IBM RT Series Hardware Reference Manual page 198

Mode Control Register
This is a write-only register pointed to by the value in the
Attribute address register. This value must be hex 10 before
writing can take place. The processor output port address for this
register is hex 3CO.
Mode Control Register Format
Bit 7 6 5 4 3 2 1 0
Bit 0
Bit 1
Bit 2
Graphics/Alphanumeric Mode
Display Type
Enable Line Graphics Character
Codes
Select Background Intensity Or
Enable Blink
'----'-...I.---'-_ _ _ _ _ ......
~
Not Used
Graphics/ Alphanumeric Mode-A logical 0
selects selects alphanumeric mode. A logical 1
selects graphics mode.
Monochrome Display/Color Display-A logical
o
selects IBM monochrome display attributes. A
logical 1 selects color Display attributes.
Enable Line Graphics Character Codes-When
this bit is set to 0, the ninth dot will be the same
as the background. A logical 1 enables the
special line graphics character codes for the IBM
Monochrome Display adapter. This bit when
enabled forces the ninth dot of a line graphic
character to be identical to the eighth dot of the
character. The line graphics character codes for
the Monochrome Display Adapter are Hex CO
through Hex DF.
For character fonts that do not utilize the line
graphics character codes in the range of Hex CO
58 IBM Enhanced Graphics Adapter