Hi Ulrich,

I agree with you - the memory handling is a complicated matter.
A general "truth" is: One who thinks that something is easy hasn't understood the problem. That certainly applies to this matter.

I would say that it's more or less impossible to fully grasp how remapping really works, especially as it is to some degree dependent on the environment in form of chipset, BIOS and operating system.

I think one has to limit the understanding to what's normal behaviour in terms of what BIOS and operating system shows as available memory under various conditions.

I'm somewhat doubtful to your statement "In any case, whether yoy see 2GB or 3GB or a bit more, the memory isn't going yo waste..the system is using it all, albeit reserved in some cases" though. In that case there would be no use for the remapping setting in BIOS, remapping could always have been enabled.

Here follows an extract from your Microsoft link above:

For Windows Vista to use all 4 GB of memory on a computer that has 4 GB of memory installed, the computer must meet the following requirements:

•	The chipset must support at least 8 GB of address space. Chipsets that have this capability include the following: • Intel 975X • Intel P965 • Intel 955X on Socket 775 • Chipsets that support AMD processors that use socket F, socket 940, socket 939, or socket AM2. These chipsets include any AMD socket and CPU combination in which the memory controller resides in the CPU.
•	The CPU must support the x64 instruction set. The AMD64 CPU and the Intel EM64T CPU support this instruction set.
•	The BIOS must support the memory remapping feature. The memory remapping feature allows for the segment of system memory that was previously overwritten by the Peripheral Component Interconnect (PCI) configuration space to be remapped above the 4 GB address line. This feature must be enabled in the BIOS configuration utility on the computer. View your computer product documentation for instructions that explain how to enable this feature. Many consumer-oriented computers may not support the memory remapping feature. No standard terminology is used in documentation or in BIOS configuration utilities for this feature. Therefore, you may have to read the descriptions of the various BIOS configuration settings that are available to determine whether any of the settings enable the memory remapping feature.
•	An x64 (64-bit) version of Windows Vista must be used.

A change with Windows Vista Service Pack 1 is that if 4 GB are installed, they also will be reported in Vista 32/64 bit. That doesn't mean the 4 GB will actually be used by a 32-bit OS.

Ulrich

Yes, the keywords are "that address space must be reserved". The PCI cards doesn't use the physical system memory - it's only the address space that is used for e.g. a Graphics card, not the physical system memory behind that address space. The Graphics card uses its own onboard memory. Writing to the Graphics card memory also writes to the system memory with the same addresses, which makes that memory unusable to the OS. But that same memory isn't used by the hardware either! So, to my understanding, the reserved system memory is really wasted.

For comparison, when less than 3GB is installed, no system memory is lost (unless the Memory Remap Feature is enabled and a part of the address space for the third GB is reserved). This is due to that the address space reserved for hardware is taken with start from the highest 32-bit address (4GB) and lower.

The reason that no memory is lost when using a 64-bit OS is that the Memory Remap Feature (when enabled) remappes the reserved address space  for system memory above the 4GB address space, where the 64-bit OS can make full use of it (but of course is out of reach for a 32-bit OS).

The following part, I think, may be dependent on which motherboard / chipset that is used. For my P5B Deluxe with chipset P965, the User Guide (page 2-13) says:
"Due to chipset resource allocation, the system may detect less than 8GB system memory when you installed four 2 GB DDR2 memory modules".

There is an exception where a 32-bit OS is capable of using up to 64GB of memory, and that is Windows Server 2003 32-bit with PAE (Physical Address Extension) using 36 bit addresses.

(2³⁶ addresses 64GB)