IBM RT Series Hardware Reference Manual page 327

TNL SN20-9844 (March 1987) to 75X0235
Queue Counter Register
I/O DATA FIELD
Bits 00-05 NOT USED
Bits 06-15 COUNTER VALUE (0-1023)
Figure S. Queue Counter Register (Address X'D9FS04')
Note: This register is initialized to X'OOOO' with a POR or reset adapter command.
This register contains the unexecuted 'queue loads' count. The counter is normally incremented
by the system after a new command sequence has been loaded and is ready for execution by the
adapter. It is decremented, conditionally by the adapter, at completion of a command sequence.
As a part of the memory map, it is also fully read/writable by the system. The queue counter
register should only be written when graphic operation processing is stopped (queue counter
equals 0).
Extended Monochrome Graphics Adapter 15