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Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

9.1.4. MSR



Miscellaneous Output

I/O (and Memory Offset) Address: 3C2h  Write; 3CCh Read

Default: 00h

Attributes: See Address above

7 6 5 4 3 2 1 0

VSYNC

Polarity

HSYNC

Polarity

Page

Select

Reserved

(0)

Clock Select

A0000−

BFFFFh

Acc En

I/O

Address

Bit Descriptions

7 CRT VSync Polarity.

0 = Positive Polarity (default).

1 = Negative Polarity.

6 CRT HSync Polarity.

0 = Positive Polarity (default).

1 = Negative Polarity

5 Page Select. In Odd/Even Memory Map Mode 1 (GR6), this bit selects the upper or lower 64 KB page in

display memory for processor access:

0 = Upper page (default)

1 = Lower page.

Selects between two 64 KB pages of frame buffer memory during standard VGA odd/even modes

(modes 0h through 5h). Bit 1 of register GR06 can also program this bit in other modes. Note that this bit

is always set to 1 by the driver software.

4 Reserved. Read as 0.

3:2 Clock Select. These bits usually select the dot clock source for the CRT interface. The bits select the dot

clock in standard VGA modes.

00 = CLK0, 25 MHz (for standard VGA modes with 640 pixel horizontal resolution) (default)

01 = CLK1, 28 MHz. (for standard VGA modes with 720 pixel horizontal resolution)

1x = CLK2 (left “reserved” in standard VGA, used for all extended modes 6 MHz–135 MHz)

1 A0000−

−−

−BFFFFh Access Enable. VGA Compatibility bit enables access to local video memory (frame

buffer) at A0000−BFFFFh. When disabled, accesses to system memory are blocked in this region (by not

asserting DEVSEL#). This bit does not block processor access to the video linear frame buffer at other

addresses.

0 = Prevent processor access to frame buffer (default).

1 = Allow processor access to frame buffer.

0 I/O Address Select. This bit selects 3Bxh or 3Dxh as the I/O address for the CRT Controller registers,

the Feature Control Register (FCR), and Input Status Register 1 (ST01). Presently ignored (whole range

is claimed), but will “ignore” 3Bx for color configuration or 3Dx for monochrome.

0 = Select 3Bxh I/O address (MDA emulation) (default).

1 = Select 3Dxh I/O address (CGA emulation).

NOTES:

1. In standard VGA modes, bits 7 and 6 indicate which of the three standard VGA vertical resolutions the

standard VGA display should use. All extended modes, including those with a vertical resolution of 480 scan

lines, use a setting of 0 for both of these bits. This setting was “reserved” in the VGA standard.