IBM RT Series Hardware Reference Manual page 115

Bits 3-5
Bits 6-7
Bits 8-9
Bit 10
Bit 11
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 '01 O'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
Memory Mode
Bit
98
00
=
System write operation
01
=
Overlay write operation
10
=
Adapter write operation
11
=
Automatic read/write operation
Address Counter Mode
o
=
Increase the address counter
1
=
Decrease the address counter
Address Counter Stepping
o
=
Y stepping
1
=
X stepping
Advanced Monochrome Graphics Display Adapter 7