IBM RT Series Hardware Reference Manual page 99

Data Control Register (X'OlSO') Write Only
Bits 0 through 11 should not be changed for at least 1.4 microseconds after a memory operation
since the memory operation may still be in progress.
Bits 0-2 Rotate Count
Bits 3-5
Bits 6-7
The rotate count determines the number of bits that the data read from the bit
map is shifted to the left prior to being written back into the bit map. This
value has no effect during system write or overlay write modes. It has effect.
only during adapter write or automatic write modes.
Logic Unit Function Control
Bit
543
000 - Pass through B
010 - Pass through A
100 - Pass through 'Not' B
101-A 'OR'B
110 - Pass through 'Not' A
111 - A 'NOR' B
Logic unit function control bits (bits 3-5) determine how data previously read
from the bit map or written from the system are merged before being written
into the bit map. When set to '101 'B, the bit fields masked off by the data
mask register are merged with system data before being written back into the
bit map memory. This is the function normally needed during adapter write
and automatic write modes. When set to '010'B, the bit field masked off by
data mask register 1 (DMl) is written into the bit map memory. This is the
function normally needed in system write and overlay write modes. When set
to 'OOO'B, the bit field masked off by data mask register 2 (DM2) is written
into the bit map memory. This function can be used in adapter write and
automatic write modes when no bit shift is used. In each case, an inverting
form of the operation exists to allow data to be inverted as it is written to the
bit map. See "Bit Map Memory Operations" on page 5 for further
explanations.
Reserved
Advanced Color Graphics Display Adapter 7