Why does Windows only show about 3.5 GB of my 4 GB of RAM?
You can't:
See Dude, Where's My 4 Gigabytes of RAM?
if you want to fit memory and devices into a 32-bit address range: not all of the available 4GB of address space can be given over to memory.
So what actually happens if you go out and buy 4GB of memory for your PC?
There's a hole in your memory map for the IO. (Now it's only 25% of the total address space, but it's still a big hole.) So the bottom 3GB of your memory will be available, but there's an issue with that last 1GB.
The only practical solution is to install a 64-bit operating system. In Windows Vista and later, 32-bit and 64-bit license keys are interchangeable. If you can get Windows installation media for the 64-bit version of your operating system, you can reinstall using your original license key.
First, Windows XP (32bit) only supports 4 GBs. That doesn't just apply to Windows XP. Instead, it applies to 32bit desktop Windows - 32bit Linux systems with PAE and many 32bit Windows Server editions support more than 4GB per the linked article. You will never see over 4 GBs if you are using a 32-bit Windows XP. However, I did find a more detail reason why your system shows less available ram than what is actually installed.
From The 3GB-not-4GB RAM problem (Microsoft MSDN blog):
Due to an architectural decision made long ago, if you have 4GB of physical RAM installed, Windows is only able to report a portion of the physical 4GB of RAM (ranges from ~2.75GB to 3.5GB depending on the devices installed, motherboard's chipset & BIOS).
This behavior is due to "memory mapped IO reservations". Those reservations overlay the physical address space and mask out those physical addresses so that they cannot be used for working memory. This is independent of the OS running on the machine.
Significant chunks of address space below 4GB (the highest address accessible via 32-bit) get reserved for use by system hardware:
- BIOS – including ACPI and legacy video support
- PCI bus including bridges etc.
- PCI Express support will reserve at least 256MB, up to 768MB depending on graphics card installed memory
What this means is a typical system may see between ~256MB and 1GB of address space below 4GB reserved for hardware use that the OS cannot access. Intel chipset specs are pretty good at explaining what address ranges gets reserved by default and in some cases call out that 1.5GB is always reserved and thus inaccessible to Windows.
There is more information if you check out the source. However, you may be able to take advantage of the full 8 GBs installed if you followed these instructions.
Speaking from personal experience, this is no new problem. I ran in to this same problem a few years ago when I built a 4 GBs 32-bit Windows Vista system when Vista was first released. There were countless forum posts online related to this same topic.
Read also Microsoft KB 929605 - The system memory that is reported in the System Information dialog box in Windows Vista is less than you expect if 4 GB of RAM is installed.
If you are using a 32Bit Windows XP, then it's not possible. Since there is not enough Address Space available for making use of the extra RAM
The best solution would be to upgrade to a 64-Bit Version of Windows as it supports 4+ GB of RAM.