Before the installation of Windows Vista SP1, Windows Vista displayed how much system memory was available to the operating system. For computers that have 3 GB or less of physical memory installed, the reported memory value can sometimes be somewhat less than the memory physically that is installed. These differences are because of the BIOS and possibly driver reservations of physical memory. For example, some BIOS implementations allocate some physical memory for the graphics adapter. Typical sizes are from 8 MB to 128 MB for graphics. Other sizes are possible. The BIOS and some drivers may reserve physical memory for other peripherals or purposes. These reservations are reasonable and are part of the usual operation for a computer. However, they do reduce how much physical memory is available for use by the operating system and applications. These reservations affect both 32-bit and 64-bit computers.
On computers that have a 32-bit operating system, more than 3 GB of system memory, and with a version of Windows that is earlier than Windows Vista SP1, users will see a larger difference in how much memory is reported as available to the operating system compared to how much physical memory is installed. This is because some physical address space must be reserved as I/O regions for memory mapped peripherals. These I/O regions are allocated between the 3 GB physical address and the 4 GB upper physical address limit.
Physical memory addresses that are mapped to these I/O regions cannot be used to address physical system memory. These addresses also cannot be used to prevent the operating system from using some physical memory that would ordinarily be accessed between the 3GB physical address and the 4GB upper physical address limit. The size of these I/O regions varies from system to system because they determine the type and configuration of the system’s peripherals.