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Byte Swapping and Frame Buffers

Another example of byte swapping is what happens to multibyte pixels in a frame buffer. Macintosh software is compatible with several multibyte pixel formats, of which 16-bit pixels provide a good example of the effects of byte swapping. The Macintosh 16-bit RGB format interprets a half word as consisting of a 1-bit alpha value followed by three 5-bit red, green, and blue color components. The diagram in Figure A-7 shows how these pixels are packed into a word in big-endian memory.

Figure A-7 Big-endian RGB 16-bit pixel format

When this data is moved across the little-endian PCI bus, data swapping makes the data appear as shown in Figure A-8.

Figure A-8 Little-endian RGB 16-bit pixel format

Notice two effects of the byte swapping process:

• The relative location of the pixels is correct for the little-endian PCI; this is a direct consequence of maintaining address invariance.
• The data within the pixels has been partly rearranged. For example, the green component has been split into two pieces because it spans a byte boundary.