximum memory support of 4GB only. This is because they share same core with their big bro and can address over 4GB, while the desktop ones(2000, XP and Vista 32bit) don’t.

When memory remapping is enabled in BIOS, the MB will map some memory to address above 4GB. So using an OS without good memory remapping support will lose more RAM.  For example, ASUS MBs will map memory from 2GB-4GB to address space 4GB-6GB. So when it’s on, 32bit Windows XP or Vista will get a total RAM of 2GB ONLY compared to 3GB without remapping.