Is there a way to get Windows XP to recognize more than 3 or 3.5 GB of memory?
The difference between the 4GB of physical RAM and the 3.5 GB of RAM seen by Windows is the consequence of system BIOS needing to use that much address space for Memory-Mapped IO. In a nutshell, what this means is that a 32-bit computer can address a maximum of 4GB of memory space. The computer also needs to use some of this memory space to assign to hardware devices in the computer: USB ports, PCI buses, etc. It uses the upper-most portions of memory to do so. Note that “address space” is distinct from “physical memory” — it is possible for the BIOS to assign 3.5GB-4GB address space to hardware even when there is only, say, 2GB of physical memory installed. So it’s only when the physical memory encroaches on this MMIO address space that you see a loss of memory reported to windows: the computer has a choice between using that address space for hardware or for RAM, and it chooses hardware. This is also a distinct phenomenon from the physical memory assigned to integrated video adapter
