Intel 815 manual 8. GR06Miscellaneous Register, Bit, Graphics/Text Mode

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

R

9.3.8.GR06Miscellaneous Register

I/O (and Memory Offset) Address: 3CFh (Index=06h)
Default:				0Uh (U=Undefined)
Attributes:				Read/Write
7				4	3	2	1	0
				Reserved (0000)		Memory Map Mode	Chain	Graphics /
							Odd/Even	Text
								Mode
		Bit			Description
	7:4		Reserved. Read as 0s.
	3:2		Memory Map Mode. These 2 bits control the mapping of the frame buffer into the processor address
			space as follows:
			Bit [3:2]	Frame Buffer Address Range
			00	A0000h − BFFFFh
			01	A0000h − AFFFFh
			10	B0000h − B7FFFh
			11	B8000h − BFFFFh
			Notes:
			1. This function is both in standard VGA modes and in extended modes that do not provide linear frame
			buffer access.
			2. Software must set to the proper value.
	1		Chain Odd/Even. This bit provides the ability to alter the interpretation of address bit A0, so that it may
			be used in selecting between the odd-numbered memory planes (planes 1 and 3) and the even-
			numbered memory planes (planes 0 and 2).
			0 = A0 functions normally.
			1 = A0 is switched with a high order address bit, in terms of how it is used in address decoding. The
			result is that A0 is used to determine which memory plane is being accessed
			(A0=0 for planes 0 and 2 and A0=1 for planes 1 and 3).
	0		Graphics/Text Mode.
			0 = Text mode.
			1 = Graphics mode.

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