IBM RT Series Hardware Reference Manual page 199

Bit 3
through Hex DF, bit 2 of this register should be a
logical O. Otherwise unwanted video information
will be displayed on the CRT screen.
Enable Blink/Select Background Intensity-A
logical 0 selects the background intensity of the
attribute input. This mode was available on the
Monochrome and Color Graphics adapters. A
logical 1 enables the blink attribute in
alphanumeric modes. This bit must also be set to
1 for blinking graphics modes.
Overscan Color Register
This is a write-only register pointed to by the value in the
Attribute address register. This value must be hex 11 before
writing can take place. The processor output port address for this
register is hex 3CO.
Overscan Color Register Format
Bit 7 6 5 4 3 2 1 0
Bit O-Bit 5
Selects Blue Border Color
Selects Green Border Color
Selects Red Border Color
Selects Secondary Blue
Border Color
Selects
I
ntensified or
Secondary Green
Selects Secondary Red
Border Color
Not Used
Overscan Color-This 6-bit register determines
the overscan (border) color displayed on the
CRT screen. For monochrome display this
register should be set to a value of O. A logical 1
selects the appropriate color.
IBM Enhanced Graphics Adapter 59