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Vertical Retrace Start Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 10. The processor output port
address for this register is hex 3B5 or hex 3D5.
Vertical Retrace Start Register Format
Bit
7
6
5
4
3
2
1
0
I I I I I I I I
Low Order Vertical Retrace Pulse
Bit O-Bit 7
Vertical Retrace Start-This is the low-order 8
bits of the vertical retrace pulse start position
programmed in horizontal scan lines. Bit 8 is in
the overflow register location hex 07.
Light Pen High Register
This is a read-only register pointed to when the value in the CRT
Controller address register is hex 10. The processor input port
address for this register is hex 3B5 or hex 3D5.
Light Pen High Register Format
Bit
7
6
5
4
3
2.
1
0
I I I I I I I I
~
High Order Memory Address
Counter
Bit O-Bit 7
Light Pen High-This is the high order 8 bits of
the memory address counter at the time the light
pen was triggered.
Vertical Retrace End Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 11. The processor output port
36 IBM Enhanced Graphics Adapter
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