Read by the routines at PO_ATTR and CLASS_07, and updated by the routines at PO_SCR, TEMPS, OUT_FLASH, CLASS_09 and CO_TEMP_1. Holds the attribute mask that is used when printing to the main part of the display. Set bits in the mask preserve the corresponding bits in the original (on-screen) attribute; reset bits in the mask replace the corresponding bits in the original attribute with those in the target attribute (ATTR-T).