Intel 815 manual Basic Graphics Data Considerations, Contiguous vs. Discontinuous Graphics Data

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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6.2.Basic Graphics Data Considerations

6.2.1.Contiguous vs. Discontinuous Graphics Data

Graphics data stored in memory, particularly in the frame buffer of a graphics system, has organizational characteristics that often distinguish it from other varieties of data. The main distinctive feature is the tendency for graphics data to be organized in a discontinuous block of graphics data made up of multiple sub-blocks of bytes, instead of a single contiguous block of bytes.

Figure 12. Representation of On-Screen Single 6-Pixel Line in the Frame Buffer

(0, 0)	(639, 0)
	63	32 31	0
			270F8h
			28100h
			28108h
256, 256	261, 256
	256th Scan Line
	Note: Drawing is not to scale
(0, 479)	(639, 479)
		b_blt5.vsd

The figure above shows an example of contiguous graphics data — a horizontal line made up of six adjacent pixels within a single scan line on a display with a resolution of 640x480. Presuming that the graphics system driving this display has been set to 8 bits per pixel, and that the frame buffer’s starting address of 0h corresponds to the upper left-most pixel of this display, then the six pixels that make this horizontal line starting at coordinates (256, 256) would occupy six bytes starting at frame buffer address 28100h, and ending at address 28105h.

In this case, there is only one scan line’s worth of graphics data in this single horizontal line, so the block of graphics data for all six of these pixels exists as a single, contiguous block comprised of only these six bytes. The starting address and the number of bytes are the only pieces of information that a BLT engine would require to read this block of data.

The simplicity of the above example of a single horizontal line contrasts sharply to the example of discontinuous graphics data depicted in the figure below. The simple six-pixel line of the figure above is now accompanied by three more six-pixel lines placed on subsequent scan lines, resulting in the 6x4 block of pixels shown.

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