Network Working Group 4691
RFC-5 Jeff Rulifson
June 2, l969
DEL
:DEL, 02/06/69 1010:58 JFR ; .DSN=1; .LSP=0; ['=] AND NOT SP ; ['?];
dual transmission?
ABSTRACT
The Decode-Encode Language (DEL) is a machine independent language
tailored to two specific computer network tasks:
accepting input codes from interactive consoles, giving immediate
feedback, and packing the resulting information into message
packets for network transmissin.
and accepting message packets from another computer, unpacking
them, building trees of display information, and sending other
information to the user at his interactive station.
This is a working document for the evolution of the DEL language.
Comments should be made through Jeff Rulifson at SRI.
FORWARD
The initial ARPA network working group met at SRI on October 25-26,
1968.
It was generally agreed beforehand that the runmning of interactive
programs across the network was the first problem that would be
faced.
This group, already in agreement about the underlaying notions of
a DEL-like approach, set down some terminology, expectations for
DEL programs, and lists of proposed semantic capability.
At the meeting were Andrews, Baray, Carr, Crocker, Rulifson, and
Stoughton.
A second round of meetings was then held in a piecemeal way.
Crocker meet with Rulifson at SRI on November 18, 1968. This
resulted in the incorporation of formal co-routines.
and Stoughton meet with Rulifson at SRI on Decembeer 12, 1968. It
was decided to meet again, as a group, probably at UTAH, in late
January 1969.
The first public release of this paper was at the BBN NET meeting in
Cambridge on February 13, 1969.
NET STANDARD TRANSLATORS
NST The NST library is the set of programs necessary to mesh
efficiently with the code compiled at the user sites from the DEL
programs it receives. The NST-DEL approach to NET interactive system
communication is intended to operate over a broad spectrum.
The lowest level of NST-DEL usage is direct transmission to the
server-host, information in the same format that user programs
would receive at the user-host.
In this mode, the NST defaults to inaction. The DEL program
does not receive universal hardware representation input but
input in the normal fashion for the user-host.
And the DEL 1 program becomes merely a message builder and
sender.
A more intermediate use of NST-DEL is to have echo tables for a
TTY at the user-host.
In this mode, the DEL program would run a full duplex TTY for
the user.
It would echo characters, translate them to the character set
of the server-host, pack the translated characters in messages,
and on appropriate break characters send the messages.
When messages come from the server-host, the DEL program would
translate them to the user-host character set and print them on
his TTY.
A more ambitious task for DEL is the operation of large,
display-oriented systems from remote consoles over the NET.
Large interactive systems usually offer a lot of feedback to
the user. The unusual nature of the feedback make it
impossible to model with echo table, and thus a user program
must be activated in a TSS each time a button state is changed.
This puts an unnecessarily large load on a TSS, and if the
system is being run through the NET it could easily load two
systems.
To avoid this double overloading of TSS, a DEL program will
run on the user-host. It will handle all the immediate
feedback, much like a complicated echo table. At appropriate
button pushes, message will be sent to the server-host and
display updates received in return.
One of the more difficult, and often neglected, problems is the
effective simulation of one nonstandard console on another non-
standard console.
We attempt to offer a means of solving this problem through
the co-routine structure of DEL programs. For the
complicated interactive systems, part of the DEL programs
will be constructed by the server-host programmers.
Interfaces between this program and the input stream may
easily be inserted by programmers at the user-host site.
UNIVERSAL HARDWARE REPRESENTATION
To minimize the number of translators needed to map any facility's
user codes to any other facility, there is a universal hardware
representation.
This is simply a way of talking, in general terms, about all the
hardware devices at all the interactive display stations in the initial
network.
For example, a display is thought of as being a square, the
mid-point has coordinates (0.0), the range is -1 to 1 on both
axes. A point may now be specified to any accuracy, regardless of
the particular number of density of rastor points on a display.
The representation is discussed in the semantic explanations
accompanying the formal description of DEL.
INTRODUCTION TO THE NETWORK STANDARD TRANSLATOR (NST)
Suppose that a user at a remote site, say Utah, is entered in the
AHI system and wants to run NLS.
The first step is to enter NLS in the normal way. At that time
the Utah system will request a symbolic program from NLS.
REP This program is written in DEL. It is called the NLS
Remote Encode Program (REP).
The program accepts input in the Universal Hardware
Representation and translates it to a form usable by NLS.
It may pack characters in a buffer, also do some local
feedback.
When the program is first received at Utah it is compiled and
loaded to be run in conjunction with a standard library.
All input from the Utah console first goes to the NLS NEP. It is
processed, parsed, blocked, translated, etc. When NEP receives a
character appropriate to its state it may finally initiate
transfers to the 940. The bits transferred are in a form
acceptable to the 940, and maybe in a standard form so that the
NLSW need not differentiate between Utah and other NET users.
ADVANTAGES OF NST
After each node has implemented the library part of the NST, it
need only write one program for each subsystem, namely the
symbolic file it sends to each user that maps the NET hardware
representation into its own special bit formats.
This is the minimum programming that can be expected if
console is used to its fullest extent.
Since the NST which runs the encode translation is coded at the
user site, it can take advantage of hardware at its consoles to
the fullest extent. It can also add or remove hardware
features without requiring new or different translation tables
from the host.
Local users are also kept up to date on any changes in the system
offered at the host site. As new features are added,
the host programmers change the symbolic encode program. When
this new program is compiled and used at the user site, the new
features are automatically included.
The advantages of having the encode translation programs
transferred symbolically should be obvious.
Each site can translate any way it sees fit. Thus machine code
for each site can be produced to fit that site; faster run
times and greater code density will be the result.
Moreover, extra symbolic programs, coded at the user site, may
be easily interfaced between the user's monitor system and the
DEL program from the host machine. This should ease the
problem of console extension (e.g. accommodating unusual keys and
buttons) without loss of the flexibility needed for man-machine
interaction.
It is expected that when there is matching hardware, the symbolic
programs will take this into account and avoid any unnecessary
computing. This is immediately possible through the code
translation constructs of DEL. It may someday be possible through
program composition (when Crocker tells us how??)
AHI NLS - USER CONSOLE COMMUNICATION - AN EXAMPLE
BLOCK DIAGRAM
The right side of the picture represents functions done at the
user's main computer; the left side represents those done at the
host computer.
Each label in the picture corresponds to a statement with the
same name.
There are four trails associated with this picture. The first
links (in a forward direction) the labels which are concerned
only with network information. The second links the total
information flow (again in a forward direction). The last two
are equivalent to the first two but in a backward direction.
They may be set with pointers t1 through t4 respectively.
[">tif:] OR I" >nif"]; ["nif(encode)]
Encode maps the semi-raw input bits into an input stream in a
form suited to the serving-host subsystem which will process the
input. [>nif(hrt)tif(net mode)nif(urt)>tif(imp ctrl)tif(urt) nif(d ctrl)>tif(prgm ctrl)nif(display)tif(dctrl)=" sum /
'< sum /
'> sum /
'= sum /
'" sum /
.empty);
The conjunct construct is rigged in such a way that a conjunct
which is not a sum need not have a value, and may be evaluated
using jumps in the code. Reference to the conjunct is made only
in places where a logical decision is called for (e.g. if and
while statements).
We hope that most compilers will be smart enough to skip
unnecessary evaluations at run time. I.e a conjunct in which the
left part is false or a disjunct with the left part true need not
have the corresponding right part evaluated.
ARITHMETIC EXPRESSION
SYNTAX
statement = conditional / unconditional;
unconditional = loopst / cases / cibtrikst / uist / treest /
block / null / exp;
conditional = "IF" conjunct "THEN" unconditional (
"ELSE" conditional /
.empty);
block = "begin" exp $('; exp) "end";
An expressions may be a statement. In conditional statements the
else part is optional while in expressions it is mandatory. This
is a side effect of the way the left part of the syntax rules are
ordered.
SEMI-TREE MANIPULATION AND TESTING
SYNTAX
treest = setpntr / insertpntr / deletepntr;
setpntr = "set" "pointer" pntrname "to" pntrexp;
pntrexp = direction pntrexp / pntrname;
insertpntr = "insert" pntrexp "as"
(("left" / "right") "brother") /
(("first" / "last: ) "daughter") "of" pntrexp;
direction =
"up" /
"down" /
"forward" /
"backward: /
"head" /
"tail";
plantree = "replace" pntrname "with" pntrexp;
deletepntr = "delete: pntrname;
tree = '( tree1 ') ;
tree1 = nodename $nodename ;
nodename = terminal / '( tree1 ');
terminal = treename / buffername / point ername;
treename = id;
treedecl = "pointer" .id / "tree" .id;
Extra parentheses in tree building results in linear subcategorization,
just as in LISP.
FLOW AND CONTROL
controlst = gost / subst / loopstr / casest;
GO TO STATEMENTS
gost = "GO" "TO" (labelv / .id);
assignlabel = "ASSIGN" .id "TO" labelv;
SUBROUTINES
subst = callst / returnst / cortnout;
callst = "CALL" procname (exp / .emptyu);
returnst = "RETURN" (exp / .empty);
cortnout = "STUFF" exp "IN" pipename;
cortnin = "FETCH" pipename;
FETCH is a builtin function whose value is computed by envoking
the named co-routine.
LOOP STATEMENTS
SYNTAX
loopst = whilest / untilst / forst;
whilest = "WHILE" conjunct "DO" statement;
untilst = "UNTIL" conjunct "DO" statement;
forst = "FOR" integerv '- exp ("BY" exp / .empty) "TO" exp
"DO" statements;
The value of while and until statements is defined to be false
and true (or 0 and non-zero) respectively.
For statements evaluate their initial exp, by part, and to part
once, at initialization time. The running index of for
statements is not available for change within the loop, it may
only be read. If, some compilers can take advantage of this
(say put it in a register) all the better. The increment and
the to bound will both be rounded to integers during the
initialization.
CASE STATEMENTS
SYNTAX
casest = ithcasest / condcasest;
ithcasest = "ITHCASE" exp "OF" "BEGIN" statement $(';
statement) "END";
condcasest = "CASE" exp "OF" "BEGIN" condcs $('; condcs)
"OTHERWISE" statement "END";
condcs = conjunct ': statement;
The value of a case statement is the value of the last case executed.
EXTRA STATEMENTS
null = "NULL";
I/O STATEMENTS
iost = messagest / dspyst ;
MESSAGES
SYNTAX
messagest = buildmes / demand;
buildmest = startmes / appendmes / sendmes;
startmes = "start" "message";
appendmes = "append" "message" "byute" exp;
sendmes = "send" "message";
demandmes = "demand" "Message";
mesinfo =
"get" "message" "byte"
"message1" "length" /
"message" empty: '?;
mesdecl = "message" "bytes" "are" ,byn "bits" long" '..
DISPLAY BUFFERS
SYNTAX
dspyst = startbuffer / bufappend / estab;
startbuffer - "start" "buffer";
bufappend = "append" bufstuff $('& bufstuff);
bufstuff = :
"parameters" dspyparm $('. dspyparm) /
"character" exp /
"string"1 strilng /
"vector" ("from" exp ':exp / .empty) "to" exp '. exp /
"position" (onoff / .empty) "beam" "to" exp '= exp/
curve" ;
dspyparm F :
"intensity" "to" exp /
"character" "width" "to" exp /
"blink" onoff /
"italics" onff;
onoff = "on" / "off";
estab = "establish" buffername;
LOGICAL SCREEN
The screen is taken to be a square. The coordinates are
normalized from -1 to +1 on both axes.
Associated with the screen is a position register, called
PREG. The register is a triple where x and y
specify a point on the screen and r is a rotation in
radians, counter clockwise, from the x-axis.
The intensity, called INTENSITY, is a real number in the
range from 0 to 1. 0 is black, 1 is as light as your
display can go, and numbers in between specify the relative
log of the intensity difference.
Character frame size.
Blink bit.
BUFFER BUILDING
The terminal nodes of semi-trees are either semi-tree names
or display buffers. A display buffer is a series of logical
entities, called bufstuff.
When the buffer is initilized, it is empty. If no
parameters are initially appended, those in effect at the
end of the display of the last node in the semi-tree will be in
effect for the display of this node.
As the buffer is built, the logical entities are added to it.
When it is established as a buffername, the buffer is
closed, and further appends are prohibited. It is only a
buffername has been established that it may be used in a tree
building statement.
LOGICAL INPUT DEVICES
Wand
Joy Stick
Keyboard
Buttons
Light Pens
Mice
AUDIO OUTPUT DEVICES
.end
SAMPLE PROGRAMS
Program to run display and keyboard as tty.
to run NLS
input part
display part
DEMAND MESSAGE;
While LENGTH " O DO
ITHCASE GETBYTE OF Begin
ITHCASE GETBYTE OF %file area uipdate% BEGIN
%literal area%
%message area%
%name area%
%bug%
%sequence specs%
%filter specs%
%format specs%
%command feedback line%
%filer area%
%date time%
%echo register%
BEGIN %DEL control%
DISTRIBUTION LIST
Steve Carr
Department of Computer Science
University of Utah
Salt Lake City, Utah 84112
Phone 801-322-7211 X8224
Steve Crocker
Boelter Hall
University of California
Los Angeles, California
Phone 213-825-4864
Jeff Rulifson
Stanford Research Institute
333 Ravenswood
Menlo Park, California 94035
Phone 415-326-6200 X4116
Ron Stoughton
Computer Research Laboratory
University of California
Santa Barbara, California 93106
Phone 805-961-3221
Mehmet Baray
Corey Hall
University of California
Berkeley, California 94720
Phone 415-843-2621