Intel 815 manual Graphics Controller Registers, 7. SR07Horizontal Character Counter Reset

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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9.2.7.SR07Horizontal Character Counter Reset

I/O (and Memory Offset) Address:	3C5h (index=07h)
Default:	00h
Attributes:	Read/Write

Writing this register with any data causes the horizontal character counter to be held in reset (the character counter output will remain 0) until a write occurs to any other sequencer register location with SRX set to an index of 0 through 6.

The vertical line counter is clocked by a signal derived from the horizontal display enable (which does not occur if the horizontal counter is held in reset). Therefore, if a write occurs to this register during the vertical retrace interval, both the horizontal and vertical counters will be set to 0. A write to any other sequencer register location (with SRX set to an index of 0 through 6) may then be used to start both counters with reasonable synchronization to an external event via software control.

This is a standard VGA register that was not documented by IBM.

Bit	Description
7:0	Horizontal Character Counter.

9.3.Graphics Controller Registers

The graphics controller registers are accessed via either I/O space or Memory space. To access the registers the VGA Graphics Controller Index Register at I/O address 3CEh (or memory address 3CEh) is written with the index of the desired register. Then the desired register is accessed through the data port for the graphics controller registers at I/O address 3CFh (or memory address 3CFh).

9.3.1.GRXGRX Graphics Controller Index Register

I/O (and Memory Offset) Address:				3CEh
Default:				000UUUUUb (U=Undefined)
Attributes:				Read/Write
7				4	3	0
			Reserved (0000)		Graphics Controller Register Index
		Bit			Description
	7:4		Reserved. Read as 0s.
	3:0		Sequencer Register Index. This field selects any one of the graphics controller registers (GR[00:1F]) to
			be accessed via the data port at I/O location 3CFh.

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