Indeed, one feature of active-matrix (and even passive-matrix) displays is that it needs only m + n signal lines to address a pixel in an m + n display. To change the color of a pixel, a signal goes out over the lines corresponding to the row and column of the addressed pixel, selecting it; and then another signal is transmitted over another line to actually change the value of that pixel. In this scheme, it would be impossible to address all pixels simultaneously. Nor would you actually want to, as this would require millions of control lines to drive the display!