Intel 815 ST01Input Status, Bit Descriptions, Vertical Retrace/Video, Display Enable Output

Models: 815

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Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

R

9.1.2.

ST01Input Status 1

 

 

I/O (and Memory Offset) Address:

3BAh/3DAh

 

Default:

00h

 

Attributes:

Read Only

The address selection is dependent on CGA or MDA emulation mode as selected via the MSR register.

 

7

 

6

5

4

3

2

1

0

 

 

Reserved

 

Reserved

Video Feedback

 

Vertical

 

Reserved (00)

Display

 

 

(0)

 

 

(0)

 

 

Retrace

 

 

Enable

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bit

 

 

 

 

 

Descriptions

 

 

 

 

 

 

 

 

 

 

 

 

 

7

 

Reserved (as per VGA specification). Read as 0s.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6

 

Reserved. Read as 0.

 

 

 

 

 

 

 

 

 

 

 

5:4

 

Video Feedback 1, 0. These are diagnostic video bits that are selected by the Color Plane Enable

 

 

 

Register. These bits that are programmably connected to 2 of the 8 color bits sent to the palette. Bits 4

 

 

 

and 5 of the Color Plane Enable Register (AR12) selects which two of the 8 possible color bits become

 

 

 

connected to these 2 bits of this register. The current software normally does not use these 2 bits. They

 

 

 

exist for EGA compatibility.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

Vertical Retrace/Video.

 

 

 

 

 

 

 

 

 

0 = VSYNC inactive (Indicates that a vertical retrace interval is not taking place).

 

 

 

 

 

1 = VSYNC active (Indicates that a vertical retrace interval is taking place).

 

 

 

 

 

Note:

 

 

 

 

 

 

 

 

 

 

Bits 4 and 5 of the Vertical Retrace End Register (CR11) can program this bit to generate an interrupt at

 

 

 

the start of the vertical retrace interval. This ability to generate interrupts at the start of the vertical retrace

 

 

 

interval is a feature that is largely unused by current software.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2:1

 

Reserved. Read as 0s.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

Display Enable Output.

 

 

 

 

 

 

 

 

 

0 = DE inactive. Active display area data is being drawn on the display. Neither a horizontal retrace

 

 

 

 

interval or a vertical retrace interval is currently taking place.

 

 

 

 

 

 

1 = DE active. Either a horizontal retrace interval or a vertical retrace interval is currently taking place.

 

 

 

 

 

 

 

 

 

 

 

 

 

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Page 81
Image 81
Intel 815 manual ST01Input Status, Bit Descriptions, Vertical Retrace/Video, Display Enable Output