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Offset Register Format
Bit
7
6
5
4
3
2
1
0
I I
Bit O-Bit 7
I I I
~
Logical line width of the screen
Offset-This register specifies the logical line
width of the screen. The starting memory
address for the next character row is larger than
the current character row by this amount. The
Offset Register is programmed with a word
address. Depending upon the method of clocking
the CRT Controller, this word address is either a
word or double word address.
Underline Location Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 14. The processor output port
address for this register is hex 3B5 or hex 3D5.
Underline Location Register Format
Bit
7
6
5
4
3
2
1
0
III
Bit O-Bit 4
I I
:
Horizontal row scan where
underline will occur
Not Used
Underline Location-This register specifies the
horizontal row scan on which underline will
occur. The value programmed is one less than
the scan line number desired.
Start Vertical Blanking Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 15. The processor output port
IBM Enhanced Graphics Adapter 39
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