IBM RT Series Hardware Reference Manual page 196

Attribute Controller Registers
Name Port Index
Address Register
3CO
-
Palette Registers 3CO OO-OF
Mode Control Register
3CO
10
Overscan Color Register 3CO 11
Color Plane Enable Register 3CO
12
Horizontal Pel Panning Register 3CO 13
Attribute Address Register
This is a write-only register. The processor output port is hex
3CO.
Palette Registers Hex 00 through Hex OF Format
Bit
7
6
5
Bit O-Bit 4
4 3 2 1 0
~
Blue Video
Green Video
Red Video
Secondary Blue/Mono Video
,
Secondary Green/Intensity
,
Secondary Red Video
, Not Used
Attribute Address Bits-The Address Register is
a pointer register located at hex 3CO. This
register is loaded with a binary value that points
to the attribute data register where data is to be
written. The Attribute Controller does not have
an address bit input to control selection of the
address and data registers. An internal address
flip-flop controls selection of either the address
or data registers. To initialize the flip-flop, an
lOR instruction is issued to the Attribute
Controller at address 3BA or 3DA. This clears
the flip-flop, and selects the Address Register.
After the Address Register has been loaded, the
56 IBM Enhanced Graphics Adapter