supdup.mss

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  @pageheading(even, right "The SUPDUP Protocol", left "Page @ref(page)")
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@case[draft, memo={
@Center(MASSACHUSETTS INSTITUTE OF TECHNOLOGY
ARTIFICIAL INTELLIGENCE LABORATORY)
@blankspace(.4inches)
@flushleft(AI Memo 644@>@Value(Date))
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@Majorheading(The SUPDUP Protocol)
@MajorHeading(by)
@MajorHeading(Richard M. Stallman)},
loser={
@modify(majorheading,below 0)
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@MajorHeading(The SUPDUP Protocol)
@Heading(Richard M. Stallman
Artificial Intelligence Lab
Massachusetts Institute of Technology
Cambridge, MA 02139)
}]

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@Begin(Abstract)
The SUPDUP protocol provides for login to a remote
system over a network with terminal-independent output, so that only the
local system need know how to handle the user's terminal.
It offers facilities for graphics and for local assistance to
remote text editors.  This memo contains a complete description
of the SUPDUP protocol in fullest possible detail.

@memo<
@B(Keywords:) Communications, Display, Networks.
>
@end(Abstract)

@Begin(ResearchCredit)
This report describes work done at the Artificial Intelligence Laboratory
of the Massachusetts Institute of Technology.  Support for the laboratory's
research is provided in part by the Advanced Research Projects Agency of the
Department of Defense under Office of Naval Research contract N00014-80-C-0505.
@End(ResearchCredit)

@Unnumbered(Introduction)

The SUPDUP protocol is a superior replacement for the TELNET protocol.
Its name means "Super Duper".

The basic purpose of the two protocols is the same: allow a user to log
in to a remote system connected to his local system by a network, as if
his terminal were directly connected to the remote system.  Both
protocols define a "virtual terminal" which the remote system is
expected to use.  The difference is that TELNET's "Network Virtual
Terminal" is an obsolete teletype, whereas SUPDUP's virtual terminal is
a display, capable (optionally) of powerful operations on
text, drawing pictures, and invisibly sharing the work of a text editor
running on the computer, all totally under the computer's control.

The cost associated with SUPDUP is that the remote system must be
capable of communicating with SUPDUP's virtual terminal.
Most operating systems understand nothing more than simple printing
terminals, leaving everything else to user programs.  The SUPDUP virtual
display terminal is not a superset of the simple printing terminal, so
these operating systems cannot serve as the remote system for SUPDUP.
They can support user programs which make SUPDUP connections to other
systems.  Since these operating systems have the deficiency that every
user program which does display must know the details of operation of
each type of terminal to be used, they are inferior anyway; a modern
operating system which provides terminal-independent display support
will have no trouble supporting SUPDUP, or anything like SUPDUP.


SUPDUP can operate over any pair of streams which transmit 8-bit
bytes.

The SUPDUP protocol was created initially by taking the formats
of data in the terminal input and output buffers of ITS,@footnote(The
Incompatible Timesharing System) and the ITS terminal characteristics
words.  It has grown by layer on layer of extensions, and ought to be
redesigned from scratch for simplicity.  If you do not need to
communicate with systems that support the existing SUPDUP protocol,
you might as well design the low-level details anew.  The higher level
design decisions of SUPDUP are reasonably good to follow.

All numbers except number of bits are octal unless followed by a
decimal point; thus, 12 and 10@. are the same.
Bits in a word are numbered in decimal from the sign bit (bit 0) to
the least significant bit (bit 35, in a 36-bit word).  Sizes of words
and fields are also decimal.

Data words, and bits and fields within them, are referred to by the
names that are conventional on ITS.  The names of bits and fields are
those provided for assembler language programs on ITS.

@Section[SUPDUP Initialization]

A SUPDUP connection is created using the standard connection-creation
mechanism for the network in use.  For the Arpanet, this is the ICP.
For the Chaosnet, this would be the exchange of an RFC, an OPN and a
STS packet.  The SUPDUP protocol itself becomes concerned only after
this has been done.

The initialization of a SUPDUP connection involves sending terminal
characteristics information to the remote system and a greeting
message to the local system.  The greeting message is ASCII text, one
character per byte, terminated with a %TDNOP code (see below).  It
should simply be printed on the console by the local system.  The
terminal characteristics information is made up of several 36-bit
words, which are precisely the internal terminal characteristics words
of ITS.  Each word is
broken up into 6 6-bit bytes, which are sent most significant first,
each in one 8-bit byte.  The words are

@enumerate[
A count.  The high 18 bits of this word contain minus the number of
words that follow.  This permits new words of terminal characteristics
to be defined without requiring all local-system SUPDUP programs to
be changed.  The server will supply a default value for any
characteristics words defined in the future and not mentioned below.

The TCTYP variable.  This should always contain 7.  If the server is an
ITS, this tells it to assume that its terminal is a SUPDUP virtual
terminal.

The TTYOPT variable.  This contains many bits and fields.

The height of the screen.

The width of the screen minus one.  The reason that one is subtracted is
that if one column is used (as on ITS) to indicate line continuation,
this is the effective width of the screen for the user's text.

The number of lines by which the terminal will scroll automatically if an attempt is
made to Linefeed past the bottom of the screen.  This value
is under the control of the physical terminal being used,
but the local system is expected to know the value by virtue
of knowing what type of terminal is in use.  It then informs
the remote system by passing this value. For most terminals,
this should be 1.

The TTYSMT variable.  This contains bits and fields that describe the
graphics and local editing capabilities of the terminal.
]

The TTYOPT word contains these bits:

@description[

%TOCLC (bit 2)@\The remote system should convert lower case characters
to upper case.  This is the initial value of a user option.  If the
local system has such an option as well, the local setting should be
propagated.  Otherwise, this bit should be zero.

%TOERS (bit 3)@\The terminal can erase selectively;
that is, it supports the output commands %TDEOL, %TDDLF, and
optionally %TDEOF (see below).

%TOMVB (bit 5)@\The terminal can move its cursor backwards.
(When SUPDUP was invented, model 33 teletypes were still in use!)

%TOSAI (bit 6)@\The terminal can handle codes 000 through 037, and
177, as printing characters.

%TOSA1 (bit 7)@\The remote system should output codes 000 through 037,
and 177, as themselves, expecting them to be printing characters.
This is the initial value of a user-settable option, and it should not
be set unless %TOSAI is also set.

%TOOVR (bit 8)@\The terminal can overprint.  That is, if two printing
characters are output at the same position with no erasing in between,
they will both be visible.  Any terminal which can move the cursor
back and @i[cannot] overprint will be assumed to handle %TDDLF, since
it can do so by means of the sequence Backspace Space Backspace.

%TOMVU (bit 9)@\The terminal can move its cursor upwards.  That is, it
is a display terminal with some form of cursor control.

%TOMOR (bit 10)@\The remote system should pause, in whatever fashion
it likes to do so, at the bottom of a screen of output.  This is the
initial value of a user option, and it should be set to @i[one].

%TOROL (bit 11)@\The remote system should scroll, as opposed to wrap
around, when output goes past the bottom of the screen.  This is the
initial value of a user option, and can be set whichever way the local
system expects its users to prefer.  On ITS, the default is to wrap.

%TOLWR (bit 13)@\The terminal can generate lower-case characters as
input.

%TOFCI (bit 14)@\The terminal keyboard has Control and Meta keys and
can generate all the meaningful codes of the 12-bit character code
(see below).

%TOLID (bit 16)@\The terminal can do insert/delete line; it supports
the %TDILP and %TDDLP commands.

%TOCID (bit 17)@\The terminal can do insert/delete character; it
supports the %TDICP and %TDDCP commands.

%TPCBS (bit 30)@\The local system is sending 034-escape sequences on
input.  This bit @i[must be set].

%TPORS (bit 32)@\The local system should be notified with a %TDORS
when the remote system attempts to abort output.  This bit @i[should be
set].

%TPRSC (bit 33)@\The local system implements the region-scrolling
commands %TDRSU and %TDRSD, which scroll an arbitrary portion of the
screen.
]

The TTYSMT variable contains these bits and fields:

@begin[description]
%TQMCH (bits 0-2)@\CPU type.
1 indicates a PDP-11.  2 indicates an Imlac PDS1.  3 indicates a PDS4.
0 indicates anything else.  4 through 7 are undefined.

%TQHGT (bits 3-7)@\Character height in pixels, for the SUPDUP Graphics Protocol.
This is the interline spacing for the font used for ordinary character output.
The total usable screen height is this times the screen height in characters
which applies to ordinary character output.

%TQWID (bits 8-11)@\Character width in pixels, for the font used for
ordinary character output.  The total usable screen width in pixels is
this parameter times the screen width in characters, as used for
ordinary character output.

%TQVIR (bit 12)@\The local system supports virtual coordinates in the
SUPDUP Graphics Protocol.

%TQBNK (bit 13)@\The local system supports blinking objects in the
SUPDUP Graphics Protocol.

%TQXOR (bit 14)@\The local system supports XOR mode in the
SUPDUP Graphics Protocol.

%TQREC (bit 15)@\The local system supports rectangle commands in the
SUPDUP Graphics Protocol.

%TQSET (bit 16)@\The local system supports multiple object sets in the
SUPDUP Graphics Protocol.

%TQGRF (bit 17)@\The local system supports the SUPDUP Graphics
Protocol.

%TRGIN (bit 18)@\The local system supports graphics input in the
SUPDUP Graphics Protocol.

%TRGHC (bit 19)@\The local system supports a hardcopy output device in the
SUPDUP Graphics Protocol.

%TRLED (bit 20)@\The local system supports the Local Editing Protocol.

%TRSCN (bit 21)@\The local system supports scan-line output in the
SUPDUP Graphics Protocol.

%TRLSV (bits 22-24)@\If this value @i<n> is nonzero, the local system
supports line saving.  Furthermore, the largest allowed label is
4@+[@i<n>].

%TRANT (bits 25-27)@\If this value @i<n> is nonzero, the local system
supports approximately 4@+[@i<n>] lines of anticipatory output.
@end[description]

@Section[SUPDUP Input]

The SUPDUP input language provides for the transmission of MIT
extended ASCII, described in detail below.  This is a 12-bit character
code, but not all possible 12-bit codes are assigned a meaning.  Some
of those that are not are used as special commands by the local
editing protocol.

To transmit 12-bit data on an 8-bit connection, it must be encoded.
The encoding used is that the 8-bit byte value 034 is an escape code.
To transmit the value 034, send two 034's.  Codes 200 and up are
encoded into sequences starting with 034.  Specifically, to encode
@i[m]*200+@i[n], send 034, @i[m]+100, @i[n].  Other sequences starting
with 034 serve other purposes, described below.

The %TOFCI bit in TTYOPT should be set if the terminal actually has the
ability to generate all the meaningful codes of extended ASCII.  If
%TOFCI is zero, SUPDUP input is restricted to a subset which is
essentially ASCII (but 034 must still be encoded as a sequence of two
034's).  The Local Editing Protocol assumes that %TOFCI is one, and uses
a 9-bit subset of the 12-bit character set for its editing commands.

A character in the 12-bit character set is composed of a @i[basic
character], which makes up the bottom 7 bits, and 5 @i[bucky bits].  The
2000 bit and the 1000 bit are currently unused.  The 400 bit stands for
the @i[Meta] shift key, and the 200 bit stands for the @i[Control] shift
key.  These two bits can be added to any character, and are significant
in editing commands.  The 4000 bit is used to extend the set of basic
characters.

The 7-bit basic character is usually interpreted as an ASCII
character, except that only these codes in the range 0 through 037 are
allowed:

@itemize[
Backspace (code 010)

Tab (code 011)

Linefeed (code 012)

VT (code 013)

Formfeed (code 014)

Return (code 015)

Call (code 032)

Altmode (code 033)

Backnext (code 037)
]

Backspace, Tab, Linefeed, Formfeed, and Return were derived from the
ASCII formatting control characters with the same codes, and they will
often echo like those ASCII characters.

The 4000 bit gives a non-ASCII interpretation to the basic
character.  For example, codes 4000 through 4037 are used for
additional printing characters for symbols which are not in ASCII.
They represent the characters produced, on output, by codes 0 through
37 (which, in the SUPDUP output language, must signify printing
characters if they mean anything at all).  Local system SUPDUP programs
do not
need to support these characters, and most do not (so we do not list
their pictorial appearances).  If the local-system SUPDUP program does
provide
for output of codes 0 through 37 as printing characters, the %TOSAI bit
in TTYOPT should be one.  Otherwise, it should not
ever send codes 4000 through 4037 as input.

4177 is another additional printing character, which is echoed as code
177 (again, used only if %TOSAI is set).  4101, 4102, 4103 and 4110 are
used for four special command keys, which are called @i[escape],
@i[break], @i[clear] and @i[help].  Only @i[help] receives much use.

All other combinations of the 4000 bit with various basic characters
are used for special purposes of the protocol, or are undefined. 
Codes 4105 (4000 plus E) and 4123 (4000 plus S) are used by the Local
Editing Protocol.  Code 4124 (4000 plus T) is used by the Line Saving
Protocol.  Codes 4130 (4000 plus X) and 4131 (4000 plus Y) are used by
the SUPDUP Graphics Protocol.

A server system that has no application for MIT extended ASCII should
convert it into ordinary ASCII as follows:
@itemize[
Discard all but the low 8 bits of the 12-bit character.

If the 200 bit is set, clear it, and the 100 bit and the 40 bit.  Thus,
control-bit plus A is converted into ASCII control-A.
]

The encoded SUPDUP input consists entirely of byte values less than 200.
Byte values 200 and above are used for special commands that are not
"terminal input".  On ITS, these commands are processed by the network
server program, whereas 034 escape sequences are processed by the
system's terminal driver.

@itemize[
The two byte sequence 300 301 says to log out the remote job and break
the connection.

The two byte sequence 300 302 introduces "console location" text.
This text is meant for human consumption, and describes the location
in some sense of the local system or the user's terminal or both.  It
is supplied to the remote system so that system can use it to respond
to inquiries about the user's location.  The console location text
should be made up of ASCII printing characters, and is terminated with
a zero byte.]

@Section[SUPDUP Output]

Once the SUPDUP connection is initialized, all output uses the SUPDUP
output language.  In this language, codes 0 through 177 are used only
to represent printing characters which are expected to advance the
cursor one position to the right.  Most terminals support only codes
40 through 176, and only those codes will be sent.  If the terminal
supports other codes as printing characters, that fact is indicated by
the %TOSAI bit in TTYOPT.  In any case, codes 011
through 015 do @i[not] have their ASCII formatting significance.

All other output operations are represented by commands with codes
from 200 to 377.  Some of these commands are followed by extra argument
bytes.  The contents of an argument byte may be anything from 0 to
377, and its meaning is determined by the command
it follows.  The traditional names of these commands are six
characters long and start with %TD.  The commands are:

@begin[description]
%TDMOV (code 200)@\Four arguments, the old vertical and horizontal
position followed by the new vertical and horizontal position.  Move
the cursor to the new position, assuming that it was at the specified
old position.  The local system should forget its opinion of the old
cursor position and use what the %TDMOV says.  This would be absurd on
a display terminal; %TDMOV is used only for printing terminals.  For
actual displays, %TDMV0 (see below) is used.

%TDEOF (code 202)@\No arguments.  Erase to end of screen.  The cursor
does not move.

%TDEOL (code 203)@\No arguments.  Erase to end of line.  The cursor
does not move.

%TDDLF (code 204)@\No arguments.  Erase the character after the
cursor.  The cursor does not move.

%TDCRL (code 207)@\No arguments.  Advance the cursor to the next line
and clear it.  At the bottom of the screen, this is expected to scroll
the text on the screen by a fixed number of lines.  The number of
lines the terminal will scroll by is not expected to be under
the control of the remote system.  Instead, the TTYROL terminal
characteristics word is used to tell the remote system how many lines
the terminal @i[will] scroll.

%TDNOP (code 210)@\No arguments.  Do nothing.

%TDORS (code 214)@\No arguments.  This command marks the place in the
data stream at which the remote system executed an abort-output
operation.  See the next section.

%TDQOT (code 215)@\One argument, which should be passed on directly to
the terminal.  This is used for transmitting 8-bit data, such as a
program being downloaded into a terminal attached to the local system.

%TDFS (code 216)@\No arguments.  Move the cursor right one position.
This could be done just as well with a %TDMV0; %TDFS is used for data
compression.

%TDMV0 (code 217)@\Two argument bytes: the vertical position and the
horizontal position.  Just move the cursor.

%TDCLR (code 220)@\No arguments.  Clear the screen and move the cursor
to the top left corner.

%TDBEL (code 221)@\No arguments.  Ring the terminal's bell.

%TDINI (code 222)@\No arguments.  Informs the terminal that the remote
system has just been started.  This is only sent to hard-wired
terminals.  Network connections will never receive this command,
because no network connections exist when the system is started.

%TDILP (code 223)@\One argument, a number.  Insert that many blank
lines at the vertical position of the cursor, pushing the following
lines down.  The cursor does not move.

%TDDLP (code 224)@\One argument, a number.  Delete that many lines at
the vertical position of the cursor and below, moving the following
lines up, and creating blank lines at the bottom of the screen.  The
cursor does not move.

%TDICP (code 225)@\One argument, a number.  Insert that many blank
character positions after the cursor, pushing following text on the
line to the right.  The last few positions on the line are pushed off
the right margin and discarded.  Other text lines are not affected.
The cursor does not move.

%TDDCP (code 226)@\One argument, a number.  Delete that many character
positions after the cursor, moving following text on the line left to
fill them up, and creating blank positions at the right margin.  Other
lines are not affected, and the cursor does not move

%TDBOW (code 227)@\No arguments.  Display printing characters that
follow as white-on-black.

%TDRST (code 230)@\No arguments.  Reset %TDBOW mode and any other such
options as may be defined later.

%TDGRF (code 231)@\Variable number of arguments.  This introduces a
sequence of SUPDUP Graphics Protocol operations.  The SUPDUP Graphics
Protocol is documented in a separate section below.

%TDRSU (code 232)@\Two arguments, the number of lines in the region to
be scrolled, and the number of lines to scroll it by.  The region
consisting of the specified number of lines, starting with the line
that the cursor is on, is scrolled up by an amount specified by the
second argument.  Lines scrolled out of the top of the region are
lost, and blank lines are created at the bottom of the region.
This operation is equivalent to a %TDDLP at the top of the region
followed by a %TDILP above the place where the blank lines should be
created, above the bottom of the region.  However, %TDRSU can avoid
disturbing the lines below the region even momentarily, which looks
better.

%TDRSD (code 233)@\Two arguments.  Like %TDRSU but scrolls the region
down instead of up.
@end[description]

The Local Editing Protocol introduces several additional %TD codes.  See
the section on the Local Editing Protocol for details on implementing
them.

@description[
%TDSYN (code 240)@\Do-Local-Echoing.  Two arguments, a
resynchronize code from the last resynchronize received, and the
number of input characters received by the remote system since that
resynchronize.  If the synchronization constraint is satisfied, the
local system will begin local editing.

%TDECO (code 241)@\Prepare-for-Local-Editing.  No arguments.
Tells the local system to begin sending resynchronizes, which will
enable the remote system to send a %TDSYN.

@multiple[
%TDEDF (code 242)@\Define-Char.  Normally followed by two argument bytes
which encode a
9-bit input character and a 5-bit function code; the function code
specifies the editing action of the input character when the user types
it.  The bottom seven
bits of each byte are taken, and the two are combined (first byte most
significant) into a 14-bit number.  The top 5 bits of this number are
the function code.
Function code 37 serves as an escape: a third argument byte follows,
which contains the actual function code.  This is how function codes
larger than 37 are specified.

The other 9 bits are normally the character whose local editing function
is being defined.  It is a 12-bit MIT extended ASCII character whose top
three bits are taken as zero.  This means that not all 12-bit codes can
be specified; so the local editing protocol only applies to input
characters with codes 777 and below.  Any input character which
contains the 4000 bit is always handled remotely.  (The 1000 and 2000
bits are unused in the 12-bit character set.)

If the terminal does
not have Control and Meta keys, and transmits ASCII (%TOFCI is not
set), %TDEDF will still speak in terms of 9-bit characters.
Definitions specified for 9-bit characters in the range 300 through
337 should be applied to the ASCII control characters 000 through 037.
Definitions for 9-bit characters in the range 040 through 177 should
be applied to the ASCII characters with the same values.

@tag(caseindirect)
A counterintuitive feature of MIT extended ASCII is that Control-A and
Control-a are two different characters.  Their codes are 301 and 341.
Most programs interpret these two characters the same way, and most
editors will want to tell the local system to interpret the two the same
way.  In the Local Editing Protocol this is done by defining the
lower-case character with function code 22, which means "use the
definition of a related character".  For characters whose basic
character is a lower case letter, the related character used is the
corresponding upper case character.  Thus, defining character 341
(Control-a) with code 22 tells the local system to use the definition of
character 301 (Control-A).  For a non-lower-case character with the
Control bit (200) set, the related character is obtained by turning off
bits 200 and 100.  Thus, Control-I (311) turns into Tab (011, which is
ASCII Control-I).  Code 22 should be used only with lower case characters
and control characters.

Some function codes are special, and do something other than define a
character.  With these, the 9 bits are used to transmit arguments
associated with the operation being performed.  Here is how.

@begin[description]
Code 31@\Set Word Syntax.  The syntax is specified for the ASCII
character which is the low 7 bits of the accompanying 9-bit character.
The 200 bit of the 9-bit character says what the syntax @i[is]: if it
is 1, the character is a separator.  If it is 0, the character is part
of a word.

Code 32@\Set Insertion Mode.  The 9 bits contain the insertion mode.

Code 33@\Initialize.  The word syntax table is set so that only
letters and digits are part of words.  Insertion mode is set to 1.
The fill column is set to 0.  All character definitions are set to
code 0 except these:
@begin[itemize]
Lower case letters, with or without Control or Meta, are set to code
22 (use the definition of the related upper case character).  These are
characters 140 through 172, 340 through 372, 540 through 572, and 740
through 772.

All printing characters except lower case letters are set to code 7
(self-insert).  These are characters 40 through 140, and 173 through 176.

Digits, with either Control, Meta or both, are set to code 27 (specify
repeat count).  These are characters 260 through 271, 460 through 471,
and 660 through 671.


@end[itemize]

Code 34@\Set Margin.  The top 2 bits of the 9 say which margin to set:
0, 1, 2, 3 stand for left, top, right, bottom.  The remaining 7 bits
specify that margin, as a distance from the actual screen edge behind
it.

Code 41@\Set Fill Column.  The 9 bits contain the value of the fill
column, or zero if there is to be no fill column.

@end[description]
]
%TDNLE (code 243)@\Stop-Local-Editing.  No arguments.

%TDTSP (code 244)@\Space-for-Tab.  No arguments.  Used to output spaces
which are part of the representation of an ASCII tab character in the
text being edited.

%TDCTB (code 245)@\Line-Beginning-Continued.  No arguments.  States that
the beginning of this screen line is not at the beginning of a line of
the text being edited.

%TDCTE (code 246)@\Line-End-Continued.  No arguments.  States that
the line of text which appears on this screen line extends beyond the
end of the screen line.

%TDMLT (code 247)@\Multi-Position-Char.  Two arguments, the width of
the character about to be displayed, and its character code.  The
width specifies the number of
character positions, following the cursor, which belong to the
representation of a single
character of the text being edited.  This command sequence should be
followed immediately by output to fill up those positions.
]

The Line Saving Protocol introduces four additional commands:

@description[

%TDSVL (code 250)@\Save-Lines.  Three arguments: a number of lines
@i<n>, followed by two 7-bit bytes of label number @i<l> (low byte first).
@i<n> lines starting with the line the cursor is on have their contents
saved under labels @i<l>, @i<l>+1, ... @i<l>+@i<n>-1.

%TDRSL (code 251)@\Restore-Lines.  Three arguments bytes, the same as for
%TDSVL.  The contents saved under @i<n> labels starting at label @i<l> are
restored onto the screen on lines starting with the one the cursor is
on.  If any of the labels is not defined, one or more Label-Failure
commands are sent to the remote system to report this.

%TDSSR (code 252)@\Set-Saving-Range.  Two arguments which specify the
range of columns of line contents to save and restore in %TDSVL and
%TDRSL.  The first argument is the first column to save.  The second
is the last column to save, plus one.

%TDSLL (code 253)@\Set-Local-Label.  Two arguments, the low and high
7-bit bytes of a label number.  This label number, and numbers
immediately below it, will be used label the saved contents of any
lines pushed off the screen by locally handled editing commands.
]

One more command is used together with either local editing or line
saving for anticipatory output.

@description[
@begin[multiple]
%TDMCI (code 254)@\Move-Cursor-Invisible. One argument, a logical
vertical position which is a 14-bit @i[signed] number transmitted low
7 bits first.  Positions the cursor at an invisible line so that
anticipatory text transmission can be done.  Output to the invisible
line continues until a cursor motion %TD command is received.

The specified vertical position
signifies the relationship between the text on this line and that on
the screen: if lines 2 through 20 are in use for editing (according to
the editing margins), then an invisible line with vertical position 30
contains the text that would appear 10 lines below the last text
actually displayed.  This information is for the use of locally-handled
scrolling commands.

The invisible lines displayed this way will be remembered by the
local system  for the use of locally handled scrolling commands, if
the local system can handle any.  Also, the contents can be saved
under a label and then restored into actual visibility under the
control of the remote editor.
@end[multiple]

]

@Section[Aborting Output in SUPDUP]

When a SUPDUP server program is asked to abort (discard) output
already transmitted, it sends a %TDORS character.  At the same time,
it sends an interrupt to the local system if the
network between them permits this.  The local system should keep a
count of the number of such interrupts minus the number of %TDORSes
received.  When this count is positive, all output received must be
discarded and only scanned for %TDORSes.  It is legitimate for this
count to become negative.

When the %TDORS is read which decrements the count back to zero, the
local system must transmit the current cursor position to the remote
system.  The remote system is waiting for this, because it does not
know how much output the local system actually discarded, and
therefore it does not know where the terminal cursor ended up.  The
cursor position is transmitted using a four byte sequence:
034 020 @i<vpos> @i<hpos>.  The remote system should hold all output and
not transmit any more until this command is received.

If the network does not provide for interrupts (high-priority messages),
the local system should send the cursor position whenever a %TDORS is
received.

@Section[Non-Network Terminals]

SUPDUP can also be used to communicate with directly wired or dial-up
terminals, which we call non-network terminals.  The key point of
distinction is that "networks" provide buffering, flow control and
error recovery, so that SUPDUP as used over networks need not be
concerned with these things.  When SUPDUP is used with non-network
terminals, they must all be provided for.

The buffering is provided in the remote system's terminal controller
and in the terminal itself.  The flow control, and associated error
recovery, are provided by an extension of the SUPDUP protocol.  This
flow control mechanism was inspired by TCP [see TCP] and is
immeasurably superior to the XON/XOFF mechanism by which many
terminals deprive the user of the use of two valuable editing
commands.

Flow control works by means of an @i[allocation], stored in the remote
system, which is the number of characters which the remote system can
send to the terminal without overloading it.  After this many have
been sent, the remote system must cease transmission until it is sent
an input command giving more allocation.  The terminal sends this
command when it has made room in its buffer by processing and removing
characters from it.

To begin using flow control, the terminal should send the sequence 034
032, which initializes the allocation to zero.  Then it should send
the sequence 034 001 @i[n], where @i[n] is the number of characters of
buffer space available for output.  This increments the allocation by @i[n].
Several 034 001 @i[n] sequences can be used to set the allocation to a
value above 177.

The remote system should remember the allocation it has received and
decrement it each time a character is transmitted.  The terminal
should count the number of output characters processed and removed
from the terminal's buffer for output; every so often, another 034 001
@i[n] sequence should be sent containing the number of characters
processed since the last 034 001 @i[n].  For example, the terminal could
send 034 001 030 each time it processes another 30 characters of
output.  By sending this sequence, it gives the remote system
permission to fill up that part of the buffer again.

This flow control mechanism has the advantage that it introduces no
delay if the buffering capacity does not become full, while never
requiring the remote system to respond quickly to any particular input
signal.  An even greater advantage is that it does not impinge on the
character set available for input from the terminal, since it uses the
034 escape mechanism, and 034 can be transmitted with the sequence 034
034.

Because line noise can cause allocate commands to be garbled, if the
terminal receives no output for a considerable period of time, it
should send another 034 032 followed by 034 001 @i[n].  This will make
sure that the remote system and the terminal agree on the allocation.
This is the only error recovery provided.  To achieve error recovery
for data as well, one must essentially implement a network
protocol over the line.  That is beyond the scope of SUPDUP.

ITS performs an additional encoding on SUPDUP output data over
non-network lines.  It encodes the data into 7-bit bytes so that it
can be sent through old terminal controllers which cannot output 8-bit
bytes.  Other systems should not need to use this encoding.  Codes 0
through 176 are transmitted as is.  Code 177 is transmitted as 177
followed by 1.  Code 2@i[nn] is transmitted as 177 followed by @i[nn]+2.
All cursor positions are transmitted with 1 added to them to reduce
the frequency with which bytes containing zero must be sent; this is
also to avoid problems with old controllers.

The 300-sequences may not be used as input and the terminal should not
send them.  This is because they are normally processed by the network server,
which is not in use for a non-network line.

@Unnumbered[SUPDUP Graphics Protocol]

The SUPDUP Graphics Protocol allows pictures to be transmitted and
updated across an ordinary SUPDUP connection, requiring minimal support
from the remote operating system.  Its features are

@itemize[
It is easy to do simple things.

Any program on the server host can at any time begin
outputting pictures.  No special preparations are needed.

No additional network connections are needed.  Graphics
operations go through the normal text output connection.

It does not require a network.  It is suitable for use with locally
connected intelligent display terminals, and by programs which need not
know whether they are being used locally or remotely.  It can be the
universal means of expression of terminal graphics output, for whatever
destination.

Loss or interpolation of output, due to a "silence" command typed by
the user or to the receipt of a message, does not leave the local
system confused (though it may garble the interrupted picture itself).

The local system is not required to remember the
internal "semantic" structure of the picture being
displayed, but just the lines and points, or even just bits
in a bit matrix.

Like the rest of the SUPDUP protocol, SUPDUP Graphics is terminal-independent.
]

The terminal capabilities associated with the SUPDUP Graphics Protocol are
described by bits and fields in the TTYSMT variable, described above.
One of these bits is %TQGRF, which indicates that the local system is able
to handle the SUPDUP Graphics Protocol in the first place.

Graphics output is done with one special %TD command, %TDGRF.  This
command is followed by any number of graphics operations, which are
characters in the range 0 through 177.  These are just the ordinary
printing characters, but in graphics mode they receive a special
interpretation described below.  Characters in their role as graphics operations
have symbolic names starting with %GO, such as %GOMVA and %GOELA.
Some graphics operations themselves take arguments, which follow the operations
and are composed of more characters in the range 0 through 177.

Any character 200 or above leaves graphics mode and then is interpreted
normally as a %TD command.  This way, the amount of misinterpretation of output
due to any interruption of a sequence of graphics operations is bounded.
Normally, %TDNOP is used to terminate a sequence of graphics operations.

Some other %TD commands interact with SUPDUP Graphics.
The %TDRST command should reset all graphics modes to their normal
states, as described below.  %TDCLR resets some graphics state information.

@section[Graphics Coordinates]

     Graphics mode uses a cursor position which is remembered from one
graphics operation to the next while in graphics mode.  The graphics
mode cursor is not the same one used by normal type-out: graphics
protocol operations have no effect on the normal type-out cursor, and
normal type-out has no effect on the graphics mode cursor.  In
addition, the graphics cursor's position is measured in pixels rather
than in characters.  The relationship between the two units (pixels and
characters) is recorded by the %TQHGT and %TQWID fields of the TTYSMT
variable of the terminal, which contain the height and width in pixels
of the box occupied by a normal-size character.  The size of the screen in either
dimension is assumed to be the size of a character box times the
number of characters in that direction on the screen.  If the screen
is actually bigger than that, the excess may or may not be part of
the visible area; the remote system will not know that it exists, in any
case.

Each coordinate of the cursor position is a 14-bit signed number,
with zero at the center of the screen.  Positive coordinates go up
and to the left.  If the screen dimension is even, the visible
negative pixels extend one unit farther than the positive ones, in
proper two's complement fashion.  Excessively large values of the
coordinates will be off the screen, but are still meaningful.
These coordinates in units of pixels are called @i[physical
coordinates].

Local systems may optionally support @i[virtual coordinates] as well.  A
virtual coordinate is still a 14-bit signed number, but instead of being
in units of physical pixels on the terminal, it is assumed that +4000
octal is the top of the screen or the right edge, while -4000 octal is
the bottom of the screen or the left edge.  The terminal is responsible
for scaling virtual coordinates into units of pixels.  The %TQVIR bit in
the TTYSMT variable indicates that the local system supports virtual
coordinates.  When the remote system wants to use virtual coordinates, it
should send a %GOVIR; to use physical coordinates again, it should send
a %GOPHY.  For robustness, every %TDGRF should be followed by a %GOVIR
or %GOPHY right away so that following commands will be interpreted
properly even if the last %GOVIR or %GOPHY was lost.

The virtual coordinates are based on a square.  If the visible
area on the terminal is not a square, then the standard virtual range
should correspond to a square around the center of the screen, and the
rest of the visible area should correspond to virtual coordinates
just beyond the normally visible range.

     Graphics protocol operations take two types of cursor position
arguments, absolute ones and relative ones.  Operations that take
position arguments generally have two forms, one for relative position
and one for absolute.  A relative address consists of two offsets,
delta-X and delta-Y, from the old cursor position.  Each offset is a
7-bit two's complement number occupying one character.  An absolute
address consists of two coordinates, the X followed by the Y, each being
14 bits distributed among two characters, with the less significant 7
bits in the first character.  Both types of address set the running
cursor position which will be used by the next address, if it is
relative.

  Relative addresses are
provided for the sake of data compression only.  They do not specify a
permanent constraint between points; the local system is not expected to
have the power to remember objects and constraints.  Once an object has
been drawn, no record should be kept of how the cursor positions were
specified.

     Although the cursor position on entry to graphics mode remains
set from the last exit, it is wise to reinitialize it with a %GOMVA
operation before any long transfer, to limit the effects of lost output.

It is perfectly legitimate for parts of objects to go off the screen.
What happens to them is not terribly important, as long as it is not
disastrous, does not interfere with the reckoning of the cursor
position, and does not cause later objects, drawn after the cursor
moves back onto the screen, to be misdrawn.

@section[Graphics Operations]

All graphics mode operations have codes between 0 and 177, and symbolic
names which start with %GO.  Operations fall into three classes: draw
operations, erase operations, and control operations.  The draw operations
have codes running from 100 to 137, while the erase operation codes run
from 140 to 177.  The
control operations have codes below 100.

If an operation takes a cursor position argument, the 20 bit in the
operation code usually says whether the cursor position should be
relative or absolute.  The 20 bit is one for an absolute address.

     Operations to draw an object always have counterparts which erase
the same object.  On a bit matrix terminal, erasure and drawing are
almost identical operations.  On a display list terminal, erasure
involves searching the display list for an object with the specified
characteristics and deleting it from the list.  Thus, on such terminals
you can only count on erasing to work if you specify the object to be
erased exactly the same as the object that was drawn.  Any terminal
whose %TOERS bit is set must be able to erase to at least this extent.

For example, there are four operations on lines.  They all operate
between the current graphics cursor position and the specified
cursor position argument:

@itemize[
%GODLR (code 101) draw line, relative address.

%GODLA (code 121) draw line, absolute address.

%GOELR (code 141) erase line, relative address.

%GOELA (code 161) erase line, absolute address.
]

Here is how to clear the screen and draw one line.

@example(
%TDRST			;Reset all graphics modes.
%TDGRF			;Enter graphics.
%GOCLR			;Clear the screen.
%GOMVA xx yy		;Set cursor.
%GODLA xx yy		;Draw line from there.
%TDNOP			;Exit graphics.
)

     Graphics often uses characters.  The %GODCH operation is followed
by a string of characters to be output, terminated by a zero.  The
characters must be single-position printing characters.  On most
terminals, this limits them to ASCII graphic characters.  Terminals
with %TOSAI set in the TTYOPT variable allow all characters 0-177.
The characters are output at the current graphics cursor position (the
lower left hand corner of the first character's rectangle being placed
there), which is moved as the characters are drawn.  The normal
type-out cursor is not relevant and its position is not changed.  The
cursor position at which the characters are drawn may be in between
the lines and columns used for normal type-out.  The %GOECH operation is
similar to %GODCH but erases the characters specified in it.  To clear
out a row of character positions on a bit matrix terminal without
having to respecify the text, a rectangle operation may be used.


@Section(Graphics Input)

     The %TRGIN bit in the right half of the TTYSMT variable indicates
that the terminal can supply a graphic input in the form of a cursor
position on request.  Sending a %GOGIN operation to the terminal asks to
read the cursor position.  It should be followed by an argument
character that will be included in the reply, and will serve to
associate
the reply with the particular request for input that elicited it.  The
reply should have the form of a Top-Y character (code 4131), followed
by the reply code character as just described, followed by an absolute
cursor position.  Since Top-Y is not normally meaningful as input,
%GOGIN replies can be distinguished reliably from keyboard input.

Unsolicited graphic input should be sent using a Top-X instead of a
Top-Y, so that the program can distinguish the two.  Instead of a reply
code, for which there is no need, the terminal should send an encoding
of the buttons pressed by the user on his input device.  For a
three-button mouse, the low two bits contain the number of the button
(1, 2 or 3 from left to right), and the next two bits contain the
number of times the button was clicked.

@section(Sets)

The local system may optionally provide the feature of grouping objects
into sets.  Then the remote system can blink or move all the objects in a
set without redrawing them all.  Sets are not easily implemented on bit matrix
terminals, which should therefore ignore all set operations (except
for a degenerate interpretation in connection with blinking, if that
is implemented).  The %TQSET bit in the TTYSMT variable of the
terminal indicates that the terminal implements multiple sets of
objects.

  There are up to 200 different sets, each of which can contain any
number of objects.  At any time, one set is selected; objects drawn
become part of that set, and objects erased are removed from it.  An
object in another set cannot be erased without selecting that set.
%GOSET is used to select a set.  It is followed by a character whose
code is the number of the set to be selected.

A set can be made temporarily invisible, as a whole, using the %GOINV
operation, without being erased or its contents being forgotten; and it
can then be made instantly visible again with %GOVIS.  %GOBNK makes the
whole set blink.  %GOCLS erases and forgets all the objects in the
current set.

Also, a whole set can be moved with %GOMSR or %GOMSA.  A set has at all
times a point identified as its "center", and all objects in it are
actually remembered relative to that center, which can be moved
arbitrarily, thus moving all the objects in the set at once.  Before
beginning to use a set, therefore, one should "move" its center to some
absolute location.  Set center motion can easily cause objects in the
set to move off screen.  When this happens, it does not matter what
happens temporarily to those objects, but their "positions" must not be
forgotten, so that undoing the set center motion will restore them to
visibility in their previous positions.

     On a terminal which supports multiple sets, the %GOCLR operation
should empty all sets and mark all sets "visible" (perform a %GOVIS on
each one).  So should a %TDCLR SUPDUP command.  Thus, any program
which starts by clearing the screen will not have to worry about
initializing the states of all sets.

Probably only display list systems will support sets.  A sufficiently
intelligent bit matrix terminal can provide all the features of a
display list terminal by remembering display lists which are redundant
with the bit matrix, and using them to update the matrix when a %GOMSR
or %GOVIS is done.  However, most bit matrix terminals are not
expected to go to such lengths.

@section(Blinking)

The %TQBNK bit in TTYSMT indicates that the terminal supports blinking
on the screen.  Usually blinking is requested in terms of sets: the
operation %GOBNK means make the selected set blink.  All objects in it
already begin blinking, and any new objects drawn in that set also
blink.  %GOVIS or %GOINV cancels the effect of a %GOBNK, making the
objects of the set permanently visible or invisible.

Implementing blinking in terms of sets is convenient, but causes a problem.
It is not very hard for an intelligent bit matrix
terminal to implement blinking for a few objects, if it is told
told in advance, before the objects are drawn.  Supporting the use of
sets in general is much harder.  To allow bit matrix terminals to
support blinking without supporting sets fully, we provide a convention
for the use of %GOBNK which works with a degenerate implementation of
sets.

For the remote system, the convention is to do all non-blinking output
in set 0 and all blinking output in set 1, and always send the %GOBNK
@i(before) drawing an object which is to blink.

For the bit matrix terminal, which offers %TQBNK but not %TQSET, the
convention is that %GOBNK is interpreted as meaning "objects drawn from
now on should blink", and %GOSET is interpreted as meaning "objects
drawn from now on should not blink".  The argument of the %GOSET is
ignored, since sets are not implemented.

When a remote system that is following the convention draws blinking
objects, it will send a %GOSET to set 1 (which has no effect), a %GOBNK
(which enters blinking mode), draw operations for the blinking objects,
and finally a %GOSET to set 0 (which turns off blinking for objects
drawn from now on).  If the same sequence of commands is sent to a
terminal which does fully support sets, the normal definitions
of %GOBNK and %GOSET will produce the same behavior.

Erasing a blinking object should make it disappear, on any terminal
which implements blinking.  On bit matrix terminals, blinking @i(must)
be done by XORing, so that the non-blinking background is not destroyed.

     %GOCLS, on a terminal which supports blinking but not sets,
should delete all blinking objects.  Then, the convention for deleting
all blinking objects is to select set 1, do a %GOCLS, and reselect set
0.  This has the desired effect on all terminals.  This definition of
%GOCLS causes no trouble on non-set terminals, since %GOCLS would
otherwise be meaningless to them.

     To make blinking objects stop blinking but remain visible is
possible with a %GOVIS on a terminal which supports sets.  It might seem
natural that %GOVIS on non-set terminals should just make all blinking
objects become solid, but this does not work out cleanly.  For example,
what would the terminal do if another %GOBNK is output?  To be compatible with terminals
that implement sets, the formerly blinking objects would all have to be
remembered.  This is an undesirable burden for the simple blinking
terminal, so we do not require it, and the remote system should use %GOVIS
only if sets are actually supported.

@section(Bit Map Displays: Rectangles, Scan Lines and XOR Mode)

     Bit matrix terminals have their own operations that display list
terminals cannot duplicate.  First of all, they have XOR mode, in
which objects drawn cancel existing objects when they overlap.  In
this mode, drawing an object and erasing it are identical operations.
All %GOD.. operations become equivalent to the corresponding %GOE..'s.
XOR mode is entered with a %GOXOR and left with a %GOIOR.  Display
list terminals that do not implement XOR mode will ignore %GOXOR and
%GOIOR; therefore, programs originating graphics output should
continue to distinguish draw operations from erase operations even in
XOR mode.  %TQXOR indicates a terminal which implements XOR mode.  XOR
mode, when set, remains set even if graphics mode is left and
re-entered.  However, it is wise to re-specify it from time to time,
in case output is lost.

     Bit matrix terminals can also draw solid rectangles.  They can
thus implement the operations %GODRR, %GODRA, %GOERR, and %GOERA.  A
rectangle is specified by taking the current cursor position to be one
corner, and providing the address of the opposite corner.  That can be
done with either a relative address or an absolute one.  The %TQREC
bit indicates that the terminal implements rectangle operations.

Finally, bit matrix terminals can handle data in the form of
scan-lines of bits.  The %TRSCN bit in TTYSMT indicates the presence
of this capability.  This is done with the operations %GODSC, %GOESC,
%GODRN and %GOERN.  The first two draw and erase given actual
sequences of bits as arguments.  The last two take run-length-encoded
arguments which say "so many ones, then so many zeros", for data
compression.

Scan line data for %GODSC and %GOESC is grouped into 16-bit units, and
each 16-bit unit is broken into three argument characters for
transmission.  (Three characters are needed since seven bits at most
can be used in each one).  The first two argument characters transmit
six bits each, and the third transmits four bits.  The most
significant bits of the 16-bit unit are transmitted first; the least
significant four or six bits of the argument character are used.  The
end of the scan line data is indicated by 100 (octal) as an argument.

When drawing, a one bit means turn on the corresponding pixel, and
a zero bit means leave the pixel alone.  This is so that pictures output
with scan lines can overstrike with other things.  Similarly, when erasing,
clear pixels for one-bits and do not clear them for zero-bits.
In XOR mode, flip pixels for one-bits and leave alone the other pixels.

Operations %GODRN and %GOERN use run-length-encoded arguments instead of
actual patterns of bits.  Each argument character specifies a number of consecutive
one bits or a number of consecutive zero bits.  Codes 1 through 77 indicate
a number of zero bits.  Codes 101 through 177 indicate a
number of one bits.  An argument of zero terminates the %GODRN or %GOERN.
As with the scan operations, a one bit means operate on the corresponding
pixel, and a zero bit means do nothing to it.

The scan line operations do not usually go well with virtual coordinates;
you cannot escape the fact that you are dealing with the pixel size of
the display when you are controlling individual pixels.

@section[Using Only Part of the Screen]

     It is sometimes desirable to use part of the screen for picture
and part for text.  Then one may wish to clear the picture without
clearing the text.  On display list terminals, %GOCLR should do this.
On bit matrix terminals, however, %GOCLR can't tell which bits were
set by graphics and which by text display.  For their sake, the %GOLMT
operation is provided.  This operation takes two cursor positions as
arguments, specifying a rectangle.  It declares that graphics will be
limited to that rectangle, so %GOCLR should clear only that part of
the screen.  %GOLMT need not do anything on a terminal which can
remember graphics output as distinct from text output and clear the
former selectively, although it would be a desirable feature to
process it even on those terminals.

     %GOLMT can be used to enable one of several processes which
divide up the screen among themselves to clear only the picture that
it has drawn, on a bit matrix terminal.  By using both %GOLMT and
distinct sets, it is possible to deal successfully with almost any
terminal, since bit matrix terminals will implement %GOLMT and display
list terminals almost always implement sets.

     The %TDCLR operation should clear the whole screen, including
graphics output, ignoring the graphics limits.

@section(Graphics Output by Multiple Processes)

It may be useful for multiple processes, or independent programs, to
draw on the screen at the same time without interfering with each other.

     If we define "input-stream state" information to be whatever
information which can affect the action of any operation, other than
what is contained in the operation, then each of the several processes
must have its own set of input-stream state variables.

     This is accomplished by providing the %GOPSH operation.  The %GOPSH
operation saves all such input-stream information, to be restored when
graphics mode is exited.  If the processes can arrange to output
blocks of characters uninterruptibly, they can begin each block with a
%GOPSH followed by operations to initialize the input-stream state
information as they desire.  Each block of graphics output should be
ended by a %TDNOP, leaving the terminal in its "normal" state for all
the other processes, and at the same time popping the what the %GOPSH
pushed.

    The input-stream state information consists of:
@itemize[
The cursor position.

the state of XOR mode (default is OFF).

the selected set (default is 0).

the coordinate unit in use (physical pixels, or virtual)
(default is physical).

whether output is going to the display screen or to a hardcopy
device (default is to the screen).

what portion of the screen is in use (see "Using Only Part of the Screen")
(default is all).
]

     Each unit of input-stream status has a default value for the sake
of programs that do not know that the information exists; the
exception is the cursor position, since all programs must know that it
exists.  A %TDINI or %TDRST command should set all of the variables to
their default values.

     The state of the current set (whether it is visible, and where
its center is) is not part of the input-stream state information,
since it would be hard to say what it would mean if it were.  Besides,
the current set number is part of the input-stream state information,
so different processes can use different sets.  The allocation of sets
to processes is the server host's own business.

@section(Errors in Graphics Operations)

Errors in graphics operations should be ignored by the local system.
This is because there is no simple way to report an error well enough
to be useful.  Since the output and input streams are not
synchronized, the remote system would not be able to tell which of its
operations caused the error.  No report is better than a useless report.

Errors which are not the fault of any individual operation, such as
running out of memory for display lists, should also be ignored as
much as possible.  This does @i(not) mean completely ignoring the operations
that cannot be followed; it means following them as much as possible:
moving the cursor, selecting sets, etc. as they specify, so that any
subsequent operations which can be executed are executed as intended.

@section(Storage of Graphics Operations in Files)

Since graphics mode operations and their arguments are all in the range 0 through 177,
it is certainly possible to store them in files.  However, this is not as useful
as one might think.

Any program for editing pictures of some sort probably wants to have
a way of storing the pictures in files, but graphics mode operations
are probably not best for the application.  This is because the program
presumably works with many kinds of information such as constraints which
are only heuristically deduceable from actual appearance of the picture.
A good representation must explicitly provide a place for this information.
Inclusion of actual graphics operations in a file will be
useful only when the sole purpose of the file is to be displayed.

@section(Graphics Draw Operations)

Note: the values of these operations are represented as 8-bit octal
bytes.  Arguments to the operations are in lower case inside angle
brackets.  @i<p> represents a cursor position (absolute or relative,
according to the operation description).

@begin(description)

%GODLR (code 101) @i<p>@\
Draw line relative, from the cursor to @i<p>.

%GODPR (code 102) @i<p>@\
Draw point relative, at @i<p>.

%GODRR (code 103) @i<p>@\
Draw rectangle relative, corners at @i<p> and at the
current cursor position.  Only if %TQREC is set in TTYSMT.

%GODCH (code 104) @i<string> 0@\
Display the chars of @i<string> starting at the current
graphics cursor position.

%GODSC (code 105) @i<scan-bits> @i<scan-terminator>@\
Draw scan bits starting at the current graphics
cursor position.  Only if %TRSCN is set in TTYSMT.

%GODRN (code 106) @i<run-length-encodings> 0@\
Draw bits determined by @i<run-length-encodings>
starting at the current graphics cursor
position.  Only if %TRSCN is set in TTYSMT.

%GODLA (code 121) @i<p> Draw line absolute, from the cursor to @i<p>.@\
This code does the same thing as %GODLR, except that the cursor
position argument is absolute.

%GODPA (code 122) @i<p>@\
Draw point absolute, at @i<p>.

%GODRA (code 123) @i<p>@\
Draw rectangle absolute, corners at @i<p> and at the
current cursor position.  Only if %TQREC is set in TTYSMT.

@end(description)

@section(Graphics Erase Operations)

@begin(description)

%GOELR (code 141) @i<p>@\
Erase line relative, from the cursor to @i<p>.

%GOEPR (code 142) @i<p>@\
Erase point relative, at @i<p>.

%GOERR (code 143) @i<p>@\
Erase rectangle relative, corners at @i<p> and at the
current cursor position.  Only if %TQREC is set in TTYSMT.

%GOECH (code 144) @i<string> 0@\
Erase the chars of @i<string> starting at the current
graphics cursor position.

%GOESC (code 145) @i<scan-bits> @i<scan-terminator>@\
Erase scan bits starting at the current graphics
cursor position.  Only if %TRSCN is set in TTYSMT.

%GOERN (code 146) @i<run-length-encodings> 0@\
Erase bits determined by @i<run-length-encodings>
starting at the current graphics cursor
position.  Only if %TRSCN is set in TTYSMT.

%GOELA (code 161) @i<p>@\
Erase line absolute, from the cursor to @i<p>.

%GOEPA (code 162) @i<p>@\
Erase point absolute, at @i<p>.

%GOERA (code 163) @i<p>@\
Erase rectangle absolute, corners at @i<p> and at the
current cursor position.  Only if %TQREC is set in TTYSMT.
@end(description)

@section(Graphics Control Operations)

@begin(description)
%GOMVR (code 001) @i<p>@\
Move cursor to point @i<p>

%GOMVA (code 021) @i<p>@\
Move cursor to point @i<p>, absolute address.

%GOXOR (code 002)@\
Turn on XOR mode.  Only if %TQXOR is set in TTYSMT.

%GOIOR (code 022)@\
Turn off XOR mode.  Only if %TQXOR is set in TTYSMT.

%GOSET (code 003) @i<n>@\
Select set.  @i<n> is a 1-character set number, 0 - 177.  Only if %TQSET
is set in TTYSMT, except for a degenerate interpretation when %TQBNK is
set and %TQSET is not.

%GOMSR (code 004) @i<p>@\
Move set origin to @i<p>.  Only if %TQSET is set in TTYSMT.

%GOMSA (code 024) @i<p>@\
Move set origin to @i<p>, absolute address.  Only if %TQSET is set in TTYSMT.

%GOINV (code 006)@\
Make current set invisible.  Only if %TQSET is set in TTYSMT.

%GOVIS (code 026)@\
Make current set visible.  Only if %TQSET is set in TTYSMT.

%GOBNK (code 007)@\
Make current set blink.  Canceled by %GOINV or %GOVIS.  Only if %TQBNK
is set in TTYSMT.

%GOCLR (code 010)@\
Erase the graphics portion of the screen.  If possible, erase all
graphics output but not normal output.

%GOCLS (code 030)@\
Erase entire current set.  Only if %TQSET is set in TTYSMT, except for a
degenerate interpretation if %TQBNK is set and %TQSET is not.

%GOPSH (code 011)@\
Push all input-stream status information, to be
restored when graphics mode is exited.

%GOVIR (code 012)@\
Start using virtual coordinates.  Only if %TQVIR is set in TTYSMT.

%GOPHY (code 032)@\
Resume giving coordinates in units of pixels.

%GOHRD (code 013) @i<n>@\
Divert output to output subdevice @i<n>.
@i<n>=0 reselects the main display screen.
Only if %TQGHC is set in TTYSMT.

%GOGIN (code 014) @i<n>@\
Request graphics input (mouse, tablet, etc).
@i<n> is the reply code to include in the answer.
Only if %TQGIN is set in TTYSMT.

%GOLMT (code 015) @i<p1> @i<p2>@\
Limits graphics to a subrectangle of the screen.
%GOCLR will clear only that area.  The cursor positions are both
absolute!

@end(description)

@Unnumbered(Local Editing Protocol)

The purpose of the Local Editing Protocol is to improve the response
time in running a display text editor on the remote
system through SUPDUP.  It enables the local computer to
perform most of the user's editing commands, with the locus of
editing moving between the local and remote systems in a manner
invisible to the user except for speed of response.

Bits in the TTYSMT word tell the remote system that the local system
supports one or both of these subprotocols of SUPDUP.  It is up to the
remote system to use them or not.

Any local editing protocol must accomplish these things:

@enumerate[
The representation used for the remote editor's text display output
must contain sufficient information for the local system to deduce the
text being edited, at least enough to implement the desired editing
commands properly.

The remote system must be able to tell the local system when to do
local editing.  This is because the remote system is not always
running the text editor, and the editor may have modes in which
characters are not interpreted in the usual way.  If the protocol is
to work with a variety of editors, the editor must be able to tell the
local system what the definition is for each editing command, and
which ones cannot be handled locally at all.  Extensible editors
require the ability to tell the local system a new definition for any
character at any time.

The local system must be able to verify for certain that all the input
it has sent to the remote system has been processed, and all the
resulting output has been received by the local system.  We
call this process "synchronization".  To attempt local editing if this
condition is not met would cause timing errors and paradoxical
results.

The local system must be able to tell the remote system in some
fashion about any editing that has been performed locally.

Local editing must cease whenever an unpredictable event such as a
message from another user causes output which the local system will
misunderstand.
]

Each component of the Local Editing Protocol is present to satisfy one
of the design requirements listed above.  So the description of the
protocol is broken up by design requirement.
Each requirement is the subject of one section below.

@Subsection[Making Output Comprehensible for Editing]

The primary purpose of the SUPDUP protocol is to represent
display terminal I/O in a hardware-independent fashion.  A few new
%TD commands plus some conventions as to how the existing %TD commands
are used by remote editors are enough to allow the contents of the
edited text to be deduced.

The local system must not only obey these commands but also keep a
record of the characters on the screen suitable for executing the
user's editing commands.  This basically consists of keeping an
up-to-date matrix which describes what character is at each screen
position.  However, it is not as simple as that.  The problem cases are:

@enumerate[
Space characters and tab characters in the text being edited must be
distinguished.  Although a tab character may look the same as a
certain number of space characters at one particular time, it does not
behave like that number of space characters if text is inserted or
deleted to the left of it.

Control characters may be output in a fashion indistinguishable from
some sequence of printing characters.  For example, EMACS displays the
ASCII character control-A as "^A".
Editing commands which operate on single characters may be confused by
this.

One line on the screen may not correspond to one line of text being
edited.  EMACS represents a long line of text using several screen
lines.  Some other editors allow text to be invisible before the left
margin or after the right margin.

Not all of the screen space is used for displaying the text being
edited.  Some parts of the screen may be reserved for such things as
status information, command echoing, or other editing windows.  The
local system must avoid moving the cursor into them or editing them as
if they were text.

Space characters at the end of a line in the text being edited must be
distinguished from blank space following the displayed line, since some
editors (including EMACS) distinguish them.  Both the Forward Character
and End of Line commands need this information.
]

For each of these problem cases, the Local Editing Protocol has an
editor-independent solution:

@enumerate[

We enable the local system to distinguish spaces from tabs in the text
being edited by defining a output command Space-for-Tab (%TDTSP).  It is
used by the remote editor to represent the spaces which make up the
representation of a tab character.  The local system displays it just
like a space character, but records it differently.

An editor-independent solution to the problem of ambiguous display of
control characters is a new output command, %TDMLT @i<n>
@i<ch>, which says that the next @i<n> screen positions are part of the
display of one character of text, with code @i<ch>.  Using this, EMACS
outputs a control-A using five characters: %TDMLT 002 ^A ^
A.  The local system must record this, keeping track of which positions
are grouped together to represent one text character.  The record should
be cleared when the positions involved are erased.

Text lines that run over the ends of screen lines are said to be
@i(continued).  To deal with continued lines, we define two new output
commands, %TDCTB and %TDCTE These tell the local system that the
beginning or end of the screen line, respectively, is not really the
beginning or end of a line in the text being edited.  For EMACS, %TDCTE
on one line would always accompany %TDCTB on the next, but this might
not be appropriate for other editors.  Both of these commands must set
bits that accompany the lines when they are moved up or down on the
screen.  The %TDCTE bit should be cleared by anything which erases the
end of the line, and by %TDDCP within the line.  The %TDCTB bit should
be cleared by anything which erases the entire line.  EMACS always
outputs a %TDCTB for the first line on the screen, since EMACS does not
attempt to think about the text before what appears on that line.

Any parts of the screen that are being used for things other than the
text being edited can be marked off limits to local editing by setting
the editing margins.  Inside each edge of the screen there is an editing
margin.  The margin is defined by a width parameter is normally zero,
but if it is set nonzero by the remote system, then that many columns or
rows of character positions just inside the edge are made part of the
margin.  By setting the margins, local editing can be restricted to any
rectangle on the screen.  The margins are set by means of a special
kind of %TDEDF (see below).

@multiple[
Space characters at the end of a line are distinguished by means of
conventions for the use of the erasing commands %TDEOL, %TDCLR and
%TDDLF.   The convention is that %TDDLF is used
only to clear text in the middle of a line, whereas the others imply
that all the text on the line (or all the text past the point of erasure)
will be reprinted.  Although the area of screen erased by
%TDEOL or %TDCLR becomes blank, the
screen-record
matrix elements for those areas is filled with a special character,
distinct from the space character and from all graphic characters.
Character code 200 will serve for this.  Spaces in the text are output
by the editor as actual spaces and are recorded as such in the
screen-record matrix.  Blanks created in the middle of the line by
%TDICP are recorded as space characters since character insertion
is done only within the text of the line.  Blanks created at the end of
the line by %TDDCP are recorded as character 200 since they are
probably past the end of the text (or else the text line extends past
this screen line, which will be reported with %TDCTE).

If the screen record and the editor obey the above conventions, the
end of the text on the line is after the last character on the line
which is not 200.  (Note that this can only be relied upon to be the end
of a line of text if %TDCTE has not been done on this screen line).
]]

@Subsection[Defining the Editing Commands]

When a remote editor requests local editing, it must tell the local
system what editing function belongs to each character the user can
type.  In SUPDUP local editing, %TDEDF is used for this.  If character
meanings change during editing, the editor must tell the local system
about the changes also.  In most display editors, text is inserted by
typing the text, so we regard printing characters as editing commands
which are usually defined to insert themselves.

A %TDEDF command usually specifies the editing
meaning of one user input character.  The first time the remote editor
enables local editing, it must first specify the definitions of all user
input characters with %TDEDF commands.  On subsequent occasions,
%TDEDF commands need only be used for characters whose meanings
have changed.

In addition to the definitions of user input characters, the local
system needs to know certain other parameters which affect what editing
commands do.  These are the word syntax table, the fill column, and the
insertion mode.

Commands which operate on words must know which text characters to treat
as part of a word.  This can be changed by activities on the remote
system (such as, switching to editing a different file written in a
different language).  A special form of %TDEDF command (code 31) is used to
specify the word syntax bit for one text character.  See below.

EMACS has an optional mode in which lines are broken automatically, at
spaces, when they get too long.  In such a feature, certain
characters@footnote(In EMACS, the Space character) which are normally
self-inserting may break the line if the line is too long.  Such
characters cannot simply be defined as self-inserting.  Instead, they
should be defined as @i[self-inserting before the fill column] (code
40).  The fill column is a parameter whose purpose is to specify where
on the line these characters should cease to be handled locally.  A
special form of %TDEDF (code 41) sets the value of the fill column.

Since many editors, including EMACS, have modes which ordinary
printing characters either push aside or replace existing text, the
Local Editing Protocol defines an @i[insertion mode] parameter to
control this.  If the insertion mode parameter is 1, ordinary printing
characters insert.  If the insertion mode parameter is 2, they
overwrite existing text.  If the parameter is 0, all ordinary printing
characters become unhandleable.  Note that which characters are
"ordinary printing characters" is controlled by the assigned function
codes.

A special form of %TDEDF command (code 32) is used to set the insertion mode
parameter.  To switch between insert and overwrite modes, it is
sufficient to change this parameter; it is not necessary to change the
definitions of all the printing characters.  Also, simple commands which
change the insertion mode can be handled locally, though we have not
actually defined any such function. 

Another special form of %TDEDF (code 34) is used to set the editing
margins, which say what part of the screen is used for local editing.

Another special form of %TDEDF command (code 33) initializes the
definitions of all user input characters, all the word syntax table
bits, the fill column and the insertion mode to a standard state.  This
state is not precisely right for any editor, but it gets most characters
right for just about all editors.  It sets up all ASCII printing
characters to insert themselves; aside from them, the number of commands
handled locally for any given editor is small.  When an editor is first
transmitting its command definitions to the local system, it can greatly
reduce the number of %TDEDF commands needed by first using code 33 to
initialize all characters and then fixing up those command definitions
and parameters whose standard initial states are not right.

Each %TDEDF command contains a user input character and a function
code.  The function codes are established by the Local Editing Protocol
specifically for use in %TDEDF.  Normally the function code
specifies the editing definition of the associated user input
character.  A few function codes indicate the special forms of
%TDEDF command which set parameters or initialize everything.

@Paragraph[%TDEDF Function Codes]

Here is a list of function codes (all in octal) and their meanings,
plus implementation directions.

@begin[description]
0@\This character should not be handled locally.
Either it is undefined, or its definition in the remote editor has
no local equivalent.

1@\Move forward one character.  Do not handle the command locally at
the end of the text on a line.

2@\Move backward one character.  Do not handle the command locally at
the beginning of a line.

3@\Delete the following character.  Do not handle the command locally
at the end of the text on a line, or on a line which is continued at the end.

4@\Delete the previous character.  Do not handle the command locally at
the beginning of a line, or on a line which is continued at the end.

5@\Move backward one character.  Treat a tab as if it were made of
spaces.  Do not handle the command locally at the beginning of a line.

6@\Delete the previous character.  Treat a tab as if it were made of
spaces.  Do not handle the command locally at the beginning of a line,
or on a line which is continued at the end.

7@\This is the usual function code for printing characters.  The
insertion mode parameter says what characters assigned this function
code ought to do.  They can insert (push the following text over), or
replace (enter the text in place of the following character).  Or they
may be not handled at all.  See function code 32, which sets the
insertion mode.  When the current mode is "replace", if the following
character may occupy more than one position, the command should not be
handled locally.  When the insertion mode is "insert", the command
should not be handled locally on a line continued at the end.

10@\Move cursor up vertically.  Do not handle the command locally if
the cursor would end up in the middle of a tab character, or past the
end of the line, or if either the line which the cursor starts on or the
previous line is continued at the beginning.

11@\Similar, but move down instead of up.  Do not handle locally if
either the line the cursor starts on, or the following line, is continued at
the beginning.

12@\Like code 10, but treat tabs just like so many spaces.

13@\Like code 11, but move down.

14@\Move cursor to beginning of previous line.  Must not be handled
locally if either the current line or the previous line is continued at the beginning.

15@\Move cursor to beginning of next line.  Must not be handled
locally if next line has been marked as continued at the beginning.

16@\Insert a line-break at the cursor and move the cursor after it.
Should not be handled locally if the cursor horizontal position is
greater than or equal to the fill column (plus the left editing margin).

17@\Insert a line-break at the cursor but do not move the cursor.
Does not pay attention to the fill column either.

20@\Move cursor to beginning of current line.  Must not be handled
locally if this line has been marked as continued at the beginning.

21@\Move cursor to end of current line.  Must not be handled
locally if this line has been marked as continued at the end.

22@\Use the definition of another character whose code is determined
from this character.  The details and purpose of this function are
explained in the section on @ref(caseindirect).

23@\Move cursor to the end of the following word.  Do not handle
locally if the word appears to end at the end of the line and the line
is marked as continued at the end.

24@\Move cursor to the beginning of the previous word.  Do not handle
locally if the word appears to begin at the beginning of the line and
the line is marked as continued at the beginning.

25@\Delete from cursor to the end of the following word.
Do not handle locally on a line marked as continued at the end.

26@\Delete from cursor to the beginning of the previous word.  Do not
handle locally on a line marked as continued at the end, or when code
24 would not be handled locally.

27@\Specify a digit of a repeat count.  The low seven bits of the
command character with this definition should be an ASCII digit.  This
digit should be accumulated into a repeat count for the next command.
The next character which is not part of the repeat count uses the
repeat count.  The argument-specifying characters cannot be handled
locally unless the command which uses the argument is also handled
locally.  So those characters must be saved and not transmitted until
the following command has been read and the decision on handling it
locally has been made.  If it is impossible to handle all the
specified repetitions of the command locally, the command should not
be handled locally at all.

30@\Introduces digits which make up an argument.  The following
characters, as long as they are digits (or a leading minus sign),
should be saved and used as a repeat count for the first character
which is not a digit or minus sign.

31@\This function code does not actually define the accompanying
character.  Instead, it specifies the word syntax table bit for one
ASCII character.  As arguments, it requires an ASCII character and a
syntax bit.  See the section on SUPDUP
output, under %TDEDF, for further information.

32@\This function code is used to specify the insertion mode
parameter.  The accompanying "character" should have numeric code 0, 1
or 2.  This "character" is the new setting of the parameter.  0 means
that the ordinary printing characters, those characters with function
code 7, should not be handled locally.  1 means that they should
insert, and 2 means that they should replace existing text.  The
definition of character code 0, 1, or 2 is not changed.

33@\This function code ignores the accompanying character and
initializes the definitions of all command characters, as well as the
word syntax table and insertion mode.  See the section on SUPDUP
output, under %TDEDF, for further information.

34@\Set margin.  This function code specifies one of the editing
margins.  See the section on SUPDUP output, under %TDEDF, for further
information.

35@\Move cursor up vertically, ignoring continuation.  Like code 10,
but handle locally regardless of whether lines are continued.

36@\Move cursor down vertically, ignoring continuations.  Like code 35
but move down.

40@\Self-insert, within the fill column.  Interpreted like code 7
(self-insert), except do not handle locally if the current cursor
column is greater than or equal to the fill column parameter (plus the
left editing margin).

41@\Set fill column.  This function code specifies the value of the
fill column parameter.  The numeric value of the accompanying
"character" is the new value of the parameter.  If the fill column
parameter is zero, the fill column feature is inactive.  Otherwise,
certain function codes are not handled when the cursor horizontal
position is greater than the fill column (plus the current left
editing margin).  Function codes affected are 16, 40 and 42.

42@\Insert line-break, moving over it, except that if the cursor is at
the end of a line which is not continued, and a blank line follows, move
the cursor onto the blank line.  Should not be handled locally if the
current cursor column is greater than or equal to the fill column
parameter (plus the left editing margin).

43@\Scroll up.  Scrolls region of editing up a number of lines
specified by an accumulated numeric argument.  Do not handle locally
if no numeric argument accumulated locally.  Local handling is
possible only if the local system knows the contents of lines off
screen due to use of the line saving protocol or anticipatory output.

44@\Scroll down.  Similar to 43 but scrolls down instead of up.

@end[description]

All insertion and deletion functions should take special notice of any
tab characters (output with Space-for-Tab) on the line after the cursor.
Unless the local system wishes to make assumptions about the tab stops in
use by the remote system, no insertion or deletion may be done before a
tab.  In such a situation, insertion and deletion commands should not be
handled locally.

No local system is required to handle all the function codes.
For example, our preliminary implementation does not handle the
vertical motion, insertion of line breaks, or arguments.  Any function
code which the local system prefers not to handle can be treated as
code 0; the characters with that definition are simply not handled
locally.  The local system is also free to fail to handle any
command character locally for any reason at any time.

Some functions are deliberately left undefined in particular unusual
cases, even though several obvious remote definitions could easily be
simulated, so that they can be used by a wider variety of editors.  For
example, editors differ in what they would do with a command to move
forward one character at the end of the text on a line.  EMACS would
move to the beginning of the next line.  But Z would move into the space
after the line's text.  Since function code 1 is defined in this case
not to be handled locally, either EMACS or Z could use it.

@subsection[Synchronization]

The local system sees no inherent synchronization between the channels
to and from the remote system.  They operate independently with their
own buffers.

When the local system receives an input character from the user which
it cannot handle, either because its definition is not handleable or
because local editing is not going on at the moment, it sends this
character to the remote system to be handled.  After processing the
character, the remote system may decide to permit local editing.  It
must send a command to the local system to do so.  By the time the
local system receives this command, it may already have received and
sent ahead several more input characters.  In this case, the command
to permit local editing would be obsolete information, and obeying it
would lead to incorrect display.

The Local Editing Protocol contains a synchronization mechanism
designed to prevent such misbehavior.  It enables the local system,
when it receives a command to begin local editing, to verify that
the remote system and the communication channels are quiescent.

Before the remote editor can request local editing, it must ask for
synchronization.  This is done by sending the output command
%TDECO (Prepare-for-Local-Editing).  After receiving this, the local system
lays the groundwork for synchronization by sending a resynchronize
command to the remote system.  This is a two-character sequence whose
purpose is to mark a certain point in the input stream.  By counting
characters, later points in the input stream can also be marked,
without need for constant resynchronize commands.

The resynchronize command begins with a special code which was
previously meaningless in the SUPDUP input language.  The second
character of the sequence is a identifier which the remote system will
use to distinguish one resynchronize command from another.  It is best
to use 40 the first time, and then increment the identifier from one
resynchronize to the next, just to avoid repeating the identifier
frequently.

When the remote editor actually wishes to permit local editing, after
sending %TDEDF commands as necessary, it sends a %TDSYN (Do-Local-Editing)
command, also known as a resynch reply.  This command is followed by
two argument bytes.  The first one repeats the identifier specified in the
last resynchronize command received, and the second is simply the
number of input characters received at the remote system since that
resynchronize command.

When the local system sees the resynch reply, it compares the
identifier with that of the last resynch that @i[it] sent, and
compares the character count with the number of characters @i[it] has
sent since the last resynch.  If both match, then all pipelines are
empty; the remote system has acknowledged processing all the
characters that were sent to it.  Local editing may be done.

If the resynch identifier received matches the last one sent but the
character counts do not, then more input characters are in transit
from the local system to the remote system, and had not been processed
remotely when the %TDSYN was sent.  So local editing cannot begin.

If the identifiers fail to match, it could be that the remote system
is confused.  This could be because the user is switching between two
editors on the remote system.  After each switch, the newly resumed
editor will be confused in this way.  It could also be that the remote
editor sent a resynch reply for the previous resynch, while the last
one was on its way.  In either case, the proper response for the local
system is to send another resynch.  It should wait until the user
types another input character, but send the resynch before the input
character.  This avoids any chance that a resynch itself will prevent
local editing by making the pipelines active when they would have been
quiescent.

The first %TDSYN after a %TDECO is
usually preceded by many %TDEDF commands to initialize all the
editing commands.  Later %TDSYN commands are preceded by
%TDEDFs only for commands whose meanings have changed.

Since the character count in a %TDSYN cannot be larger than
177, a new resynchronize should be sent by the local system every so
often as long as resynchronization is going on.  We recommend every
140 characters.  Once again, the resynch should be sent when the next
input character is in hand and ready to be sent.  If the remote system
sees more than 177 input characters without a resynchronize, it should
send another %TDECO.

Once synchronization has been verified and local editing begins, it
can continue until the user types one character that cannot be
handled locally.  Once that character has been transmitted,
local editing is not allowed until the next valid %TDSYN.

@subsection[Reporting Results of Local Editing]

When the local system has done local editing, it must eventually report to the
remote system what it has done, so that the changes can be made
permanent, and so that following commands handled remotely can have
the proper initial conditions.

In the Local Editing Protocol, the local editing is reported by
transmitting the editing commands themselves, preceded by a special
marker which identifies them to the remote system as locally handled.
The special marker two characters long; it consists of the extended
ASCII character 4105 (4000 plus E) followed by a byte containing the
number of locally handled editing characters that follow.  Thus, a
locally handled sequence X Z Control-B Y would be sent as 4105 4 130 132
302 131.

When the remote editor receives the locally-handled commands, it
executes them as usual, but it does not actually update the display.
It only @i[pretends to itself] that it did.  It updates all its tables
of what is on the screen, just as it would if it were updating the
display, but it does not output anything.  This brings the remote
editor's screen-records into agreement with the actual screen contents
resulting from the local editing.

Local systems are recommended to output all accumulated locally
handled commands every few seconds if there is no reason to do
otherwise.  Sending a few seconds worth of input commands all at
once causes the remote editor to run for fewer long periods rather than
many short periods.  This can greatly reduce system overhead in
timesharing systems on which process-switching is expensive.

@subsection[Unexpected Output From Remote System]

If output is received from the remote system while local editing is
going on, the local system should stop doing local editing, and should
not send any more resynchs until another %TDECO is received.  If the
remote system generates output that the text editor does not know about,
it should notify the text editor to send another %TDECO.

The remote system can send the command %TDNLE (Stop-Local-Editing) as a
clean way of stopping local editing.  It has the same effect.

@Unnumbered[The Line Saving Protocol]

The Line Saving Protocol allows the remote editor to tell the
local terminal to save a copy of some displayed text, and later refer
to the copy to put that text back on the screen without having to
transmit it again.

The Line Saving Protocol reduces the amount of output required in such
situations by allowing text once displayed to be saved, and restored
whenever the same text is to be displayed again.  Moving to a part of
the file which had been displayed at an earlier time no longer requires
transmitting the text again.

@Section[The Design of the Line Saving Protocol]

The basic operations provided by the Line Saving Protocol are those of
saving and restoring the contents of individual lines on the screen.

A line on the screen is saved for later use by giving it a label.  The
line remains unchanged on the screen by this operation, but a copy of
the contents are saved.  At a later time, the saved contents can be
brought back onto the screen, on the same line or another one, by
referring to the same label.

A label number is 14 bits wide (transmitted as two bytes of 7 bits,
low bits first), but local systems are not required to be able to
handle 2@+[14] different labels.  The %TRLSV field in the TTYSMT
variable tells the remote system approximately how many different
labels the local system can remember.  The remote system should not
try to use a larger label number.  The local system ought to be able
to remember approximately that number of labels under normal
circumstances, but it is permitted to discard any label at any time
(because of a shortage of memory, perhaps).

Lines are saved with the command %TDSVL (Save-Lines).  It takes three
bytes of arguments: one for the number of lines to save, and two for
the label.  Several
consecutive lines are saved under consecutive labels, starting with
the line the cursor is on and the specified label, then moving downward
and incrementing the label number.

To restore lines, the remote system should send a %TDRSL (Restore-Lines)
command.  %TDRSL takes arguments just like %TDSVL, but it copies the
saved text belonging to the labels back onto the screen.  The meanings
of the labels are forgotten when the labels are restored.

Because some editors can use for text display a region of the screen
which does not extend to the left and right margins, there is another
command to specify which range of columns should be saved and restored.
%TDSSR (Set-Saving-Range) @i<x1> @i<x2> specifies that contents of lines
should
be saved and restored from column @i<x1> (inclusive) to column @i<x2>
(exclusive).  Zero-origin indexing is used.

Line contents should be saved and restored by actual copying; or by some
other technique with equivalent results.

If the local system is asked to restore a label for which there is no
record, it should send to the remote system the sequence 4124 (4000 plus
T) followed by the number of lines (one byte) and starting label (two
bytes).  This is called a Label-Failure sequence.  The remote system, on
receiving this, should record that those labels are no longer available
and reoutput the contents of those lines from scratch.  In order to
implement this, the remote system must remember which lines it attempted
to restore from which labels.

@section[Interaction between Local Editing and Line Saving]

Some local editing commands can push lines off the screen.
If Line Saving is in use as well, the remote system can ask for such
lines to be saved with labels in case they are useful later.

The command %TDSLL (Set-Local-Label) @i<l> sets the label to be
used for the next line to be pushed off the screen by local editing.
Successive such lines decrement this parameter by 1.  If several lines
are pushed off the screen at once, they are processed lowest first.
This tends to cause several consecutive lines which have been pushed off
the screen to have consecutive labels, whether they were pushed off
together or individually.  That way, they can be restored with a single
%TDRSL command.

Because the remote editor is always expected to know how locally handled
commands have updated the display, it can always tell when a line has
locally been given a label.

@Bigsection[Anticipatory Output]

The remote system can use the time when the user is idle to send and
label lines which are not needed on the screen now but might be useful
later.  Such @i(anticipatory output) can be brought onto the screen by a
%TDRSL command from the remote system or by a locally-handled scrolling
command.

To begin a line of anticipatory output, the remote editor sends a %TDMCI
(Move-Cursor-Invisible) command, which is followed by a 14-bit signed
number called the logical vertical position (transmitted as two 7-bit
bytes, low bits first).  The value of this number indicates the
relationship between this line of output and the text on the screen, for
the sake of local editing.  After the output is done, the contents can
be saved under a label, if the local system supports line saving.  In
any case, text output after the cursor has been moved with a %TDMCI
should not appear on the screen.  The terminal's actual cursor should
remain where it was before the %TDMCI.

When local editing is in progress, unexpected output which moves the
cursor to an invisible line, or outputs characters to such a line,
should not terminate local editing.  Only unexpected output to the
actual screen, or moving the cursor onto the screen, should do that.
Output which is actually unrelated to the editor ought to start with a
%TDMV0 and will be detected by this test.

The logical vertical position specifies the position of this line on
an infinitely long screen which contains the actual lines of locally
editable text at their actual positions.  For example, if the current
editing margins specify that lines 2 through 20 are used for local
editing, then a line output at logical vertical position -3 contains
what would appear five lines above the first displayed line of edited
text, and a command to scroll down five lines would bring it onto the
screen at line 2.  A line might be displayed at logical vertical
position 1; it would be invisible, but a command to scroll down one
line would make it visible on line 2.

If the local system does not support local editing, the value used for
the logical vertical position is immaterial; the only purpose of
outputting the line is to save it under a label.  In this case, the
local system is not required to save multiple invisible lines
according to logical vertical position.  It may keep only the last one
output.  So each transmitted line should be saved under a label as
soon as it is finished.

The remote operating system and the network are likely to have a large
buffering capacity for output.  Since anticipatory output is used
primarily on slow connections, the remote editor could easily produce
and buffer in a second an quantity of anticipatory output which will take many
seconds to transmit.  While the backlog lasts, the user would obtain no
service for his explicit commands, except those handled locally.
To prevent this, the remote editor ought to send anticipatory output
in batches of no more than two or three lines' worth, and send these
batches at an average rate less than the expected speed of the bottleneck of the
connection.  In between batches, it should check for input.

The %TRANT field of the TTYSMT word tells the remote system
approximately how many lines of anticipatory output the local system can
support for local editing use.  The remote system should use this as a
guide.  It is free to send whatever it wants, and the local system is
free to decide what to keep and what to throw away.

@Unnumbered[References]

@description[
Arpanet@\"Arpanet Protocol Handbook", Network Information Center, SRI
International.

Chaosnet@\Dave Moon, "Chaosnet", MIT Artificial Intelligence Lab memo
628, June 1981.
  
Echo Negotiation@\Bernard S. Greenberg
"Multics Emacs: an Experiment in Computer Interaction"
in Proceedings, Fourth International Honeywell Software
Conference, Honeywell, Inc., March, 1980, Bloomington, Minn.

EMACS@\Richard M. Stallman, "EMACS, the Extensible, CUstomizable,
Self-Documenting Display Editor", Artificial Intelligence Lab memo
519a, April 1981.

TCP@\Jon Postel, "DOD Standard Transmission Control Protocol", 
USC/Information Sciences Institute, IEN-129, RFC 761, NTIS ADA082609, 
January 1980. Appears in: Computer Communication Review, Special 
Interest Group on Data Communication, ACM, V.10, N.4, October 1980.

Z@\Steven R. Wood, "Z, the 95% Program Editor", in the proceedings of the SIGPLAN
conference on text manipulation, June 1981.
]