IBM RT Series Hardware Reference Manual page 154

negative vertical retrace. The IBM Monochrome
display requires a negative vertical retrace
polarity.
Feature Control Register
This is a write-only register. The processor output register is hex
3BAor3DA.
Feature Control Register Format
Bit 7 6 5 4 3 2 1 0
Bits 0 and 1
Feature Control Bit 0
Feature Control Bit
1
Reserved
Not Used
Feature Control Bits-These bits are used to
convey information to the feature connector.
The output of these bits goes to the FEAT 0 (pin
19) and FEAT 1 (pin 17) of the feature
connector.
Input Status Register Zero
This is a read-only register. The processor input port address is
hex 3C2.
14 IBM Enhanced Graphics Adapter