File: rfc186.txt

package info (click to toggle)
doc-rfc 20181229-2
  • links: PTS, VCS
  • area: non-free
  • in suites: buster
  • size: 570,944 kB
  • sloc: xml: 285,646; sh: 107; python: 90; perl: 42; makefile: 14
file content (955 lines) | stat: -rw-r--r-- 30,557 bytes parent folder | download | duplicates (5)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955






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





Michener                                                        [Page 3]

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)
   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.






Michener                                                        [Page 6]

RFC 186                A Network Graphics Loader            12 July 1971


   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



Michener                                                        [Page 7]

RFC 186                A Network Graphics Loader            12 July 1971


         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".)






























Michener                                                        [Page 8]

RFC 186                A Network Graphics Loader            12 July 1971


         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.















Michener                                                        [Page 9]

RFC 186                A Network Graphics Loader            12 July 1971


   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.








Michener                                                       [Page 10]

RFC 186                A Network Graphics Loader            12 July 1971


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.








Michener                                                       [Page 11]

RFC 186                A Network Graphics Loader            12 July 1971


   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












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


       -
      |
     -  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.






















Michener                                                       [Page 13]

RFC 186                A Network Graphics Loader            12 July 1971


   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



Michener                                                       [Page 14]

RFC 186                A Network Graphics Loader            12 July 1971


   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.




















Michener                                                       [Page 15]

RFC 186                A Network Graphics Loader            12 July 1971


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.



Michener                                                       [Page 16]

RFC 186                A Network Graphics Loader            12 July 1971


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]















Michener                                                       [Page 17]