User's Manual

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0
R
90
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.