FB Only Clear Behind Keyword
MDSP bit 200000 modifier, the bit that enables displays to remain across items with feedback. Reset if N = 0, otherwise set. All MDSP modifiers are both parameters and switches. If N is missing the bit is set and the mode is active.
Setting this MDSP bit stops the feedback from clearing any display and instead only clears the region immediately behind the feedback. It is used when people want a
persistent display across items and want feedback. With DMTG's insane screen clearing philosophy this was possible, with DMDX's default behavior of a frame implicitly clearing anything else remaining on the screen unless a no erase has been set this is not possible, you couldn't specify a no erase for the feedback, <fbocb> allows this. In addition the erasing of the feedback only clears a small rectangle on the screen where the feedback was.