|Title||Network graphics loader
Network Working Group J. Michener
Request for Comments: 186 MCG
NIC: 7130 12 July 1971
A Network Graphics Loader
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.
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
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.
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
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
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
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
(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:
| | |
| 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
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
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
Note that the following sequence of requests could be used to
simply convert line data (in the correct format) to ARDS
(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
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
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. 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
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
+-->length,,2000 ; length of display pgm
| - ; large output area
| | -----
| | -----
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
The output from the Graphics Loader for this example would be as
ACKNOWLEDGE setup,setup :(See description of ACKN
; for format in word)
ACKNOWLEDGE transmit, exq
output,,1 ; output in ARDS mode
. (in total, there are 10 sets of output.)
ACKNOWLEDGE transmit, update
. (in total, there are 20 sets of output.)
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
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:
SETUP SETUP ; Sent before data words are read.
EXQ EXQ ; Never sent in Auto-Transmit mode.
TRANSMIT EXQ ; Only sent in Auto-Transmit mode.
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.
1) A MODESET request which suppresses acknowledge messages is
never acknowledged. One which permits acknowledge messages is
2) Requests which read data words (certain SETUP, UPDATE and
MODESET requests) send acknowledge messages (unless suppressed)
_before_ reading the data words.
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.
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
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.
DSP/ First word of a 200 (octal) word area of
combined data sink and control register stack.
WCR/ Minus one plus the negative of the length of the
"WAR area" as specified in the most recent
DIR/ Stop on WCR positive.
Do not stop on Hit.
Matrix Multiplier inactive.
Two dimensional mode.
Do not "Do Twice".
SR/ 0, except for bits not under program control
like the stylus "Z" coordinate bits.
CDIR/ Scaled Coordinates to Memory
Curve mode inactive.
Minimum effort inactive.
Solid, not dashed lines.
Self mode inactive.
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]