Abstract: A computer system having a digitizing tablet overlaying the display screen. The tablet serves as a user's primary input device. Various features of the system make it possible for the user to run and interact with standard programs designed for keystroke and mouse input and not designed for use with a tablet. In addition to the main processor, on which the user's programs are executed, there is an interface processor. In addition to a standard display buffer, there is an ink plane buffer for interface display data that is combined with the data from the standard display buffer on a pixel-by-pixel basis according to data from a mask plane buffer. The interface processor manages input from the tablet, presents feedback to the user by means of the ink and mask planes, and provides keystroke and mouse data to the main processor as if from a standard keyboard controller. The interface processor presents the user with a collection of simulated devices, including standard devices such as a keyboard and a mouse.