The commonest technique in interfacing with graphics LCDs and then overlaying text on them is to setup a pixel image array in XRAM. The image array allows the fast read/write references needed to merge the pixels from an entry in a character font table into the graphics image. After an updated display image is created the buffer is then copied to the graphics display. If you write this copy loop in the tightest assembly language possible you can get the display updated quickly. Organize the pixel image buffer so that it maps the orientation of the bytes the same as the bytes are sent to the pages of the LCD controller. In this way the column addresses inside the LCD controller only need to be set once per page since all the controllers I have worked with have auto column address increment as the bytes in a page are loaded.

Michael Karas