In order to expose the entire 4GB RAM to the OS, the system hardware must actually support
MORE THAN 4GB address space and implement a feature called memory remapping or memory hoisting.
The Intel 945 chipset can decode 4GB of DRAM but there is no way for the BIOS to expose all 4GB to the OS because the chipset is limited to a maximum 4GB address space and does not support memory remapping. The lost memory address space is reserved to all
your hardware in order to function, and thus cannot be used for addressing RAM (two different things cannot utilize the same physical address). This is a hard limit of the Intel chipset.
Google "intel 945 chipset, 4GB RAM, limit" and you'll get a ton of hits on this issue. e.g. http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c00667512/c00667512.pdf
A BIOS update *might* be able to free up an additional hundred megabytes or two, but you'll need a new chipset/mobo/system to utilize any more than ~3.25 GB "ish" RAM.
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