System Information shows only (3GB) while your Windows Vista has 4 GB of memory

32-bit Windows is limited to a maximum of 4GB and cannot see any pages above 4GB. 64-bit Windows can use between 8GB and 128GB depending on SKU.

Various devices in a typical computer require memory-mapped access. This is known as memory-mapped I/O (MMIO). For the MMIO space to be available to 32-bit operating systems, the MMIO space must reside within the first 4 GB of address space. This is independent of the OS running on the machine.

For example, if you have a video card that has 256 MB of onboard memory, that memory must be mapped within the first 4 GB of address space. If 4 GB of system memory is already installed, part of that address space must be reserved by the graphics memory mapping. Graphics memory mapping overwrites a part of the system memory. These conditions reduce the total amount of system memory that is available to the operating system.

The reduction in available system memory depends on the devices that are installed in the computer. However, to avoid potential driver compatibility issues, the 32-bit versions of Windows Vista limit the total available memory to 3.12 GB. If a computer has many installed devices, the available memory may be reduced to 3 GB or less. However, the maximum memory available in 32-bit versions of Windows Vista is typically 3.12 GB.

Physical Address Extension (PAE) extends the physical address space to 36-bits if your HW supports this. For most operations, the processor execution units will only see 32-bit addresses; the MMU will take care of the translation to 36bit addresses. No swapping here, only page translations (which are used regardless of PAE being on or not), this is a fundamental feature of any virtual memory operating system.

The OS and apps only see 32-bit addresses because the registers are limited to 32-bits (hence the “32-bit” architecture nomenclature). These are linear addresses which are extended to 36-bits in the translation to physical addresses, but they never show up in registers since there’s no room. It’s all internal until the address lines coming out of the chip are toggled.

So, the OS can happily handle up to 64 GB of memory for 32-bit PAE-able systems.

The above explanations do not change for 32 bit Windows XP either.

I hope this helps explain why you can not see 4 GB of RAM in system.

For more information, please visit the Microsoft KB web link given at:

http://support.microsoft.com/kb/929605/en-us

Additionally, you may like to know about the components of User Interface in Windows Vista which report slightly different values for the total physical memory that is available on the Computer. Please visit the web link at:

http://support.microsoft.com/kb/935268/en-us