IBM RT Series Hardware Reference Manual page 163

128K of graphics memory is required to support two character
sets. 256K supports four character sets. Asynchronous reset
clears this register to O. This should be done only when the
sequencer is reset.
Memory Mode Register
This is a write-only register pointed to when the value in the
address register is hex 04. The processor output port address for
this register is 3C5.
Memory Mode Register Format
Bit
Bit 0
Bit 1
Bit 2
7 6 5 4 3 2 1 0
E
Alpha
Extended Memory
Odd/Even
Not Used
Alpha-A logical 0 indicates that a non-alpha
mode is active. A logical 1 indicates that alpha
mode is active and enables the character
generator map select function.
Extended Memory-A logical 0 indicates that the
memory expansion card is not installed. A logical
1 indicates that the memory expansion card is
installed and enables access to the extended
memory through address bits 14 and 15.
Odd/Even-A logical 0 directs even processor
addresses to access maps 0 and 2, while odd
processor addresses access maps 1 and 3. A
logical 1 causes processor addresses to
sequentially access data within a bit map. The
maps are accessed according to the value in the
map mask register.
IBM Enhanced Graphics Adapter 23