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RFC 186 - Network graphics loader


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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.

   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.

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

         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.)

   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".

   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)
   then a TRANSMIT request is simulated (using the arguments specified
   in the MODESET request which initiated Auto-TRANSMIT mode).
   Otherwise the next request is begun.

3. The TRANSMIT Request.

   Transmit takes one argument word and decodes the mode field, except
   in special cases.  Normally A1 indicates the number of words of
   display program and/or output (in the WAR area) which are to be
   transmitted (possibly after data format conversion as indicated by
   the mode field).  A2 indicates the starting address of the block of
   data to be (converted and) sent.

   3A.   Special Case:  If A2 is zero, the mode and A1 are ignored.  In
         this case an acknowledge message is sent (unless suppressed), a
         length word (containing 1) is sent (36 bits - left half word
         equals 1, right halfword equals zero) and then a word
         containing the origin address for display programs.  (This
         address should always be 1000 (octal); it might have to change,
         however, and this mechanism is provided to permit the user to
         determine what its value is.)

   3B.   The normal case is when A2 is non-zero.

         If there has never been a SETUP request, error condition 1 is
         recognized.  If the mode field is non-zero or 1, or if A1 is
         negative, or A2 is less than the origin (but not zero), or A1 +
         A2 is greater than the current "effective value of the WAR"
         (read on), then error condition 2 is recognized.

         The "effective value of WAR" always points to the word beyond
         the last word of output from the LDS-1.  Before the first
         EXECUTE request, it points to the word after the end of the
         display program (indicating that zero words have been output).
         EXECUTE requests effect the value in various ways (depending on
         error conditions, etc.) as described previously.  The effective
         WAR value is also changed to correctly reflect the effect of
         SETUP requests, which can change the size of the WAR area (type
         1.B), and UPDATE request, which can increase the size of the
         display program causing the whole WAR arla to be shifted.

         If the parameters are correct, an acknowledge message is sent
         (unless suppressed; see MODESET).

         If A1, the number of words to be reformatted (possibly) and
         sent, is zero it is understood that the block of words to be
         processed starts at A2 and terminates with the last word
         before the effective WAR value.  (Thus the number of words to
         be processed is "the effective WAR value minus A2".)

         If the mode field of the first word of the request is zero then
         no reformatting will be done: an exact copy of the core image
         will be transmitted.  If it is a one, then the block of words
         to be processed is _assumed_ to contain the coordinates of end
         points of a sequence of line segments.  (The odd numbered words
         (starting counting with one, not zero!) are assumed to
         represent _set-point_ data and the even-numbered words are
         assumed to contain draw-to data.  (Note that this is consistent
         with LDS-1 "Scaled Coordinate to Memory" mode _as_ _long_ _as_
         _no_ _dots_ _are_ _displayed_!!)  The line segments are recoded
         in ARDS format, five seven-bit ASCII characters to a word, with

         the right most bit of each 36 word unspecified.  The last word
         may contain null ASCII characters (all zero) to fill out the
         word.

         The first word transmitted (after the acknowledge message, if
         any) has a left half of 3 and a right half of: 0 (for exact
         core image) or 1 (for ARDS format).

         The next word has a left half equal to the number of words
         which follow and a right half of zero.  For ARDS format, this
         is the number of words after reformatting (of course).

         Unless zero words are to be transmitted, the next words are the
         data request.

         Note that the following sequence of requests could be used to
         simply convert line data (in the correct format) to ARDS
         format:

         (The notation "A,,B" stands for a 36 bit word whose left half
         has the value "A" and whose right half has the value "B".)

         Establish network connection
         SETUP,,0                       ; start
         2n,,0                          ; data representing "n" line
                                        ;   segments follow
         +----------------+             _
         |                |              |
         |                |               - 2n coordinate pairs
         |                |             _|
         +----------------+
         TRANSMIT,,2                    ; request ARDS transmission
         0,,1000                        ; convert and transmit the
                                        ;   whole "program"
         FLUSH,,0                       ; signoff

4. The UPDATE request.

   The UPDATE request allows a portion of the display program to be
   altered and also allows the end of the display to be extended (to
   include more data perhaps).  The mode field of the first word of the
   request indicates the number times the _EXECUTE-action_ is to be
   performed if the UPDATE request is successful.  The UPDATE request
   takes an argument word which is similar to that of TRANSMIT (i.e., A1
   is a length and A2 is an address).

   If no SETUP request has previously been executed error condition 1 is
   recognized.

   If A1 is negative or if A2 is less than the origin of the display
   program, error condition 2 is recognized.

   If A1 + A2 is greater than the end of the display program, then the
   length of the display program is increased to equal "A1 + A2 -
   origin".  The WAR area remains the same length as before, and its
   contents are shifted to their new locations.  The "effective WAR
   value" (see TRANSMIT) is increased by the amount of increase in
   display program length.

   An acknowledge message is sent (unless suppressed) if no error
   condition is recognized (and if no problem with core size limitation
   arises).  Also, the flag which is set by severe error conditions
   encountered during EXECUTE requests, is cleared by successful UPDATE
   requests.

   The next "A1" words are read and stored in consecutive locations
   starting in location "A2".  If A1 is zero, no words are read.

   If the mode field of the first word of the UPDATE request is
   positive, it is taken as an iteration count for a multiple EXECUTE
   request.  Otherwise, the next request is read from the Network.

5.  The FLUSH request.

   The Flush request takes no arguments and ignores the mode field of
   the first word of the request.  No acknowledge message is sent.  The
   network connection is broken and the process destroys itself.

6.  THE MODESET request.

   The mode field of the first request word is decoded first.  If it is
   zero or less, or if it is greater than (currently) 6, error condition
   2 is recognized.

   6A.   If the mode field is 1, then acknowledge messages and ASCII
         error messages are not to be suppressed.

   6B.   If the mode field is 2, then acknowledge messages and ASCII
         error messages are to be suppressed.

   6C.   If the mode field is 3, exit from Auto TRANSMIT mode.  (See
         6D.)

   6D.   If the mode field is 4 or 5, whenever an EXECUTE has a normal
         stop, a TRANSMIT request is to be automatically performed.

   The mode field for the transmit is to be 4 less than the mode field
   for this MODESET request.  One (more) word is read from the network.
   It contains the arguments for the TRANSMIT.  (That is, the word is
   saved and used when the TRANSMIT operation is performed.  The values
   of the arguments are checked at that time, not during the MODESET
   request.

EXAMPLE

   Suppose a display routine calculated and saved a different 3
   dimensional transformation matrix each time it was executed.  (It
   might be programmed to make an object appear to rotate.)  The user of
   the routine typically would want to set up the matrices and then "let
   'er rip" with ten or twenty (or more) executions.  This could be done
   as follows:

        SETUP,,0

    +-->length,,2000          ; length of display pgm
    |    -                    ; large output area
    |   |  -----
    |   |  -----
    +-->   -----
        | program
        |  -----
        |  -----
         -

        MODESET,,5            ; auto transmit in ARDS mode

        0,,1000+length        ; arguments for transmit

                              ;    meaning "current contents

                              ;    of output area"

        EXQ,,10               ; execute 10 times, sending

                              ;    the data each time

        UPDATE,,20            ; update, then execute 20 times

        8,,address of matrix  ; arguments for update, to

             -                ;    change a matrix
            |
   8 words -  new matrix
            |
             _
           etc.

   The output from the Graphics Loader for this example would be as
   follows:

   ACKNOWLEDGE     setup,setup     :(See description of ACKN

                                   ;  for format in word)

   ACKNOWLEDGE     modeset,modeset

   ACKNOWLEDGE     transmit, exq

        output,,1                   ; output in ARDS mode

        leng1,,0
       _
      |
     -  data
      |_

   ACKNOWLEDGE     transmit,exq

        output,,1

        leng2,,0
       _
      |
     -  data
      |_
             .               (in total, there are 10 sets of output.)
             .
             .

   ACKNOWLEDGE     update,update

   ACKNOWLEDGE     transmit, update

        output,,1

        leng11,,0

       -
      |
     -  data
      |
       -
             .
             .               (in total, there are 20 sets of output.)
             .

ACKNOWLEDGE MESSAGES

   These messages are each a single 36 bit word.  They can be suppressed
   dynamically by the MODESET request.  The left half of the 36 bit word
   identifies the fact that it is an acknowledge message by containing a
   1.  The right half is further divided into two 9-bit fields
   (quarters).  The right 9-bit field (within the right half word)
   contains the opcode field of the last request sent by the user.  Thus
   if an UPDATE request was received last, this field is a 4.

   The left 9-bit field (within the right half word) contains the number
   of the action actually being performed.  This differs from the opcode
   of the last request received only for Auto TRANSMIT mode and when a
   positive mode field is given in an UPDATE request (which is an
   EXECUTE count).

   Possible pairs of these fields are given below.  Note that Auto
   TRANSMIT mode sends only one acknowledge message which indicates both
   successful execution of the display program _and_ correct parameters
   for the TRANSMIT operation.

   Currently    Most Recent    Comments:
   Executing:   Request
                Received:

   SETUP        SETUP           ; Sent before data words are read.

   EXQ          EXQ             ; Never sent in Auto-Transmit mode.

   TRANSMIT     EXQ              ; Only sent in Auto-Transmit mode.

   TRANSMIT     TRANSMIT

   EXQ          UPDATE          ; Never sent in Auto-Transmit mode.

   TRANSMIT     UPDATE          ; Only sent in Auto-Transmit mode.

   UPDATE       UPDATE          ; Sent before data words are read.

   MODESET      MODESET         ; When setting up Auto-Transmit mode,

                                ;   this is sent before the argument

                                ;   word for TRANSMIT is read.

Notes:

   1)    A MODESET request which suppresses acknowledge messages is
         never acknowledged.  One which permits acknowledge messages is
         always acknowledged.

   2)    Requests which read data words (certain SETUP, UPDATE and
         MODESET requests) send acknowledge messages (unless suppressed)
         _before_ reading the data words.

ERROR MESSAGES

   These messages contain one or more words.  If acknowledge messages
   are suppressed, then only one word is sent.  (The assumption was made
   that brevity of response would also be desired for error messages if
   it were also desired for normal output.)

   The first (or only) word contains a left half of 2.  (Identifying
   this as an error message).  The right half contains the number of the
   error condition recognized; these numbers are summarized below.

   If the long form of the error message is being used, additional words
   are sent, the first contains a positive count of the remaining words
   in the left half and zero in the right half.  Words following the

   count word contain five 7-bit ASCII characters per word (the last
   word may end in null characters) which spell out a descriptive error
   message.  These messages are summarized below:

   Error Condition Number          Error Message

             0              UNRECOGNIZED OPERATION CODE

             1              PREVIOUS SETUP REQUIRED

             2              INVALID MODE OR OPERAND FIELD

             3              PREVIOUS EXECUTE FAILED

             4              DISPLAY PROCESSOR OCCUPIED

             5              OUTPUT AREA OVERRUN

             6              DISPLAY ERROR: TWO SECOND TIME OUT

             7              DISPLAY ERROR: IMPROPER DISPLAY STOP

             8              DISPLAY ERROR: MEMORY PROTECTION VIOLATION

OUTPUT FROM TRANSMIT

   This output consists of an identifying word (left half of 3, right
   half containing 0 for image mode and 1 for ARDS mode), a count word
   (left half indicating the number of words following this word, right
   half of zero) and zero or more data words in whatever data format the
   first word indicated.

Appendix 1

   The environment at the beginning of execution of the user's display
   program is as follows:

This LDS-1 is in program mode.

RAR/                    Origin of user's program.

WAR/                    First word of a block of core at least as long
                        as requested via SETUP operations.  This block
                        immediately follows the last word of the display
                        program.

PC/                     Origin of user's program.

SP/                     Address of first word beyond the end of a 200
                        (octal) word area of combined data sink and
                        control register stack.

P1/                     Unspecified.

P2/                     Unspecified.

DSP/                    First word of a 200 (octal) word area of
                        combined data sink and control register stack.

UR/                     Unspecified.

RCR/                    Unspecified.

WCR/                    Minus one plus the negative of the length of the
                        "WAR area" as specified in the most recent
                        SETUP request.

DIR/                    Stop on WCR positive.
                        Do not stop on Hit.
                        Matrix Multiplier inactive.
                        "Overlap" permitted.
                        Two dimensional mode.
                        Do not "Do Twice".

RSR/                    Unspecified.

SR/                     0, except for bits not under program control
                        like the stylus "Z" coordinate bits.

NEXT/                   Unspecified.

SAVELB/                 -3777,,-3777

SAVERT/                 3777,,3777

VIEWLB/                 -3777,,-3777

VIEWRT/                 3777,,3777

WINDLB/                 -3777,,-3777

WINDRT/                 3777,,3777

INSTLB/                 -3777,,-3777

INSTRT/                 3777,,3777

NAME/                   Unspecified.

CDIR/                   Scaled Coordinates to Memory
                        Curve mode inactive.
                        Minimum effort inactive.
                        Solid, not dashed lines.
                        Self mode inactive.

HITANG/                 Unspecified.

SELINT/                 Unspecified.

Matrix Multiplier/      Unspecified.

Origin of program -1/           (Should be jumped to when the
                                 display program is finished.)

        [This RFC was put into machine readable form for entry]
         [into the online RFC archives by Lorrie Shiota, 10/01]

 

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