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CRT Controller Address Register Format
Bit
7
6
5
4
3
2
1
0
Bit O-Bit 4
CRT Controller Address Bits-A binary value
pointing to the CRT Controller register where
data is to be written.
Horizontal Total Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 00. The processor output port
address for this register is hex 3B5 or hex 3D5.
Horizontal Total Register Format
Bit
7
6
5
4
3
2
1
0
~I
-J1L.........L.1-..L..1 --,--I -&.1-..L.. 1 --,--I
~.
Horizontal Total
This register defines the total number of characters in the
horizontal scan interval including the retrace time. The value
directly controls the period of the horizontal retrace output signal.
An internal horizontal character counter counts character clock
inputs to the CRT Controller, and all horizontal and vertical
timings are based upon the horizontal register. Comparators are
used to compare register values with horizontal character values
to provide horizontal timings.
Bit O-Bit 7
Horizontal Total-The total number of
characters less 2.
IBM Enhanced Graphics Adapter 25
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