Its like this.
http://support.microsoft.com/kb/929605
You may have an integrated graphics card, and you have peripheral devices. These address Memory, that is not overwritable, or "available" for other processes. hence 3.25GB of the 4GB installed is actually usable by Vista.
unused Memory is wasted Memory in Vista, so
SuperFetch monitors which applications you use the most and preloads these into your system memory so they'll be ready when you need them. Windows Vista also runs background programs, like disk defragmenting and Windows Defender, at low priority so that they can do their job but your work always comes first.
Even that 1.59GB is not unused memory. Most of it is superfetched (cached). When say you load a Game, It dumps the prefetched memory to disk and loads up with what is required to run the game or program.
In my case for example,
I have 4GB installed, but only 3.25GB available. My chipset only supports
4GB max. so since I have 256MB dedicated GPU, and peripheral devices, and this addressed memory
cannot be overwritten (i.e., the installed 4GB plus the 256MB would exceed what my chipset can support), It is taken away from the 4GB installed-hence 3.25GB available, BUT the critical thing to understand here is that even though it shows only 3.25GB available I am still using 4GB of memory (thought 256MB of it is dedicated to the GPU). A chipset must support 8GB of memory in order to have 4GB available, and be a 64-bit version of Vista.
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
