Network graphics loader (RFC0186)
Original Publication Date: 1971-Jul-12
Included in the Prior Art Database: 2002-Mar-05
Internet Society Requests For Comment (RFCs)
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.
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.
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
Depending upon the operatio...