Intel 815 manual Overlay Source Size Registers, SWID-Source Width Register

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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15.4.5.Overlay Source Size Registers

These registers provide information to the overlay engine on what data needs to be fetched from memory. If the overlay destination window is smaller than the result of the scaled up source, it will be clipped on the right and bottom of the overlay window. The source data is clipped within one QWord. Source width must not specify an additional QWord or more of data that is not required to satisfy the destination. If the scaled source is smaller than the destination window then the last pixel or last scan line accessed is used to fill the right and bottom areas. The minimum source width per operand (Y, U, and V for planar formats, packed formats have only one operand) is two QWords.

15.4.5.1.SWID—Source Width Register

Memory Address Offset:					34h (R/W)
On-chip Reg. Mem Addr Offset:					30134h (RO; debug path)
Default Value:					00h
Access:					see address offset above
Size:					32 bits
31			24	23	16	15	9	8	0
			Reserved	UV Source Width		Reserved		Y/RGB Source Width
		Bit				Description
	31:24		Reserved.
	23:16		UV Source Width. The number of bytes contained in a single line of planar UV source data. This is
			unused in packed modes and for planar modes it is used for the U and V source width (assumed to be
			the same) or in the interleaved mode (YI64) as TBD. When the last pixel is reached and the complete
			destination window has not been filled, this pixel will be repeated until the end of the destination
			window.
			When displaying YUV 4:1:1 data, this field contains the number of U bytes which identical to the
			number of V bytes and ¼ of the Y bytes.
	15:9		Reserved.
	8:0		Y/RGB Source Width. The number of bytes contained in a single line of source data. In planar modes,
			this is the Y source width.
			This should include all contributing pixel data.
			When the last pixel is reached and the complete destination window has not been filled, this pixel will
			be repeated until the end of the destination window.
			When displaying YUV 4:2:2 data, the atomic unit is a doubleword (2 Ys + U + V).
			When displaying YUV 4:1:1 data, the atomic unit is 3 doublewords (8 Ys + 2U + 2V).
			The starting offset within the buffer must reflect this restriction.

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