Unavailable Memory
The loss of usable memory caused by the PCI hole is because installed hardware devices need address space in order to communicate with the processor and the system software. 32-bit hardware only has 4 gigabytes of total addressable memory so for devices to have room to communicate, some of the real physical memory of a machine with enough memory installed to encroach into the device address space (this depends upon the machine, but is usually anything above 2.5 to 3.5GB) is hidden to provide space for the installed devices. (With 16 bits, 2 to the power of 16 = 65,536 different locations can be addressed; with 32 bits, 4,294,967,296; and with 64 bits 18,446,744,073,709,551,616.)
The amount of system memory that is hidden and unavailable varies widely with the mainboard and chipset used, the BIOS, the amount of physical memory, the amount of Video RAM installed on graphics cards, and the number and type of PCI cards installed in the system. More than a gigabyte of 32-bit system memory can be unavailable when 4 gigabytes of physical memory and multiple 3D cards with large amounts of video memory are installed; on some mainboards, the hole is always at least 1 gigabyte in size, no matter what cards are installed.
Read more about this topic: PCI Hole
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