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+Network Working Group                                         P. Deutsch
+Request for Comments: 190           Xerox Research Center, Palo Alto, CA
+NIC: 7135                                                   13 July 1971
+
+
+                DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM
+
+   This report describes an operational system for communicating textual
+   display information between a main-site computer and a remote-display
+   processor.
+
+      The main site machine is a DEC PDP-10 with the BBN paging hardware
+      (henceforth TENEX).
+
+      The remote machine is a IMLAC PDS-1 (henceforth IMLAC).
+
+      Section (1) briefly describes the IMLAC hardware configurations.
+
+      Section (II) describes the display facilities presented to the
+      user.
+
+      Section (III) describes the system calls (JSYS calls) implemented
+      in the TENEX monitor to provide these facilities.
+
+      Section (IV) describes the formats of the messages used for
+      communication between TENEX and the IMLAC.
+
+      Section (V) analyzes the division of responsibility between the
+      two machines.
+
+(1)  Hardware configurations.
+
+   The standard IMLAC is a 16-bit minicomputer with 4K of 2 microsecond
+   core, a cycle-stealing display, an input keyboard, and an
+   asynchronous serial communication interface.
+
+   The display is normally programmed to draw characters using very
+   short vectors.
+
+      The display comes in two major configurations, depending on the
+      presence or absence of hardware for drawing long vectors.
+
+      In the sequel, specifications depending on the configuration will
+      be flagged LVH or non-LVH respectively.
+
+   The I/O system normally does not provide for interrupts when
+   characters arrive from the serial interface.
+
+
+
+
+Deutsch                                                         [Page 1]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+      However, the IMLAC is barely able to keep up with the PDP-10
+      without this feature, so we were able to persuade the manufacturer
+      to implement it.
+
+   While no special hardware is required for the software described
+   here, the character interrupt and the SRI-ARC "mouse" and "keyset"
+   are highly recommended, and the software is oriented towards their
+   use.
+
+(II)  Facilities.
+
+   Each display console in the system may be in "display mode" or
+   "teletype simulation mode".
+
+      In display mode, the information displayed consists of text
+      strings at arbitrary positions on the display face.
+
+      In teletype simulation mode, the display shows the last 20-30
+      lines which would appear on a teletype listing.
+
+      A given console may switch between these modes, under program
+      control, without losing any information.
+
+   Regardless of mode, each display has a cursor string which follows
+   the position of the available pointing device,
+
+      The implemented system uses a "mouse" generally, but one console
+      uses a tablet.
+
+      The cursor string may be set by a program, for example to indicate
+      at what sort of object the user is expected to point.
+
+   In display mode, the screen of a given console is allocated to users
+   in rectangular blocks called "display areas".
+
+      This allows users to communicate via a single display split into
+      multiple areas.
+
+   When a user (program) requests a display area, he specifies how many
+   text strings he will want to display in it.
+
+      Each string has its own X-Y location, character size, font
+      (italic, underline, ...), and an arbitrary number of characters.
+
+         Each of these components is settable without disturbing the
+         others.
+
+      Each string may be manipulated without affecting the others.
+
+
+
+Deutsch                                                         [Page 2]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+(III) JSYS calls.
+
+   (ADA) Assign a display area.
+
+      JSYS ada: [440B]     allocate a display area
+
+         Accepts:
+
+         r1:
+
+            upper-left-y-cord[10], upper-left-x-cord[10], max-no-
+            string[11]
+
+         r2:
+
+            lower-right-y-cord[10], lower-right-x-cord[10], default-c-
+            size[2], default-n-inc[6], default-font[5]
+
+         Returns:
+
+            +1: Unsuccessful
+
+               r1: error code
+
+            +2: Successful
+
+               r1: da-id[18]
+
+         Function:
+
+            This jsys allocates a display area given the coordinates of
+            the diagonal, the maximum number of strings to be displayed,
+            and the default setting for the character size, font, and
+            horizontal increment.  An 18-bit da-id is returned which
+            should subsequently be used to refer to this display area.
+
+   (DDA) Deallocate a display area.
+
+      JSYS dda; [441B]     deallocate a display area
+
+         Accepts:
+
+            r1: da-id
+
+
+
+
+
+
+
+
+Deutsch                                                         [Page 3]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+         Returns:
+
+            +1: Unsuccessful
+
+               r1: error code
+
+            +2: Successful
+
+         Function:
+
+            This jsys deallocates a display area given the associated
+            da-id.
+
+   (STRDA)  STRing display:  add, delete, or change.
+
+   JSYS strda; [442B]     Manipulate (move, write, replace, delete) a
+   string in a display area
+
+      Accepts:
+
+         r1: string-id[18], da-id[18]
+
+         r2: first byte pointer or 0 or -1
+
+         r2: second byte pointer or 0
+
+         r4: y-cord[10], x-cord[10], font[6], c-size[3], h-inc[7]
+
+      Returns:
+
+         +1:  Unsuccessful
+
+               r1: error code
+
+         +2:  Successful
+
+               r1: string-id[18]
+
+      Function:
+
+         This jsys writes a new string, replaces, deletes, or moves
+         (optionally replacing) an extant string within a display area.
+         In addition, the font, character size, and horizontal increment
+         may be specified for the string.
+
+            The string may be specified by two byte pointers or by one
+            byte pointer with the string terminating with a zero
+            character.
+
+
+
+Deutsch                                                         [Page 4]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+               If first byte pointer is zero then
+
+                  if a new string is being written then
+
+                     an error code illstr is returned.
+
+                  Otherwise, the string already exists so
+
+                     delete the string from the display area.
+
+               If the first byte pointer is -1 then
+
+                  if a new string is being written then
+
+                     an error code illstr is returned.
+
+                  Otherwise, use the old string.
+
+               If the first character of the string is a zero character,
+               the string to be displayed is null, but the string is not
+               deleted.
+
+            The coordinates (optional unless the string is new or being
+            moved) are relative to the upper leftcorner of the display
+            area.
+
+               If the jsys is to effect an extant string, a zero
+               coordinate means use the old value.
+
+            For the font, c-size, and h-inc fields a field of all one's
+            indicates that the display area default value (set in the
+            ada jsys) is to be used.  A 0 means use the value which was
+            previously used for the (extant) string.
+
+            If the string is new, then an 18 bit string identifier is
+            returned.
+
+   (SCSR)  Set the Cursor StRing.
+
+      JSYS scsr; [450B]     display a string (vectors later) as the
+      cursor
+
+         Accepts:
+
+            r1:  first byte pointer or 0 or -1
+
+            r2:  second byte pointer or 0
+
+
+
+
+Deutsch                                                         [Page 5]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+            r3: font[5], c-size[2], h-inc[6]
+
+               No defaults allowed
+
+         Returns:
+
+            +1:  Unsuccessful
+
+               r1:  error code
+
+            +2:  Successful
+
+         Function:
+
+            This jsys is used to set the cursor string.  Later, a set of
+            vectors will be allowed also.  If the string length is zero
+            or the first byte pointer is 0, nothing will be displayed
+            for the cursor.  If the first byte pointer is -1 then the
+            old string will be used.  If a cursor did not previously
+            exist, an illcon error return will be executed.
+
+   (SDDA)  Suppress the display of an area.
+
+      JSYS sdda; [444B]    suppress all display in a display area
+
+         Accepts:
+
+            r1: da-id[18]
+
+            r2:  1 or 0
+
+         Returns:
+
+            +1:  Unsuccessful
+
+               r1:  error code
+
+            +2:  Successful
+
+         Function:
+
+            The display image is removed from the display area but is
+            not destroyed if r2 = 0.
+
+   (RDDA)  Restore the display of an area.
+
+      JSYS rdda; [446B]    restore all display in a display area
+
+
+
+
+Deutsch                                                         [Page 6]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+         Accepts:
+
+            r1: da-id[18]
+
+         Returns:
+
+            +1:  Unsuccessful
+
+               r1:  error code
+
+  +2:  Successful
+
+         Function:
+
+            The display image is restored in the display area.
+
+   (SDDA)  Suppress the display of a String.
+
+      JSYS ssda [445B];    suppress display of a string in a display
+      area
+
+         Accepts:
+
+            r1: string-id[18], da-id[18]
+
+         Returns:
+
+            +1:  Unsuccessful
+
+               r1:  error code
+
+            +2:  Successful
+
+         Function:
+
+            The display image for the given string is suppressed in the
+            display area
+
+   (RDSA)  Restore the display of a String.
+
+      JSYS rsda; [447B]    restore display of a string in a display area
+
+         Accepts:
+
+            r1: string-id[18], da-id[18]
+
+
+
+
+
+
+Deutsch                                                         [Page 7]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+         Returns:
+
+            +1:  Unsuccessful
+
+               r1:  error code
+
+            +2:  Successful
+
+         Function:
+
+            The display image for the given string is restored in the
+            display area.
+
+   (TSNDA)  Teletype Simulation oN.
+
+      JSYS tsnda; [451B]     turn tty simlaution on
+
+         Accepts:
+
+            none
+
+         Returns:
+
+            +1:  Always
+
+               r1:  1 if was in work station mode, 0 otherwise
+
+         Function:
+
+            Restores the tty simulation display area, and suppresses all
+            others (except the cursor).  Turns wsmode (work station mode
+            flag) off for this console and returns previous value of
+            wsmode.
+
+   (TSFDA)  Teletype Simulation ofF.
+
+      JSYS tsfda;  [452B]     turn tty simulation off
+
+         Accepts:
+
+            none
+
+         Returns:
+
+            +1:  Always
+
+
+
+
+
+
+Deutsch                                                         [Page 8]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+         Function:
+
+            Suppresses the display of the tty simulation display area,
+            restores the display of all other display areas, and sets
+            wsmode on.
+
+   (RSTDA)    Reset display areas.
+
+      JSYS rstda; [453B]     Reset display areas
+
+         Accepts:
+
+            none
+
+         Returns:
+
+            +1:  Always
+
+         Functions:
+
+            Deallocates and removes images from all display areas
+            associated with this console except the tty simulation and
+            cursor, the display of which is restored.
+
+(IV)  Message formats.
+
+   Messages are sequences of 8-bit characters, of which 7 contain useful
+   information.
+
+      The higer-order (200B) bit should contain even parity on IMLAC
+      input and is set to even parity on IMLAC output.
+
+      If the IMLAC receives an odd parity character, it halts at
+      present.
+
+      In the remainder of this document, the parity bit will not be
+      discussed.
+
+   A message may be either a character or a command.
+
+   Single-character messages from the PDP-10 to the IMLAC represent
+   program output intended for the teletype.
+
+   Commands from the PDP-10 represent display information.
+
+   Commands from the IMLAC represent characters or other input
+   information.
+
+
+
+
+Deutsch                                                         [Page 9]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+   Every command is prefixed by an internal escape character (code 33B)
+   and a character count.
+
+      The escape character will henceforth be referred to as ESC: is has
+      the same code as the ASCII escape character 33B.
+
+   Messages sent from TENEX to IMLAC:
+
+      Characters 40B-177B are directed to the teletype simulation area.
+
+      Character 12B (line feed) starts a new line in the teletype
+      simulation area.
+
+      An ESC indicates that display or control information is coming, as
+      follows.
+
+         Every message beginning with ESC contains the number of
+         following characters as its second character.
+
+         Certain constructs appear in several command messages.
+
+            (da)  A display area identifier is a pair of characters
+            containing 12 bits of information:
+
+               1st: bits (0:5) + 40B
+
+               2nd: bits (6:11) + 40B
+
+            (NSTRS)  A string count is a single character between 0 and
+            177B.
+
+            (STRID)  A string identifier is a single character between 1
+            and 177B.
+
+            (RETAIN)  The retention flag, if non-zero, specifies that an
+            existing string should be retained rather than overwritten.
+
+            (CSIZE)  A character size is a single character between 0
+            and 3:
+
+               The character sizes are respectively x1/2, x1, x2, x3.
+
+            (HINC)  A horizontal increment is a single character.
+
+               In the present implementation, HINC is ignored and a
+               standard spacing is supplied as follows.
+
+
+
+
+
+Deutsch                                                        [Page 10]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+                  LVH:
+
+                     0: 3 units
+
+                     1: 6 units
+
+                     2: 12 units
+
+                     3: 18 units
+
+                  non-LVH:
+
+                     0: 4.5 units
+
+                     1: 9 units
+
+                     2: 18 units
+
+                     3: 27 units
+
+            (FONTS)  A font specification is a single character.
+
+               In the present implementation, the font is stored but
+               does not affect the display.
+
+            (outxy)  An output X-Y coordinate pair is encoded in four
+            characters as follows:
+
+               1st: X,bits(0:5) + 40B
+
+               2nd: X,bits(6:11) + 40B
+
+               3rd: Y,bits(0:5) + 40B
+
+               4th: Y,bits(6:11) + 40B
+
+            (inxy)  An input X-Y coordinate pair is encoded in four
+            characters as follows:
+
+            LVH:  see (outxy) above.
+
+            non-LVH:  as above, except that each 12-bit coordinate is
+            actually of the form 1400B+40B*[v/9]+(v MOD 9), where the
+            actual coordinate is 0<=v<=719.
+
+                  This means there are actually fewer points on each
+                  axis.
+
+
+
+
+Deutsch                                                        [Page 11]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+            (string)  A string is just the requisite number of
+            characters.
+
+               Control characters will be displayed as a distinctive
+               blot.
+
+         Each display operation has a corresponding message.
+
+            01B - ADA (assign display area)
+
+               Followed by (da) NSTRS CSIZE HINC FONT.
+
+            02B - DDA (delete display area)
+
+               Followed by (da).
+
+            04B - STRDA (string display)
+
+               Followed by (da) STRID RETAIN (xy) FORMAT [CSIZE] [HINC]
+               [FONT] (string).
+
+                  Format specifies whether each of CSIZE, HINC, and FONT
+                  is to come from the display area default, the current
+                  value for the string, or the message.
+
+                     The bits are: 0 0 STF STI STC RDF RDI RDC.
+
+                     RDF=1 means read the FONT from the message.
+
+                     RDF=0, STF=1 means use the old value from the
+                     string.
+
+                     RDF=0, STF=0 means use the display area default.
+
+                     The pairs RDI-STI and RDC-STC specify HINC and
+                     CSIZE in the same way.
+
+            05B - SCSR (set cursor string)
+
+               Followed by RETAIN CSIZE HINC FONT (string).
+
+            06B - SDDA (suppress display of da)
+
+               Followed by (da) KILL.
+
+                  KILL#0 means delete all strings in this display area.
+
+                  KILL=0 means retain the strings.
+
+
+
+Deutsch                                                        [Page 12]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+            07B - RDDA (restore display of da)
+
+               Followed by (da).
+
+            10B - SDDA (suppress display of string)
+
+               Followed by (da) STRID KILL.
+
+                  KILL#0 means delete the string.
+
+                  KILL=0 means retain the string.
+
+            11B - RSLA (restore display of string)
+
+               Followed by (da) STRID.
+
+            12B - TSNDA (turn teletype simulation on)
+
+            13B - TSFDA (turn teletype simulation off)
+
+            14B - Long input mode
+
+               Puts the IMLAC into the mode where it sends coordinate
+               information in a message with every character.
+
+                  This is the normal operating mode for the IMLAC.
+
+            15B - Short input mode
+
+               Puts the IMLAC into the mode where it outputs characters
+               literally, just like a teletype.
+
+                  The IMLAC starts out in this mode when turned on.
+
+         A string of 10 ESC characters, followed by a non-ESC, indicates
+         an emergency - the IMLAC reinitializes itself and goes into
+         short input and teletype simulation modes.
+
+      All other (control) characters are ignored.
+
+   Messages sent from IMLAC to TENEX:
+
+      Short input mode:
+
+         Every character typed on the keyboard is transmitted literally.
+
+
+
+
+
+
+Deutsch                                                        [Page 13]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+      Long input mode:
+
+         Every message begins with ESC and a count of subsequent
+         characters.
+
+            Codes 40B-177B represent keyboard input.
+
+               Note that the IMLAC does not echo these characters on the
+               display.
+
+            Codes 00B-37B, except ESC, represent typed-in control
+            characters.
+
+               The present implementation allows the user to generate
+               all of these codes from the keyboard.
+
+            ESC may be followed by a keyset-mouse code or a control
+            character.
+
+               Code 00B represents an ESC typed on the keyboard.
+
+               Otherwise, a code 40B-77B and a code 100B-107B follow.
+
+                  This type of message is sent whenever the mouse
+                  buttons change or a character has been typed on the
+                  keyset, and the IMLAC cannot convert this to an
+                  ordinary character.
+
+                     The IMLAC converts recognizable keyset chords and
+                     mouse changes to characters; see SRI-ARC
+                     documentation for a full discussion of this
+                     hardware.
+
+                  The codes 40B-77B represent accumulated keyset chords.
+
+                     40B means no complete chord has been struck.
+
+                  The codes 100B-107B represent the state of the mouse
+                  buttons after a change:  a 1-bit corresponds to a
+                  depressed button.
+
+               Other codes should not appear.
+
+            All codes are followed by the (inxy) coordinates of the
+            mouse.
+
+               This means 7 or 8 characters are sent for each character
+               typed.
+
+
+
+Deutsch                                                        [Page 14]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+(V)  Division of responsibility
+
+   The first criterion in design of the system just described was to
+   allocate sufficient validity checking to the PDP-10 to make it
+   unnecessary for the IMLAC to send a response for each command.
+
+      Thus, the PDP-10 allocates and checks display area identifiers and
+      string numbers.
+
+      In the present implementation, display areas are numbered system-
+      wide whereas strings are numbered from 1 to N within a display
+      area.
+
+   The only errors not detectable by the PDP-10 are transmission parity
+   errors and overflow of the IMLAC's memory.
+
+      The former are presently not corrected, but could be handled by
+      any standard technique.
+
+      The latter are in principle detectable by the PDP-10, since the
+      amount of space required to store a given display is fairly simply
+      computable.
+
+         If the IMLAC runs out of space, it deletes lines from the top
+         of the teletype simulation display, until only three are left,
+         before giving up.
+
+   A secondary criterion was to hold down the number of characters
+   required to represent a display command.
+
+   We have found two problem areas and two areas in which we expect to
+   expand the IMLAC's capability.
+
+      Echoing was relegated to TENEX since we desired to avoid the
+      well-known complications associated with remote echoing.
+
+      The question of identifying a device as an IMLAC to TENEX gave us
+      a great deal of trouble.
+
+         We settled on the convention of a TENEX Executive command which
+         causes TENEX to send the "Long input mode" message.
+
+            The IMLAC starts out in short input mode.
+
+         The TENEX character input routines also may be set into either
+         long or short mode.
+
+            Their state is changed by the TSNDA and TSFDA system calls.
+
+
+
+Deutsch                                                        [Page 15]
+
+RFC 190         DEC PDP-10 -- IMLAC COMMUNICATION SYSTEM    13 July 1971
+
+
+            In short mode, (TSNDA last), short input is passed
+            literally, and only the character is passed from long input.
+
+            In long mode (TSFDA last), short input is padded with all-
+            zero coordinates, and long input is passed literally.
+
+            This arrangement allows TENEX programs which do not use the
+            special features of the IMLAC to operate correctly with
+            either an IMLAC or a teletype.
+
+            The user may restore the IMLAC to short mode with another
+            command or with a special key on the IMLAC keyboard.
+
+         Code is ready to allow the IMLAC to collect entire literal
+         strings with some internal editing before sending them to
+         TENEX.
+
+            We have not resolved the disposition of characters typed by
+            the user between the typed command initiating literal input
+            and the receipt by the IMLAC of the "collect literal"
+            message.
+
+         With 8K of core, the IMLAC can handle a significant fraction of
+         the command parsing and feedback functions of the SRI-ARC On-
+         Line System (NLS), for which this effort is principally
+         intended.
+
+
+             [This RFC was put into machine readable form for entry]
+             [into the online RFC archives by Lorrie Shiota, 10/01]
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Deutsch                                                        [Page 16]
+