IBM RT Series Hardware Reference Manual page 195

Bit O-Bit 7
Bit Mask-Any bit programmed to n causes the
corresponding bit nin each bit plane to be
immune to change provided that the location
being written was the last location read by the
processor. Bits programmed to a 1 allow
unimpeded writes to the corresponding bits in the
bit planes.
The bit mask applies to any data written by the processor (rotate,
AND'ed, OR'ed, XOR'ed, DX, and SIR). To preserve bits using
the bit mask, data must be latched internally by reading the
location. When data is written to preserve the bits, the most
current data in latches is written in those positions. The bit mask
applies to all bit planes simultaneously.
IBM Enhanced Graphics Adapter 55