|This article needs additional citations for verification. (October 2012)|
The PCI hole or PCI memory hole is a limitation of 32-bit hardware and 32-bit operating systems that causes a computer to appear to have less memory available than is physically installed. This memory addressing limitation and the later workarounds necessary to overcome it are functionally similar to the memory limits of the early 8088 IBM PC memory architecture (see Conventional memory).
Similar situations have often arisen in the history of computing, when hardware intended to have up to a certain level of resources is designed to handle several times the maximum expected amount, which eventually becomes a severe restriction as Moore's law increases resources economically available. The original IBM PC was typically supplied with 64kB of memory or less; it was designed to take a maximum of 640kB, far more than it was thought would ever be needed. In fact, Bill Gates is famously (but erroneously) thought to have pointed out that "640 KB ought to be enough for anyone." This rapidly became a restriction that had to be handled by complex DOS memory management. Similar successive restrictions in size have been imposed and overcome on hard drives.
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.
Physical address extension
A workaround first developed in the Pentium Pro, known as Physical Address Extension, allows certain 32-bit operating systems to access up to 36-bit memory addresses, even though individual programs are still limited to operating within 32 bits of address space. Each program can have its own 4 gigabyte addressing space, together utilizing up to 64 gigabytes of memory across all programs. This was fully supported in Windows XP up to the Service Pack 1 (SP1) release, but then withdrawn for SP2; the only 32-bit versions of Microsoft Windows to fully support this are certain high-end server versions of Windows Server 2003 and earlier; it remained in use by some 32-bit Linux distributions. Microsoft disabled the support in Windows XP SP2 and later operating systems because there were many compatibility problems with graphics card and other devices, which needed PAE-aware drivers, distinct from both standard 32-bit and later 64-bit drivers. Many versions of MS Windows can activate what is still called Physical Address Extensions for the purpose of using the NX bit, but this no longer extends the address space.
Filling the memory hole
In a 64-bit system memory below and above the "memory hole" is available, but 500 to 1,500 MB of RAM is unavailable because it uses memory addresses required for devices. With the decreasing cost of memory this may not be a serious issue, but there are ways to regain access to the missing memory.
Mapping devices to addresses above 4GB
The limitations of the 32-bit PCI hole can affect purely 64-bit operating systems as the system BIOS must cater for all operating systems which are supported by the hardware (16-, 32-, and 64-bit operating systems all run on the same hardware). The BIOS must be able to boot mapping all devices below 4 gigabytes, although a 64-bit system does not require this. Many BIOSes can be configured by the user to fill the memory hole by mapping devices high up in the 64-bit address space, so long as the devices, their drivers, and the chipset all support this. A machine configured this way cannot boot into a 16- or 32-bit operating system; if a machine is set up this way, the BIOS setup must be temporarily changed to boot into a 16- or 32-bit operating system, e.g. from a bootable CD or USB storage device.
Mapping memory to addresses above 4GB
Another way to remove the PCI hole, which is only useful for 64-bit operating systems and obsolescent 32-bit systems that support the Physical Address Extension method described above, is to "remap" some or all of the memory between the 2 gigabytes and 4 gigabytes limits to addresses above 4 gigabytes. This needs to be supported by the chipset of the computer and can usually be activated in the BIOS Setup. This remapping works on the level of physical addresses, unlike the higher-level remapping of virtual to physical addresses that happens inside the CPU core. Activating this for traditional 32-bit operating systems does more harm than good, as the remapped memory (often larger than the PCI hole itself) is unusable to such operating systems, even though e.g. Windows Vista will show such memory to physically exist on the "System Properties" page.
- 3 GB barrier
- Expanded memory
- PSE-36 — an alternative to PAE on x86 processors to extend the physical memory addressing capabilities from 32 bits to 36 bits
- RAM limit