PCI hole

The PCI hole, also known as the PCI memory hole, is a region in the upper area of a computer's 32-bit address space (typically between 3GB and 4GB) reserved for memory mapping of PCI devices. These devices require a dedicated space in memory to allow the CPU to communicate with them directly. The PCI hole prevents the system from utilizing the full 4GB of RAM available on a 32-bit system as the memory space occupied by the PCI devices is inaccessible to standard RAM allocation.

This limitation arose from the original design of the PCI bus and its interaction with the Intel x86 memory architecture. While newer architectures, particularly those utilizing 64-bit addressing, largely avoid the PCI hole limitation as they have significantly larger address spaces available, the concept remains relevant when discussing older systems or devices with 32-bit limitations.

The size and location of the PCI hole can vary depending on the specific hardware configuration, including the motherboard, chipset, and installed PCI devices. The actual memory space available to the operating system is therefore reduced by the amount of memory reserved for the PCI hole. This limitation led to the development of workarounds and ultimately to the adoption of 64-bit architectures.