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INTERLISP DISPLAY PRIl\lITIVES Robert F. Sproull Xerox Palo Alto Research Center 3333 Coyote H iII Road Palo Alto t California 94304 Version of July 1977 1. Introduction This report describes briefly a set of display primitives that we have developed at PARC to extend the capabilities of InterLisp[l]. These primitives are designed to operate a raster-scanned displaYt and concentrate on facilities for placing text carefully on the display and for moving chunks of an already-created display. The primitives are deliberately designed to provide a low-level interface to the display. A display output primitive will cause a specific change to appear on the display by changing the contents of a frame bufler (or some other memory) that is llsed to refresh the raster-scanned image. The primitives make no assumptions about the sorts of data structures for describing the display that an application program may wish to build. Our implementation of these primitives involves two computers: InterLisp is executed on MAXC, and communicates with a program called Chat which maintains the frame buffer that drives a 808 by 606 point raster display. Although the communications link ably provides the bandwidth necessary to achieve rapid screen changes t it nonetheless requires special treatment of synchronization. If these primitives were to be implemented on a single computer that both executes InterLisp and performs the display modifications, the synchronization problems could be ignored. The design of this system is complicated by the need to accommodate on the display ordinary "teletype" output generated by InterLisp and Tenex t as well as carefully-constructed graphic displays. The primitives resolve this problem in a reasonably effective way: the unformatted character output may be directed to a specific region of the screen under complete control of the LISP program. However, we are forced to acknowledge the existence of two independent sources of information for Chat: a stream of teletype characters emerging from Tenex and a separate graphics connection that carries the characters and graphics protocol generated by LISP. As with the Network Graphics Protocol[2], it is important that these connections be kept separate: the graphics connection must transmit highly structured protocol messages that cannot suffer interference from system messages and other uncontrolled teletype transmissions. 2. Th.e ADIS functions This report does not cover the detailed implementation of the system, but concentrates on a description of the collection of LISP functions that are provided t and their intended effects. Each function is named ADISXXX; the actual names of the functions use all upper-case INTER LISP DISPLAY PRIMITIVES 2