DEC PDP-10-IMLAC communications system
RFC 190

Document Type RFC - Unknown (July 1971; No errata)
Last updated 2013-03-02
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Network Working Group                                         P. Deutsch
Request for Comments: 190           Xerox Research Center, Palo Alto, CA
NIC: 7135                                                   13 July 1971


   This report describes an operational system for communicating textual
   display information between a main-site computer and a remote-display

      The main site machine is a DEC PDP-10 with the BBN paging hardware
      (henceforth TENEX).

      The remote machine is a IMLAC PDS-1 (henceforth IMLAC).

      Section (1) briefly describes the IMLAC hardware configurations.

      Section (II) describes the display facilities presented to the

      Section (III) describes the system calls (JSYS calls) implemented
      in the TENEX monitor to provide these facilities.

      Section (IV) describes the formats of the messages used for
      communication between TENEX and the IMLAC.

      Section (V) analyzes the division of responsibility between the
      two machines.

(1)  Hardware configurations.

   The standard IMLAC is a 16-bit minicomputer with 4K of 2 microsecond
   core, a cycle-stealing display, an input keyboard, and an
   asynchronous serial communication interface.

   The display is normally programmed to draw characters using very
   short vectors.

      The display comes in two major configurations, depending on the
      presence or absence of hardware for drawing long vectors.

      In the sequel, specifications depending on the configuration will
      be flagged LVH or non-LVH respectively.

   The I/O system normally does not provide for interrupts when
   characters arrive from the serial interface.

Deutsch                                                         [Page 1]
RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971

      However, the IMLAC is barely able to keep up with the PDP-10
      without this feature, so we were able to persuade the manufacturer
      to implement it.

   While no special hardware is required for the software described
   here, the character interrupt and the SRI-ARC "mouse" and "keyset"
   are highly recommended, and the software is oriented towards their

(II)  Facilities.

   Each display console in the system may be in "display mode" or
   "teletype simulation mode".

      In display mode, the information displayed consists of text
      strings at arbitrary positions on the display face.

      In teletype simulation mode, the display shows the last 20-30
      lines which would appear on a teletype listing.

      A given console may switch between these modes, under program
      control, without losing any information.

   Regardless of mode, each display has a cursor string which follows
   the position of the available pointing device,

      The implemented system uses a "mouse" generally, but one console
      uses a tablet.

      The cursor string may be set by a program, for example to indicate
      at what sort of object the user is expected to point.

   In display mode, the screen of a given console is allocated to users
   in rectangular blocks called "display areas".

      This allows users to communicate via a single display split into
      multiple areas.

   When a user (program) requests a display area, he specifies how many
   text strings he will want to display in it.

      Each string has its own X-Y location, character size, font
      (italic, underline, ...), and an arbitrary number of characters.

         Each of these components is settable without disturbing the

      Each string may be manipulated without affecting the others.

Deutsch                                                         [Page 2]
RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971

(III) JSYS calls.

   (ADA) Assign a display area.

      JSYS ada: [440B]     allocate a display area



            upper-left-y-cord[10], upper-left-x-cord[10], max-no-


            lower-right-y-cord[10], lower-right-x-cord[10], default-c-
            size[2], default-n-inc[6], default-font[5]


            +1: Unsuccessful

               r1: error code

            +2: Successful

               r1: da-id[18]


            This jsys allocates a display area given the coordinates of
            the diagonal, the maximum number of strings to be displayed,
            and the default setting for the character size, font, and
            horizontal increment.  An 18-bit da-id is returned which
            should subsequently be used to refer to this display area.

   (DDA) Deallocate a display area.

      JSYS dda; [441B]     deallocate a display area


            r1: da-id

Deutsch                                                         [Page 3]
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