Intel 815 manual 38. CR70Interlace Control Register, 39. CR80I/O Control, Interlace Enable

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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9.6.38.CR70Interlace Control Register

I/O (and Memory Offset) Address:				3B5h/3D5h (index=70h)
Default:				00h
Attributes:				Read/Write
7			6	0
		Interlace		CRT Half-Line Value
		Enable
		Bit		Description
	7		Interlace Enable.
			0 = Selects non-interlaced CRT output (default).
			1 = Selects interlaced CRT output.
	6:0		CRT Half-Line Value. When interlaced CRT output has been selected, the value in this register
			specifies the position along the length of a scan line at which the half-line vertical sync pulse occurs for
			the odd frame. This half-line vertical sync pulse begins at a position between two horizontal sync pulses
			on the last scan line, rather than coincident with the beginning of a horizontal sync pulse at the end of a
			scan line.

9.6.39.CR80I/O Control

I/O (and Memory Offset) Address:				3B5h/3D5h(index 80h)
Default:				00h
Attributes:				Read/Write
7				2	1	0
			Reserved (000000)		Attr Cntl	CRT Cntl
					Ext Enbl	Int Enbl
		Bit		Description
	7:2		Reserved.
	1		Attribute Controller Extensions Enable. Controls whether the attribute registers are accessed with
			both index and data at 3C0h (strict VGA mode), or whether they are accessed with 3c0h as the index
			and 3C1h is data. It is possible that BIOS software or driver software might not use the non-VGA mode.
			Either method should work, but it needs to be the method the software is using.
			0 = Disable, i.e., strictly VGA compatible mode (default)
			1 = Enable Attribute Controller extensions
			Index and Data of the Attribute Controller registers are accessible at 3C0h in standard VGA. When the
			Attribute Controller Extensions are enabled, Index and Data are accessible at addresses 3C0h and
			3C1h, respectively.
	0		CRT Controller Interpretation Enable. This bit modifies responses/functionality to registers CR30 and
			beyond, except for CR41.
			0 = Registers have strict VGA Interpretation (default)
			1 = Registers have extended VGA Interpretation (i.e., access to 4G space)

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