Even if you did everything right while installing new memory, you can find especially with older systems the phenomenon that the operating system displays less than the installed RAM. The most notorious case is that 4 GB or more physical memory is available, but only 3,25 GB or less can be addressed.
What is the reason?
Due to PC architectural requirements such as motherboard resources and OS limitations, platforms using larger memory may be unable to take full advantage of all memory installed on the system.
Basically, for the usage of more than 4GB memory a 64-bit operating system is necessary. Because of the computer architecture, a 32-bit operating system cannot address more than 232 Byte, which limits the use of memory to 4 GB.
If this restriction is no possible reason for less displayed RAM, this is due to the “4-GB problem“: When systems with chipsets, that support 4GB or more of system memory, are populated with 4GB or more, the Operating System (OS) may report a lower amount of available memory. Standard PC Architecture System Resources require addressing which overlaps physical memory below 4GB:
- System BIOS
- Motherboard Resources (I/OxAPIC)
- Memory Mapped I/O
- PCI Express Configuration Space
- Additional PCI Device Memory (Graphics Aperture)
- VGA Memory
- Others as included, etc..
These requirements may reduce the addressable memory space available to and reported by the Operating System. These memory ranges, while unavailable to the OS, are still being utilized by subsystems such as I/O, PCI Express and Integrated Graphics and are critical to the proper functioning of the PC.
x86 chipsets that support more than 4 GB of RAM typically also support memory remapping (referred to in some BIOS setup screens as “memory hole remapping”).
In this scheme, the BIOS detects the memory address conflict and in effect relocates the interfering RAM so that it may be addressed by the processor at a new physical address that does not conflict with MMIO.
Please find detailed explanations about this topic from the view of Microsoft here: