rfc186.txt

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Network Working Group                                        J. Michener
Request for Comments: 186                                            MCG
NIC: 7130                                                   12 July 1971


                       A Network Graphics Loader

MOTIVATION

   The facility described herein will permit remote users on the ARPA
   network to obtain graphics output from programs they write for the
   Evans and Sutherland Line Drawing System Model 1 (LDS-1) located at
   the DMCG computer.  Also, users at that computer can employ the
   facility to do graphics on their ARDS and IMLAC consoles.

   INTRODUCTION

   The Graphics Loader on the Project MAC Dynamic Modeling/Computer
   Graphics PDP-10 is for use with the E&S LDS-1 display.  Display
   programs can be shipped to it and executed repeatedly.  The output,
   which would normally be visible at the PDP-10 installation, is
   transmitted to the originating site in digital form.

   Corrections and alterations to display programs can be transmitted so
   that the bulk of the program need be sent only once.  Any data or
   parameters which vary may be sent whenever they change.

   The originating site may request to have any part of its program or
   data transmitted back to it from the Graphics Loader.  With this
   feature it is possible to debug a display program which is
   incorrectly modifying itself.

   In order to simplify the Graphics Loader, it is assumed that the
   display program should occupy a contiguous block of core starting at
   location 1000 octal (i.e., it has been assembled absolutely), that
   its first executable instruction is at the same place, and that, when
   one frame is complete, it jumps to location 777 octal.

   The E&S LDS-1 has the capability of writing into memory the
   coordinates of endpoints of the line segments which would be visible
   to a user sitting at the LDS-1 display device.  A register called the
   Writer Address Register (WAR) is used to indicate an area of memory
   to contain these coordinates.  Various submodes are available for
   output to memory, but for the submode of greatest interest, "Scaled
   Coordinates to Memory" mode, each "visible" line segment causes two
   words of coordinate data to be stored.  The contents of the WAR are
   incremented for each word stored.




Michener                                                        [Page 1]

RFC 186                A Network Graphics Loader            12 July 1971


   For each execution of the display program, the Graphics Loader sets
   the proper output mode (suppressing output to the cathode ray tube at
   the DGSD machine), initializes the WAR before execution and saves the
   final value of the WAR after execution.  Thus it is easy for the
   Graphics Loader to transmit to the user only the "visible output" of
   the display program.

DESCRIPTION OF REQUESTS FROM THE REMOTE USER PROGRAM

   Request are in the form of 36 bit words.  The first word of a request
   is interpreted as two 18 bit fields.  The left half contains a number
   identifying which of six operations is being requested.  The right
   half is either a mode or is ignored, depending on the requested
   operation.  (If the left half is not a valid operation number, an
   error message is sent and the next word is considered to begin a new
   request.)

   Depending upon the operation requested and upon the mode, one word of
   argument data may or may not be read.  This word is (also) treated as
   two 18 bit halves.  The interpretation of the halves depends on the
   operation.  In the description of individual operations, the left
   half will be called A1; the right half will be called A2 (standing
   for Arguments 1 and 2).

   Error checking of the arguments is performed next.  If an error
   condition is present, error information is sent to the user program
   at the originating site and the Graphics Loader prepares itself for
   the next command.  If no error condition is present, an acknowledging
   message is sent unless the Suppress Acknowledgement mode prevails.
   For certain requests, the operation is performed before the
   acknowledgement is transmitted.

   For those operations involving a transfer of display program
   information (either to or from the Graphics Loader), this transfer is
   done next, after the error checking of arguments has been performed
   and after an acknowledge message has been sent.

   This done, the Graphics Loader reads the next command.













Michener                                                        [Page 2]

RFC 186                A Network Graphics Loader            12 July 1971


SPECIFICATIONS

0.  The valid operations are currently:

      SETUP            indicated by an operation number of 1

      EXECUTE          indicated by an operation number of 2

      TRANSMIT         indicated by an operation number of 3

      UPDATE           indicated by an operation number of 4

      FLUSH connection indicated by an operation number of 5

      MODESET          indicated by an operation number of 6


   An invalid operation number is an error condition (condition number
   0).

1.  The SETUP request.

   The mode field of the first word is ignored.  SETUP requires an
   argument word.  The arguments A1 and A2 are both treated like
   lengths, so both must be non-negative numbers.  If they are not,
   error condition 2 is recognized.

   1A.   If A1 is _strictly_ _positive_, then this request describes a
         whole new display program, and any previous display program
         from this user is to be forgotten.  In this case, A1 is the
         total length of the display program, exclusive of the area to
         be addressed by the Write Address Register (WAR).  A2 is the
         length of the area to be addressed by the WAR.  As such, A2
         must be at least twice the greatest possible number of visible
         line segments to be displayed.  (If the LDS-1 programmer feels
         sure of himself, he may set up his own "WAR area" and set his
         own "Output To Memory" modes.  He would not need to use the A2
         parameter at all.)

         An acknowledge message is sent (unless suppressed; see
         MODESET).  Then the display program is read (which consists of
         (A1) words).

   1B.   If A1 is _zero_ then this request is for a change in the length
         of the area to be addressed by the WAR.  A2 contains the new
         length.  A2 may be larger or smaller than the current length of





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RFC 186                A Network Graphics Loader            12 July 1971


         the area.  If no previous 1A type of SETUP request has been
         processed, error condition 1 is recognized.  Otherwise an
         acknowledge message is sent (unless suppressed, see MODESET).

         (This request would typically be used if an initial estimate on
         the number of words required were too low.  Error condition 5,
         described under EXECUTE may be indicate a low estimate.)

2. The EXECUTE request.

   The EXECUTE request does not take a parameter word, but the mode
   field is used to specify the number of times that the "EXECUTE
   action" is to be performed.  (This "action" is described in detail
   following this paragraph.  Briefly, it is a single execution of the
   display program.)  If the mode field is zero or negative, then one
   EXECUTE action is performed.  Whenever an error is encountered during
   a multiple EXECUTE, the iteration is immediately stopped.  This way,
   the status of the display program after the error is not destroyed,
   and no flood of error messages is ever sent, only a single one.

   The EXECUTE action is as follows:

   If no previous SETUP request has been processed, error condition
   number 1 will be recognized.

   An attempt is made to seize the E&S LSD-1 display processor.  (If a
   previous EXECUTE has succeeded in seizing it, then this will also
   succeed.  If some other user of the DMCG machine has control of the
   display processor, this will fail and error condition 4 will be
   recognized.)

   The display program is now executed.  The environment at the
   beginning of execution of the display program is given in Appendix 1.

   If a previous execute failed in a way indicating a programming error
   in the display program (error conditions 6,7 and 8), then an SETUP or
   an UPDATE request must be executed before another request will be
   processed.

   (If no SETUP of UPDATE is given before another EXECUTE, then error
   condition 3 is recognized.)

   If the LDS-1 runs for two seconds without causing an interrupt or
   jumping to the "finish" location (the word before the origin of the
   display program) then it is assumed the program is running away.  The
   LDS-1 is stopped, and error condition 6 is recognized.  (A SETUP or
   an UPDATE is required before another EXECUTE is permitted.)




Michener                                                        [Page 4]

RFC 186                A Network Graphics Loader            12 July 1971


   If the LDS-1 stops because too many output words are stored (i.e., if
   the WCR becomes positive or zero), error condition 5 is recognized.
   The number of output words made available to the user is as specified
   by A2 of the most recent SETUP request.

   If the LDS stops in any manner other than either those described
   above or by jumping to the word before the origin (the "finish"
   location), then error condition 7 is recognized.  (A SETUP or an
   UPDATE is required before another EXECUTE is permitted.)

   If the LDS-1 stops by jumping to the finish location, then the value
   of the WAR at the time determines the amount of output in the "WAR
   area" which the user may have access to.

   If the WAR has been altered so that it contains an address smaller
   than its initial value, then the effective value of the WAR is its
   initial value.  If the WAR contains a value greater than the address
   of the end of the area for output, then a WCR stop error is imitated
   (the effective value of the WAR is the maximum allowed by SETUP and
   error condition 5 is recognized).  The situation in which no error is
   recognized will be called a "normal stop".






























Michener                                                        [Page 5]

RFC 186                A Network Graphics Loader            12 July 1971


   Summary for Normal Stop:

   Origin                     |
   ----->+-----------------+  |
         |                 |  |
         | Display Program |   - <--If WAR here, it is set to here._
         |                 |  |                                     |
         +-----------------+ _|                                     |
                                                                    |
                              <-------------------------------------+
                             _
         +-----------------+  |
         |                 |  |
         | WAR output area |   - <--If WAR here, it is left alone.
         |                 |  |
         +-----------------+ _|

                             _ <-----------------------------------+
                              |                                    |
                              |                                    |
                               - <--If WAR here, it is set to here,+
                              |     and error condition 5 is
                              |     recognized.
                              |


   If no error condition is recognized for an EXECUTE request, an
   acknowledge message is sent (unless suppressed; see MODE SET).  The
   _effective_ value of the WAR is saved for later use in determining
   how much output the display program generated, but it is saved only
   for normal stops and WRC positive stops.

   After normal stop, if Auto-TRANSMIT mode is set (see MODESET, below)