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In some situations, a frame buffer on a PCI expansion card may need to support data accesses in more than one format. For example, a frame buffer may need to store frame buffer data from a big-endian source in three different formats--RGB, a little-endian source in RGB, and a YUV data format. To provide multiple formats on the fly, a PCI card can create multiple apertures of its frame buffer.
An aperture is a logical view of the data in a frame buffer, organized in a specific way. The PCI card converts its frame buffer contents into the required format for each aperture, and maps each aperture into a different range of memory addresses.
Each aperture is defined by specifying its starting address in memory, its width and height in pixels, and the format and size of each pixel description. The aperture definition may also include a row bytes value, giving the address offset between successive rows. Although each aperture normally has a different pixel description, the arrangement of pixels in the frame is the same for all apertures; this arrangement starts with the upper-left pixel and proceeds as described in the previous section. An aperture may represent the whole frame buffer or any region within it.
One important use for apertures is to provide both big-endian and little-endian views of a frame buffer. Providing both views can eliminate the need for support of the byte-swapping operations. For example, in a PCI card's memory space of 16 MB, 8 MB could be allocated for a big-endian aperture and registers and 8 MB could be allocated for a little-endian aperture and registers. Mac OS running on the PowerPC processor would access the big-endian aperture, while a frame-grabber PCI master card that supported a little-endian pixel format would access the little-endian aperture.
Apertures are supported by the device drivers associated with a PCI card, which must respond to calls that query and select the card's aperture capabilities. Each aperture can be treated as a virtual device, to be opened and closed separately from other apertures. A driver can treat the physical organization of the frame buffer as an aperture as well, without subjecting it to mapping or format conversion.
For more information on apertures see PCI Multimedia Design Guide, published by the PCI SIG. You can contact the PCI SIG at the address given for the PCI Special Interest Group.