Intel 815 23. CR15Vertical Blanking Start Register, 24. CR16Vertical Blanking End Register

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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9.6.23.CR15Vertical Blanking Start Register

I/O (and Memory Offset) Address:			3B5h/3D5h (index=15h)
Default:			Undefined
Attributes:			Read/Write
	Bit		Description
	7:0	Vertical Blanking Start Bits [7:0]. This register provides the 8 least significant bits of either a 10-bit or
		12-bit value that specifies the beginning of the vertical blanking period relative to the beginning of the
		active display area of the screen. Whether this value is described in 10 or 12 bits depends on the setting
		of bit 0 of the I/O Control Register (CR80).
		In standard VGA modes, where bit 0 of the I/O Control Register (CR80) is set to 0, the vertical blanking
		start is specified with a 10-bit value. The most and second-most significant bits of this value are
		supplied by bit 5 of the Maximum Scan Line Register (CR09) and bit 3 of the Overflow Register (CR07),
		respectively.
		In extended modes, where bit 0 of the I/O Control Register (CR80) is set to 1, the vertical blanking start
		is specified with a 12-bit value. The 4 most significant bits of this value are supplied by bits [3:0] of the
		Extended Vertical Blanking Start Register (CR33).
		This 10-bit or 12-bit value should be programmed to be equal the number of scan lines from the
		beginning of the active display area to the beginning of the vertical blanking period. Since the active
		display area always starts on the 0th scan line, this number should be equal to the number of the scan
		line on which vertical blanking begins.

9.6.24.CR16Vertical Blanking End Register

I/O (and Memory Offset) Address:	3B5h/3D5h (index=16h)
Default:	Undefined
Attributes:	Read/Write

This register provides an 8-bit value that specifies the end of the vertical blanking period relative to its beginning.

Bit	Description
7:0	Vertical Blanking End Bits [7:0]. This 8-bit value should be set equal to the least significant 8 bits of
	the result of adding the length of the vertical blanking period in terms of the number of scan lines that
	occur within the length of the vertical blanking period to the value that specifies the beginning of the
	vertical blanking period (see the description of the Vertical Blanking Start Register for details).

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