Intel 815 manual 10. CR08Preset Row Scan Register, Bit Description

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

R

Bit	Description
0	Vertical Total Bit 8. The vertical total is a 10-bit or 12-bit value that specifies the total number of scan
	lines. This includes the scan lines both inside and outside of the active display area.
	In standard VGA modes, where bit 0 of the I/O Control Register (CR80) is set to 0, the vertical total is
	specified with a 10-bit value. The 8 least significant bits of this value are supplied by bits [7:0] of the
	Vertical Total Register (CR06), and the most and second-most significant bits are supplied by bit 5 and
	this bit, respectively, of this register.
	In extended modes, where bit 0 of the I/O Control Register (CR80) is set to 1, the vertical total is
	specified with a 12-bit value. The 8 least significant bits of this value are supplied by bits [7:0] of the
	Vertical Total Register (CR06), and the 4 most significant bits are supplied by 3-0 bits of the Extended
	Vertical Total Register (CR30). In extended modes, neither this bit, nor bit 5 of this register are used.
	This 10-bit or 12-bit value should be programmed to be equal to the total number of scan lines, minus 2.

9.6.10.CR08Preset Row Scan Register

I/O (and Memory Offset) Address:					3B5h/3D5h (index=08h)
Default:					0UUU UUUUb (U=Undefined)
Attributes:					Read/Write
7			6	5	4	0
		Reserved		Byte Panning	Starting Row Scan Count
		(0)
		Bit			Description
	7		Reserved. Read as 0s.
	6:5		Byte Panning. This field holds a 2-bit value that selects number of bytes (up to 3) by which the image is
			shifted horizontally to the left on the screen. This function is available in both text and graphics modes.
			In text modes with a 9-pixel wide character box, the image can be shifted up to 27 pixels to the left, in
			increments of 9 pixels. In text modes with an 8-pixel wide character box, and in all standard VGA
			graphics modes, the image can be shifted up to 24 pixels to the left, in increments of 8 pixels.
			The image can be shifted still further, in increments of individual pixels, through the use of bits [3:0] of
			the Horizontal Pixel Panning Register (AR13).
				Number of Pixels Shifted
			Bit [6:5]	9-Pixel Text	8-Pixel Text & Graphics
			00	0	0
			01	9	8
			10	18	16
			11	27	24
	4:0		Starting Row Scan Count. This field specifies which horizontal line of pixels within the character boxes
			of the characters used on the top-most row of text on the display will be used as the top-most scan line.
			The horizontal lines of pixels of a character box are numbered from top to bottom, with the top-most line
			of pixels being number 0. If a horizontal line of the these character boxes other than the top-most line is
			specified, then the horizontal lines of the character box above the specified line of the character box will
			not be displayed as part of the top-most row of text characters on the display. Normally, the value
			specified by these 5 bits should be 0, so that all of the horizontal lines of pixels within these character
			boxes will be displayed in the top-most row of text, ensuring that the characters in the top-most row of
			text do not look as though they have been cut off at the top.

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