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Mode Control Register
This is a write-only register pointed to when the value in the CRT
Controller address register is hex 17. The processor output port
address for this register is hex 3B5 or hex 3D5.
Mode Control Register Format
Bit
7
6
5
4
3
2
1
a
Bit 0
CMSO
Select Row Scan Counter
Horizontal Retrace Select
Count by Two
'---------l~
Output Control
'--_ _ _ _ _ _
~
Address Wrap
'----------i,~
Word/Byte Mode
'-------------1~~
Hardware Reset
Compatibility Mode Support- When this bit is
a logical 0, the row scan address bit 0 is
substituted for memory address bit 13 during
active display time. A logical 1 enables memory
address bit 13 to appear on the memory address
output bit 13 signal of the CRT Controller. The
CRT Controller used on the IBM
Color/Graphics Monitor Adapter is the 6845.
The 6845 has 128 horizontal scan line address
capability. To obtain 640 by 200 graphics
resolution, the CR TC was programmed for 100
horizontal scan lines with 2 row scan addresses
per character row. Row scan address bit 0
became the most significant address bit to the
display buffer. Successive scan lines of the
display image were displaced in memory by 8K
bytes. This bit allows compatibility with the
6845 and Color Graphics AP A modes of
operation.
IBM Enhanced Graphics Adapter 41
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