IBM RT Series Hardware Reference Manual page 200

Color Plane Enable Register
This is a write-only register pointed to by the value in the
Attribute address register. This value must be hex 12 before
writing can take place. The processor output port address for this
register is 3CD.
Color Plane Enable Register Format
Bit 7 6 5 4 3 2 1 0
11a....-.....L-11_11_1~1:
Enable Color Plane
Video Status MUX
Not Used
Bit O-Bit 3
Bit 4-Bit 5
COLOR PLANE
Enable Color Plane-Writing a logical 1 in any
of bits 0 through 3 enables the respective display
memory color plane.
Video Status MUX-Selects two of the six color
outputs to be available on the status port. The
following table illustrates the combinations
available and the color output wiring.
INPUT STATUS
ENABLE REGISTER REGISTER ONE
Bit5
Bit4 Bit5 Bit4
0 0 Red Blue
0 1 Secondary Blue Green
1 0 Secondary Red Secondary Green
1 1 Not Used Not Used
Horizontal Pel Panning Register
This is a write-only register pointed to by the value in the
Attribute address register. This value must be hex 12 before
writing can take place. The processor output port address for this
register is hex 3CO.
60 IBM Enhanced Graphics Adapter