RAM limit In non-PAE modes of 32-bit x86 processors, the usable RAM may be limited to less than 4 GB. Limits on memory and address space vary by platform and operating system. Limits on physical memory for 32-bit platforms also depend on the presence and use of Physical Address Extension (PAE), which allows 32-bit systems to use more than 4 GB of physical memory. PAE and 64-bit systems may be able to address up to the full address space of the x86 processor. See also DOS memory management Motherboard References External links MSDN Article: Memory Limits for Windows Releases 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 explains the issue Windows Vista SP1 includes reporting of Installed System Memory (RAM) details about the RAM limit Computer memory X86 memory management DOS memory management