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address for this register is hex 3B5 or hex 3D5.
Start Vertical Blanking Register Format
Bit
7
6
5
4
3
2
1
0
I I I I I I I •
Start Vertical Blanking
Bit O-Bit 7
Start Vertical Blank-These are the low 8 bits of
the horizontal scan line count, at which the
vertical blanking signal becomes active. Bit 8 bit
is in the overflow register hex 07.
End Vertical Blanking Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 16. The processor output port
address for this register is hex 3B5 or hex 3D5.
End Vertical Blanking Register Format
Bit
7
6
5
4
3
2
1
0
I I I
Bit O-Bit 4
I
I
I I
:
End Vertical Blanking
Not Used
End Vertical Blank-This register specifies the
horizontal scan count value when the vertical
blank output signal becomes inactive. The
register is programmed in units of horizontal scan
lines. To obtain a vertical blank signal of width
W, the following algorithm is used: Value of Start
Vertical Blank Register
+
width of vertical blank
signal in horizontal scan units
=
5-bit result to be
programmed into the End Vertical Blank
Register.
40 IBM Enhanced Graphics Adapter
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