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Bit O-Bit 7
Start Horizontal Blanking-The horizontal
blanking signal becomes active when the
horizontal character counter reaches this value.
End Horizontal Blanking Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 03. The processor output port
address for this register is hex 3B5 or hex 3D5.
End Horizontal Blanking Register Format
Bit
7
6
5
4
3
2
1
0
II I
I I I I I
:~
End Blanking
.
.
Display Enable Skew Control
L...-_ _ _ _ _ _ _
----'_~
Not Used
This register determines when the horizontal blanking output
signal becomes inactive. The row scan address and underline scan
line decode outputs are multiplexed on the memory address
outputs and the cursor outputs respectively during the blanking
interval. These outputs are latched external to the CRT
Controller with the falling edge of the BLANK output signal.
The row scan address and underline signals remain on the output
signals for one character count beyond the end of the blanking
signal.
Bit O-Bit 4
End Horizontal Blanking-A value equal to the
five least significant bits of the horizontal
character counter value at which time the
horizontal blanking signal becomes inactive
(logical 0). To obtain a blanking signal of width
W, the following algorithm is used: Value of
Start Blanking Register
+
Width of Blanking
signal in character clock units
=
5-bit result to be
programmed into the End Horizontal Blanking
Register.
IBM Enhanced Graphics Adapter 27
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