IBM RT Series Hardware Reference Manual page 343

TNL SN20-9844 (March 1987) to 75X0235
Normal operation of the command queue begins with the system loading a command or list of
commands into the queue. This group is called a queue load. The last command is an 'Execute'
instruction which contains a bit known as the decrement queue counter flag. This flag tells the
graphic operation processor to decrement the queue counter register by one after the execution
of the queue load is complete. If this is the first or only queue load, the queue pointer points to
the first queue load command word.
Initially assume that the queue counter register is 0. Following the complete loading of the
queue load (and the queue pointer if necessary), the system processor performs an I/O Write
cycle to the increment queue counter register, causing the queue counter to be incremented by
one. Once the queue counter has incremented past 0, the graphic operation processor begins
fetching command words from the queue, starting at the address designated by the queue
pointer register. As each command is processed, the queue pointer register is either decremented
by one or loaded with a new queue location (branch address). The queue pointer always points
to the next command word in the queue.
If the processed command word is an 'execute' type and the decrement queue counter flag is set,
then the queue counter is decremented on completion of the graphic operation. If the processed
command word is either a register load or branch type, or if the decrement queue counter flag is
not set, then the queue counter is not changed by the graphic operation processor.
As queue loads are added to the queue by the system processor, the queue counter is
incremented. As queue loads are completed, the counter is decremented. When the queue
counter is decremented to zero, graphic operation processing is halted.
Extended Monochrome Graphics Adapter 31