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read mode 1 on the graphics chips, and does a
memory read, the data returned from the memory
cycle will be a 1 in each bit position where the 4
bit planes equal the color compare register.
Data Rotate Register
This is a write-only register pointed to by the value in the
Graphics 1 and 2 address register. This value must be hex 03
before writing can take place. The processor output port address
for this register is hex 3CF.
Data Rotate Register Format
Bit
7
6
5
4
3
2
1
0
Bit O-Bit 2
Bit 3-Bit 4
Rotate Count
Rotate Count 1
Rotate Count 2
Function Select
Not Used
Rotate Count-These bits represent a binary
encoded value of the number of positions to
rotate the processor data bus during processor
memory writes. This operation is done when the
write mode is O. To write unrotated data the
processor must select a count of O.
Function Select-Data written to memory can
operate logically with data already in the
processor latches. The bit functions are defined
in the following table.
IBM Enhanced Graphics Adapter 49
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