In computing, an aperture is a portion of physical address space (i.e. physical memory) that is associated with a particular peripheral device or a memory unit.
Apertures may reach external devices such as ROM or RAM chips, or internal memory on the CPU itself.
The set of selector address ranges of the apertures are disjoint.
When the CPU presents a physical address within the range recognized by an aperture, the aperture unit routes the request (with the address remapped to a zero base) to the attached device.
Thus, apertures form a layer of address translation below the level of the usual virtual-to-physical mapping.