IBM RT Series Hardware Reference Manual page 177

address for this register is hex 3B5 or hex 3D5.
Vertical Retrace End Register Format
Bit 7 6 5
I I
I
Bit O-Bit 3
Bit 4
Bit 5
4 3 2 1
0
I
I I I I
~
Vertical Retrace End
O=Clear Vertical Interrupt
O=Enable Vertical Interrupt
Not Used
Vertical Retrace End-These bits determine the
horizontal scan count value when the vertical
retrace output signal becomes inactive. The
register is programmed in units of horizontal scan
lines. To obtain a vertical retrace signal of width
W, the following algorithm is used: Value of Start
Vertical Retrace Register
+
width of vertical
retrace signal in horizontal scan units = 4-bit
result to be programmed into the End Horizontal
Retrace Register.
Clear Vertical Interrupt-A logical 0 will clear a
vertical interrupt.
Enable Vertical Interrupt-A logical 0 will
enable vertical interrupt.
Light Pen Low Register
This is a read-only register pointed to when the value in the CRT
Controller address register is hex 11. The processor input port
address for this register is hex 3B5 or 3D5.
IBM Enhanced Graphics Adapter 37