            A Virtual Terminal Management Model

            RFC 782

            prepared for

            Defense Communications Agency
            WWMCCS ADP Directorate
            Command and Control Technical Center
            11440 Isaac Newton Square
            Reston, Virginia 22090

            by
            Jose Nabielsky
            Anita P. Skelton

            The MITRE Corporation
            MITRE C(3) Division
            Washington C(3) Operations
            1820 Dolley Madison Boulevard

                              TABLE OF CONTENTS

                                                                    Page

    LIST OF ILLUSTRATIONS                                             vi

    1.0  INTRODUCTION                                                  1
    1.1  The Workstation Environment                                   1
    1.2  Virtual Terminal Management                                   2
    1.3  The Scope                                                     3
    1.4  Related Work                                                  4

    2.0  THE VTM MODEL                                                 5
    2.1  The VTM Model Components                                      7
    2.2  The Virtual Terminal Model                                   10
         2.2.1  Virtual Terminal Connectivity                         11
         2.2.2  Virtual Terminal Organization                         11
                2.2.2.1  The Virtual Keys                             12
                2.2.2.2  The Virtual Controller                       12
                2.2.2.3  The Virtual Display                          12
         2.2.3  Virtual Terminal Architecture                         13
                2.2.3.1  Communication Variables                      13
                2.2.3.2  Virtual Display with File Extension          13
                2.2.3.3  Virtual Display Windows                      14
    2.3  The Workstation Model                                        17
         2.3.1  The Adaptation Unit                                   17
         2.3.2  The Executive                                         18

    REFERENCES                                                        19

                                     iii

                            LIST OF ILLUSTRATIONS

                                                                    Page

    Figure Number

         2.1       The Virtual Terminal Model                          7
         2.2       The Workstation Model                               8
         2.3       VT 0 (expanded from previous figure)                9
         2.4       The Domains                                        14

                                      v

    1.0  INTRODUCTION

         Recent advances in micro-electronics have brought us to the  age
    of the inexpensive, yet powerful, microprocessor.  Closely resembling
    the advances of the 1960's which brought about  the  transition  from
    batch  processing  to time-sharing, this technological trend suggests
    the birth of decentralized architectures where the  processing  power
    is  shifted  closer  to  the user in the form of intelligent personal
    workstations.  The virtual terminal model described in this  document
    caters to this anticipated personal computing environment.

    1.1  The Workstation Environment

         A personal workstation is a computing engine which  consists  of
    hardware  and  software dedicated to serve a single user.  As part of
    its architecture, the workstation can invoke the resources of  other,
    physically  separate  components, effectively extending this personal
    environment well beyond the bounds of the single workstation.

         In this personal environment,  processing  resources  previously
    shared  among  multiple  users  now become dedicated to a single one,
    with a large part of these resources summoned to provide an effective
    human-machine  interface.   As a consequence, modalities of input and
    output that were unfeasible under the time-shared regime now become a
    part of a conversational language  between user and workstation.  Due
    to the availability of processing cycles, and the  closeness  of  the
    user devices to these cycles, the workstation can support interactive
    devices, and dialogue modes using these devices, which could  not  be
    afforded before.

         The workstation can provide the  user  with  the  mechanisms  to
    conduct  several  concurrent  conversations  with user-agents located
    elsewhere in the global architecture.   One  such  mechanism  is  the
    partitioning  of  the  workstation  physical  display  into  multiple
    logical  displays,  with  one  or  more  of  these  logical  displays
    providing a dedicated workspace between user and agent.

         The nature of the conversations on these logical  displays  need
    not  be  limited  to  conventional  alphanumeric  input  and  output.
    Conversations using input tools  such  as  positioning  and  pointing
    devices  (e.g.,  mouse,  tablet, and such), and using high-resolution
    graphics objects for output (e.g., line drawings, raster  blocks  and
    images,  possibly  intermixed with text) should be possible on one or
    more of these screens.

         Moreover, as long as the technological trend  continues  in  its
    predicted  path,  one can postulate a workstation which could support
    by the mid 1980's multi-media conversations using  voice  and  video,

                                      1

    synchronized   with  text  and  graphics.   At  present,  multi-media
    information   management   (i.e.,   acquisition,   processing,    and
    dissemination)  is  an  active  research area, but eventually it will
    become an engineering problem which, when  solved,  will  add  a  new
    dimension  to  already feasible modes of interaction between user and
    workstation.

    1.2  Virtual Terminal Management

         All virtual terminal protocols  (VTPs)  provide  a  vehicle  for
    device-independent,     bi-directional,     8-bit    byte    oriented
    communications between two VTP users.  Most Vo so by invoking  a
    device abstraction of real terminals, called a virtual terminal.

         As with a real device, a virtual  terminal  has  a  well-defined
    architecture  with  its  own character sets and functions. A VTP uses
    the architectural features of  the  virtual  terminal  to  provide  a
    common  language,  an  intermediate  representation,  between its two
    communicating entities.  However a  VTP  user  does  not  communicate
    directly  with  this  virtual  terminal.   A function of a VTP is the
    local mapping between the site-specific order codes and  the  virtual
    terminal  domain,  thus allowing this adaptation to be transparent to
    the VTP users.

         The model of a personal workstation as a dedicated  device  with
    considerable   resources    affects  the  way  we  conceptualize  the
    architecture of virtual terminals,  both  in  breadth  and  depth  of
    function.   It also affects the way we view the virtual terminal vis-
    a-vis its local correspondents, the personal  workstations,  and  its
    remote correspondents, the other virtual terminals.

         This document presents a radical view of  virtual  terminals  as
    resource  sharing  devices.   The  classical  concept  of  a  virtual
    terminal as a two-way device with a  limited  architecture  has  been
    dismissed.   Instead,  we  view a virtual terminal as an n-way device
    with multiple correspondents sharing access to its virtual "keyboard"
    and  "display."  In  this  model, a virtual terminal has two kinds of
    correspondents:  adaptation units, and other virtual terminals.   The
    adaptation  units  serve  as  interface  agents  between  the virtual
    terminal and its users, providing the step transformation between the
    user-specific   order   codes  and  the  virtual  terminal  interface
    language.  In turn,  the  other  virtual  terminals  are  cooperating
    co-equals  of the  virtual  terminal, interacting with it to maintain
    global control and data store synchrony. Resembling the administrator
    of  a  local  copy  of  a distributed data base, the virtual terminal
    interacts with the other virtual  terminals  (the  remote  data  base
    managers)  and  with  the  local  adaptation  units  (the  data  base
    transformers) to provide read, write, and modify access to its  local

                                      2

    data  store  (the  local  copy  of  the distributed data base), while
    providing concurrency control to maintain a "single user  view"  when
    so desired.

         To communicate with its correspondents, a virtual terminal  uses
    two virtual languages. In the case where the correspondent is another
    virtual terminal, it  uses  the  language  of  the  virtual  terminal
    protocol;  in the case where the correspondent is an adaptation unit,
    it uses an interface language closer to the physical architecture  of
    the end-user, but a virtual language nevertheless.

         In essence, the virtual terminal has become a device in its  own
    right,  free  from  a  single physical realization and also dedicated
    ownership. As a result, a single workstation not only may request any
    number  of  virtual  terminals,  but  a  number  of  workstations may
    share -- and interact with -- a particular virtual terminal.

         The functional breadth of virtual terminals has  been  augmented
    by  the  concept  of  virtual  terminal  classes.   Each  class is an
    abstraction of a particular device architecture.  There  are  stream,
    line,  logical  page,  physical page, and graphics virtual terminals,
    all made up of:  a class-constrained data structure and its attendant
    operations  (the virtual display); a general controlling element (the
    virtual controller); and an input selector (the virtual keys).

         Finally, the functional depth of the virtual terminal  has  been
    extended  by  architectural  features  previously  unavailable.   The
    virtual terminal becomes a  multi-user  device  with  a  non-volatile
    virtual  display available for selective viewing.  These concepts are
    discussed is some detail in the chapter that follows.

    1.3  The Scope

         An overview of the virtual terminal model and the management  of
    communicating  virtual  terminals  is  presented.   A detailed design
    description  of  the  data  structures  and  accompanying  addressing
    functions  has been completed.  The operations and control mechanisms
    are less complete.  Before  the  design  is  solidified,  an  initial
    mimimal implementation will be made to validate the model.

         This document represents work in progress; current international
    interest  in  virtual  terminal  protocols has motivated us to submit
    this as an example of  mechanisms  that  a  virtual  terminal  should
    support.   The  model  provides a framework for supporting device and
    processing  capabilities  not  yet  commonly  available.   A  virtual
    terminal  protocol standardization effort may not want to include all
    the mechanisms that are described here, but it is our contention that
    one should not preclude these extensions for the future.

                                      3

    1.4  Related Work

         The concepts presented in this document  are  the  offspring  of
    previous  work  in  the  area  of  personal  computing,  and  of user
    interfaces to (distributed) systems.  The bibliography at the end  of
    the  document  collects  this  material.  In  particular,  we want to
    acknowledge the work done at the University of Rochester  on  virtual
    terminals,(6)   work  which  has  influenced to a large degree how we
    view user interfaces through a display.

                                      4

    2.0  THE VTM MODEL

         This section describes a virtual terminal management (VTM) model
    whose  architecture  not  only  derives  from  a  quest  for  device-
    independent, terminal-oriented communications, but  more  importantly
    from a desire to provide effective human-machine interfaces.

         The VTM architecture  is  a  multi-user  structure  which  spans
    several  building blocks. The underlying foundation to this structure
    is provided by the cooperating  virtual  terminals.   Under  the  VTM
    model,  these  cooperating  virtual  terminals  are  viewed as device
    abstractions, all with  a  common  architecture,  exchanging  virtual
    terminal  protocol  items  to  update each other's view of the world.
    Resting on this foundation lie the adaptation units.  Associated with
    a   single   end-user,   an   adaptation   unit   provides  the  step
    transformation between user and virtual  domains.   In  a  sense  the
    adaptation  unit  is  also  a virtual terminal, although one which is
    much closer to the architecture of the end-user.  Finally, on top  of
    this  supporting  structure  are  the  end-users, the application and
    human processes, all interacting towards a common goal.

         Before embarking on a description of the VTM  model  components,
    we  present  the  set of capabilities the VTM model provides its end-
    users, either human or application.  After all,  the  motivation  for
    the  model  and  its  underlying  concepts  stems  from our desire to
    provide productive user environments.

         HUMAN  <--->  WORKSTATION

         o   Multiplexing the workstation physical display both  in  time
             and space.

             The workstation assigns to each user conversation a  logical
             terminal  with  a well-distinguished logical display.  Under
             the  user  control,  the  workstation  maps  these   logical
             displays  on  non-overlapping areas of the physical display,
             providing   a   dedicated   workspace   between   user   and
             correspondents.   Limited  only  by the area of the display,
             many logical displays could be  mapped  at  one  time,  each
             providing  display updates when so required.  Since the area
             of the  display  is  a  scarce  resource,  not  all  logical
             displays  need  be  mapped at the same time.  Therefore, the
             workstation may roll-out and roll-in selected displays under
             the  user  control,  thereby  also multiplexing the physical
             display in time.

         o   Multiplexing the workstation input devices in time.

                                      5

             The input devices always map to a single  user  conversation
             (i.e.,  a  single  logical terminal).  However, the user can
             select  a  new  logical  terminal   by   some   well-defined
             interaction  (e.g.,  depressing  a  function  key,  using  a
             pointing  device,  and  such),  effectively  switching   the
             ownership of the input tools.

         o   Concurrent multi-mode use of the workstation.

             The capabilities of the  workstation  limit  the  scope  and
             character   of   the   individual   conversations.   If  the
             workstation  supports   rubout   processing   (i.e.,   erase
             operations  on  lines  and  characters),  then  the  logical
             terminals can be independent,  scrolling  "terminals,"  some
             page-oriented, others line-oriented.  If the architecture of
             the  workstation  supports  graphics  objects  as  primitive
             objects  then so can the individual logical terminals.  As a
             consequence, while some logical  terminal  displays  may  be
             dedicated  to alphanumeric output, others may include raster
             graphics and imaging data together with positioned text.

         o   The sharing of  a  single  logical  terminal  among  several
             users.

             Several end-users may link to  a  single  logical  terminal.
             All linked parties are viewed by the shared "device" as both
             input sources and  output  sinks.   As  a  consequence  this
             device  sharing  need  not be limited only to the sharing of
             device output. In general, each linked party may  have  full
             read  and  write  access  to  the logical terminal, if it so
             desires.

         o   Selective viewing on a logical terminal display.

             In the user's view, a logical terminal display  is  a  user-
             specified  window  on  a  potentially  larger structure, the
             "device"  display.   This  window  provides  the  "peephole"
             through  which the device display is viewed.  The portion of
             the device display mapped on this window is not  limited  to
             its   "present   contents."  Under  the  user  control,  the
             workstation may invoke the viewing of  past  activity  on  a
             logical  terminal  display  when  the  device display is I/O
             file-extended.  Since the window mechanism  is  an  integral
             part  of  the  device  architecture,  it is available on all
             logical terminal displays.  Furthermore, the viewing of past
             activity  does  not  affect  others  sharing  access  to the
             device.

                                      6

         o   Discarding, suspending, and resuming the output of a logical
             terminal always under user control.

             As part of the  user  interface,  the  workstation  provides
             simple  "keys" through which the user controls the output on
             a logical terminal display.  These workstation  "keys"  need
             not  be  physical  keys, but could be other input tools used
             for this purpose (e.g., analog dials, hit-sensitive areas on
             the  physical display, and such).  In any event, through the
             auspices of the workstation,  the  user's  control  requests
             translate   into   the   proper  commands  to  the  "device"
             associated with the logical terminal.

         APPLICATION  <--->  ADAPTATION UNIT

         o   A logical view of real devices.

             For  each  real   terminal   architecture,   one   canonical
             representation:  a logical device.

         o   For  a   particular   logical   device,   several   possible
             interaction paradigms.

             Some logical devices are intrinsically half-duplex (e.g.,  a
             page-oriented  logical  device), some are full-duplex (e.g.,
             communicating  processes  using  a  stream-oriented  logical
             device), and some may be either half or full-duplex (e.g., a
             line-oriented logical  device).   Some  full-duplex  logical
             devices  can  provide  no  echoing, remote echoing, or local
             echoing.   Those  that  interface  with  applications   that
             support command completion (e.g., command-line interpreters)
             can shift the locus of echoing as a function  of  a  dynamic
             break character set.

         o   One application communicating with several logical devices.

             As  part  of  an  application's  model  of  interaction,  an
             application may "own" several logical devices.  For example,
             an editor could use a line-oriented logical device to gather
             top-level  commands,  and  a page-oriented logical device to
             provide editing workspace.

    2.1  The VTM Model Components

         The virtual terminal management  model  consists  of  two  major
    components:   the  virtual  terminal model, and the workstation model
    (see Figures 2.1, 2.2, and 2.3 respectively).

                                      7

                                  AU1
                                   |
                             AU0   |    AU2
                              |    |     |
                             _______________
                             |             |
                             |     VT2     |
                             |             |
                             |             |
                             _______________
                                    |       _______________
                                    |       |             |----AU0
                                    |_______|     VT0     |
                                    |_______|             |
                                    |       |             |----AU1
                                    |       _______________
                                    |
                             ________________
                             |              |
                             |              |
                             |     VT1      |
                             |              |
                             ________________
                              |     |     |
                             AU0    |    AU2
                                    |
                                   AU1

    VT = VIRTUAL TERMINAL
    AU = ADAPTATION UNIT

                   FIGURE 2.1 - THE VIRTUAL TERMINAL MODEL

                                      8

                        ___  ___               ___  ___
                       |VT1||VT2|             |VT1||VT2|
                       ____ _____             _____ ____
                        |     |                 |    |
                      __|_____|_________________|_____|__
                      | |     | |             | |     |  |
                      |  REMOTE | -CONTROLLER-|  REMOTE  |
                      |   KEYS  |             | DISPLAYS |
                      |         |             |          |
                      | VIRTUAL |             |   DATA   |
                      |   KEYS  |             |  STORE   |
                      |         |<----------->|          |
                      |  LOCAL  |             |   LOCAL  |
                      |   KEYS  |             | DISPLAYS |
                      |         |             |          |
                      __|_____|__________________|_____|__
                        |     |                  |     |
                       ____ ____               _____ ____
                      |AU0||AU1|               |AU0||AU1|
                       ____ ____               _____ ____

              FIGURE 2.2 -- VT0 (expanded from previous figure)

                                      9

                                  +--------------------+
                                  |                    |
                                o-|-------------------|
                                  |     EXECUTIVE      |
                                  |--------------------|
       Screen        +-------+  o-|--------------------|      +-----+
    +---------+     /|OUTPUT |    |  ADAPTATION UNIT 0 |<---->| VT0 |
    |EXECUTIVE|    / |       |<---|--------------------|      +-----+
    |---------|   /  |HANDLER|  o-|--------------------|      +-----+
    |   AU0   |  /   |-------|    |  ADAPTATION UNIT 1 |<---->| VT1 |
    |---------| /    | INPUT |    |--------------------|      +-----+
    |   AU1   |/     |       |  o-|--------------------|
    |---------|      |HANDLER|    |         .          |
    |         |      |    /--|o   |         .          |
    ~         ~      +-------+   ~         .          ~
    ~         ~         /        ~                    ~
    |---------|        /        o-|--------------------|      +-----+
    |   AUK   |       /           |  ADAPTATION UNIT K |<---->| VTK |
    +---------+      /            +--------------------+      +-----+
                    /             |                    |
    +---------+    /              +--------------------+
    |Keyboard |   /
    +---------+  /
    |[] [] [] | /
    |[] [] [] |/
    +---------+

                     FIGURE 2.3 - THE WORKSTATION MODEL

    The first component embodies the canonical device, while  the  second
    component   includes   the   adaptation   unit   and  its  associated
    environment.  Each component will be described in turn below.

    2.2  The Virtual Terminal Model

         The objective of virtual terminal protocols is  to  provide  the
    users  of  the service with a common, logical view of terminals.  The
    common user  view  is  attained  through  a  standard,  protocol-wide
    representation  of  a canonical terminal, the virtual terminal.  This

                                     10

    permits the exchanges between users of the protocol  to  be  free  of
    device-specific encodings.

         The design postulates an integrated virtual terminal model which
    extends  the  nature  and  scope  of this canonical device in several
    important ways.  The major aspects of the  model,  its  connectivity,
    its organization, and its architecture are described below.

         2.2.1  Virtual Terminal Connectivity

         Most virtual terminal protocols only cater to two-way  dialogues
    in  which  a  single  virtual  terminal  terminates  each  end of the
    communication path.

         We define the virtual terminal as a n-way device  where  one  or
    more  of  the  correspondents  to  this device are local users of the
    service, and the remaining correspondents (if any) are  peer  virtual
    terminals.   Each  correspondent  to the virtual terminal has its own
    bi-directional path to produce virtual input to, and receive  virtual
    output from, the virtual terminal.  This bi-directional path provides
    the vehicle for a virtual terminal session between user  and  virtual
    terminal.   Globally, the cooperating virtual terminals and these bi-
    directional paths span a dendritic (tree-like) topology.

         It is important to note  that  we  have  decoupled  the  virtual
    terminal  from  its  physical  realization,  a  single real terminal.
    Indeed, a virtual terminal does not map necessarily to just one  real
    device, but possibly to many real devices.

         The virtual terminal is viewed ultimately as a well-defined data
    structure  which  provides  its  correspondents  with a non-dedicated
    virtual terminal service.  And these  correspondents  may  have  read
    only, write only, or read/write access rights to this data structure.

         2.2.2  Virtual Terminal Organization

         The virtual terminal is an abstraction;  its  organization,  the
    building  blocks which make up the virtual terminal, is the result of
    a feature extraction of the real terminal  that  it  is  tailored  to
    support.

         We have conceptualized the virtual terminal as  a  meta-terminal
    (i.e.,  the terminal of terminals).  The meta-terminal is composed of
    three well-distinguished building  blocks: virtual  keys,  a  virtual
    controller, and a virtual display.

                                     11

         2.2.2.1  The Virtual Keys.  The analog of the  virtual  keys  is
    the  physical keyboard of real terminals.  However, while the keys of
    a physical terminal are controlled by a single manual process,  these
    virtual  keys  can be activated by multiple, concurrent entities (the
    virtual terminal correspondents).  Each correspondent of the  virtual
    terminal, be it a user of the service or a peer virtual terminal, has
    its input stream to the meta-terminal terminated at the virtual keys.
    The  virtual  keys  provide the control of access of input streams to
    the meta-terminal.

         2.2.2.2    The Virtual Controller.    The   virtual   controller
    provides   virtual  terminal  session  management.   It  manages  the
    establishment and termination of a virtual terminal  session  with  a
    correspondent; supports the possible negotiation and renegotiation of
    the session  attributes;  and  enables  the  deactivation  and  later
    activation  of  the  session.   The  virtual controller also provides
    virtual terminal  signalling  control  by  managing  the  out-of-band
    signals addressed to the virtual terminal.

         2.2.2.3   The Virtual Display.   The  virtual  display  is   the
    dynamic  component in the meta-terminal organization.  For each class
    of  real  device  (e.g.  stream,  line,  page,  or  graphics-oriented
    devices)  there  is  a  corresponding  virtual  terminal  class.  The
    organization  of  the  virtual  terminal  data  structure  is  class-
    specific.  A virtual terminal models a particular terminal class when
    it is 'fitted' with the proper  data  structure  manager  or  virtual
    display.   This  binding  need  not  be  static  (e.g.,  a line-class
    specialist, and so forth), but could be result of decisions  made  at
    "run-time" by applying the principle of negotiated options.

         The virtual display manages the data structure  associated  with
    the  meta-terminal  and  performs  operations on the control and data
    elements  of  the  structure.  As  a  direct  consequence  of   these
    operations  on  the meta-terminal data structure, the virtual display
    may  generate  display  updates  to  one,  some,  or   all   of   the
    correspondents.  All virtual terminal output streams originate at the
    virtual display.

         Different virtual terminal  classes  are  spawned  by  different
    "kinds" of virtual displays, and this is realized in one of two ways.
    For character-oriented virtual devices,  it  is  possible  to  use  a
    single,  wide-scoped  virtual  display with a character-oriented data
    structure by constraining it to conform to the model  of  the  device
    class (e.g., line-oriented devices must be constrained to line-access
    rules).  For non character-oriented virtual devices  (e.g.,  graphics
    devices),  an  altogether different virtual display must be used with

                                     12

    properties better suited for the new domain (e.g., a graphics virtual
    display based on a structured display file).

         2.2.3  Virtual Terminal Architecture

         The commands, and associated parameters, which are available  to
    the  users  of  the  virtual terminal constitute the virtual terminal
    architecture.  The commands available to a user  --  to  request  the
    virtual  controller  to  establish,  abort,  or  close a session, and
    discard, suspend, or resume output -- remain invariant to the virtual
    terminal  class.  However, as one would expect, the user interface to
    the virtual display depends on the nature of this data structure.

         Three important architectural features of the meta-terminal are:
    the concept of communication variables, the notion of a file-extended
    virtual display, and the concept of virtual display windows. Each  of
    these  concepts  are a part of the meta-terminal architecture because
    they are apparent to the users of the virtual terminal.

         2.2.3.1  Communication Variables.  Each component of  the  meta-
    terminal  (i.e.,  virtual  keys,  controller,  display) is assigned a
    standard, protocol-wide name which we call a communication  variable.
    The communication variable is a part of the header of each command to
    the  virtual  terminal  (i.e.  protocol  item).   It  permits  better
    management  of  the  virtual  terminal  command  name space, and also
    provides the virtual keys  with  an  easy  mechanism  to  select  the
    destination  of  the  request.   It must be noted that nothing in the
    model precludes the addition of more virtual entities  to  the  meta-
    terminal,  such  as auxiliary virtual devices and signalling devices.
    The use of communication variables provides a naming hierarchy  which
    alleviates   the  problems  of  device  selection  and  command  name
    allocation in the case of such extensions.

         2.2.3.2    Virtual Display with File Extension.    The   virtual
    display is the immediate manager of the meta-terminal data structure.
    When the virtual display is provided with an I/O file  extension,  it
    is   possible  to  introduce  the  concept  of  a  stable-store  data
    structure, a data structure whose  contents  are  stored  in  backing
    store  (e.g.,  disk).   If  the virtual display is provided with this
    file  extension  capability  (a  local  option  with  no   end-to-end
    significance),  then  the  meta-terminal  data structure inherits the
    spatial and temporal attributes (dimensions and time-to-live) of  the
    associated file.  Such a virtual display, coupled with the concept of
    virtual display windows below, provides the users of the service with
    a very powerful tool.

                                     13

         2.2.3.3  Virtual Display Windows.  To communicate with a virtual
    terminal,  each  real device uses an adaptation unit as its interface
    entity (this adaptation unit is a part of the workstation model,  see
    section  2.3).  What is important to note is that the adaptation unit
    provides the  transition  between  the  device-specific  domain,  the
    device workspace,  and  the virtual domain, the master workspace (see
    Figure 2.4).

                                     14

     |                                 |                                   |
     |        VIRTUAL TERMINAL         |         ADAPTATION UNIT           |
     |<------------------------------->|<--------------------------------->|
     |             DOMAIN              |              DOMAIN               |
     |                                 |                                   |

     + - - - - - - - - - +   + - - - - - - - - - +        - - - - - - - - -
     |  +--->  x(m)      |   |                   |       /                /|
     |  |                |   |            x(i)   |      /                / |
     |  v  y(m)          |   | +---------------> |      - - - - - - - - -  |
     |                   |   | |              |  |     | +------------+ |  |
     | +--------------+  |   | |              |  |     | | VIEWPORT 1 | |  |
     | |              |  |   | |              |  |     | |            | |  |
     | |              |  |   | |              |  |     | |            | |  |
     | |              |  |   | |              |  |     | |            | |  |
     | |              |  |   | |              |  |     | |            | |  |
     | |              |  |   | |   A<---------|--|-----|-|->A         | |  |
     | |              |  |   | |  / \         |  |     | |            | |  |
     | |     <--------|--|---|-|->   \        |  |     | |            | |  |
     | |    /         |  |   | |      \       |  |     | |        <---|-|--|+
     | |    A         |  |   | |       \      |  |     | +------------+ |  ||
     | |              |  |   | |        \     |  |     |                |  ||
     | |     WINDOW   |  |   | |         \    |  |     | +------------+ |  ||
     | |              |  |   | |          \   |  |     | | VIEWPORT 2 | |  ||
     | |              |  |   | |-----------\--+  |     | |            | |  ||
     | |              |  |   | |            \    |     | |            | |  ||
     | +--------------+  |   | v  y(i)       \   |     | +------------+ |  ||
     |                   |   |                \  |     |                | / |
     |                   |   |                 \ |     |                |   |
     |                   |   |                  \|      - - - - - - - -     |
     |     /             |   |       /           |            |  |          |
     + - -/- - - - - - - +   + - - -/- - - - - - +\           |  |          |
         /                         /               \     - - - - - - - -    |
        /                         /                 \   |    KEYBOARD   |   |
      MASTER WORKSPACE         INSTANCE WORKSPACE    \  + - - - - - - - +   |
                                                      <-/   []  []  [] /|   |
                                                       /   []  []  [] / |   |
                                                      + - - - - - - - - +   |
                                                                            |
                                                PHYSICAL DEVICE WORKSPACE --+

                               FIGURE 2.4 -- THE DOMAINS

                                     15

    However  a  device  need  not  be  interested  in  the  whole  master
    workspace,  only  in  a  portion  of  it.   As  part  of  its session
    attributes, each adaptation unit has a window, a  rectangular  region
    in  the  virtual  display, which delimits its area of interest in the
    master.  This portion of the master domain will be  referred  as  the
    instance workspace.   Then,  for  each  adaptation  unit, there is an
    instance workspace whose spatial attributes (dimension  and  position
    within the master) are those of its window definition.

         All adaptation  units  communicate  with  the  virtual  terminal
    "relative"  to  their  own instance workspace.  As far as the virtual
    terminal is concerned,  each  instance  workspace  defines  a  "real"
    terminal,  although in fact it is just an intermediate representation
    of the real device.   In  essence,  the  instance  workspace  is  the
    coordinate  space  where  both  virtual  terminal and adaptation unit
    rendezvous. (See section 2.3 for a discussion of  how  this  instance
    workspace is mapped onto the device workspace).

         The window dimensions are the exclusive choice of the adaptation
    unit  that  owns  it.   With  these  dimensions  the  adaptation unit
    specifies to the virtual terminal how much of the  master  is  to  be
    viewed; data  elements  not  contained  within  the boundaries of the
    window are clipped.  Varying the dimension of the window  results  in
    corresponding changes on the amount of the master that is viewed.

         In contrast, the position of the window on the master might  not
    be  under  direct  control of the adaptation unit.  To understand the
    dynamics of a window, we introduce the notion of a master cursor  and
    an instance cursor.  The master cursor is a read/write pointer, which
    is a part of the virtual display architecture.  In turn, the instance
    cursor  is a pointer owned by the adaptation unit, which is a part of
    the state information maintained by the virtual  display.   Normally,
    both master and instance cursors are bound together so that motion of
    one cursor translates into an equivalent motion  of  the  other.   As
    long  as  the adaptation unit does not explicitly unbind its instance
    cursor from the master cursor, the active region of the master (i.e.,
    the position where the master cursor lies) is guaranteed to be always
    within the instance  space,  and  thus  viewable.   This  means  that
    certain  operations  on  the virtual display will implicitly relocate
    the window of an adaptation unit within  the  bounds  of  the  master
    workspace  to  insure the tracking of the master cursor.  (The actual
    algorithm which enforces  this  tracking  rule,  called  the  viewing
    algorithm,  has  not  been included here.)  This window relocation is

                                     16

    viewed  at  the  real  terminal  as  either  vertical  or  horizontal
    scrolling.

         However, an adaptation unit has the choice to bypass  this  rule
    by detaching its instance cursor from the master, effectively getting
    complete control of its cursor to view other portions of  the  master
    space.   If  the  virtual display has an I/O file extension, then the
    adaptation unit can pan its window on the  file-extended  space  well
    beyond  the  present  contents of the master space.  Therein lies the
    power of a stable-store data structure when coupled with the  concept
    of windowing.

    2.3  The Workstation Model

         The workstation model is composed  of  one  or  more  adaptation
    units,  and  a workstation monitor, which we will call the executive.
    Each will be  described  in  turn  below.   In  addition,  the  model
    includes  input  and output handlers, and an underlying multi-tasking
    operating system of unspecified architecture.

         2.3.1  The Adaptation Unit

         An adaptation unit embodies an instance of a  virtual  terminal,
    and  since  the  workstation model postulates possibly many different
    such  instances  per  physical  workstation,  then  potentially  many
    adaptation units will be co-located at a workstation.

         The adaptation unit can be viewed as the workstation agent which
    provides the mapping between instance workspace and device workspace.
    To define this mapping, we introduce the notion of a  viewport  as  a
    rectangular  area of the physical screen allocated for the viewing of
    a virtual terminal instance.  An adaptation  unit  has  the  task  of
    mapping  the  totality of the instance workspace onto the viewport, a
    mapping which is a device-specific concern totally removed  from  the
    domain  of  discourse  of the virtual terminal.  Thus the position of
    the viewport determines the relocation of the selected data structure
    elements   on  the  viewing  unit,  and  the  viewport  dimensions  a
    (potential) scaling transformation.

         The adaptation unit also produces virtual input to  the  virtual
    terminal   by  translating  the  user  input  into  virtual  terminal
    commands.  It implements the service side of  the  interface  to  the
    virtual terminal.

                                     17

         2.3.2  The Executive

         This conceptual entity performs the task and resource management
    required to create and destroy virtual terminal instances, and to map
    these virtual terminal instances to the screen viewports.

         It must provide at least a minimal  user  command  interface  so
    that  its  tools may be accessed (one of them being the management of
    screen real estate).

         Finally, the executive provides the mechanism for  the  end-user
    to  switch  viewport  contexts  through  the use of some input device
    (e.g., function key, pointing or positioning  device).   Following  a
    user  interaction  which indicates a change of context, the executive
    makes the newly selected  virtual  terminal  instance  the  dedicated
    owner of the input devices.

                                     18

                                 REFERENCES

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    2.   Alan Branden, et al.  "Lisp Machine Project Report,"  Artificial
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    3.   John Day.  "TELNET Data Entry  Terminal  Option,"  ARPA  Network
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    4.   Douglas Gerhart and D. L. Parnas.  WINDOW  A  formally specified
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    8.   Allen Newell, Scott  Fahlman,  Bob  Sproull.   "A  Proposal  for
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    9.   "PERQ,"  Three  Rivers  Computer  Corp.,  160  N.   Craig   St.,
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    10.  Jon  Postel  and  Dave  Crocker.   "TELNET   Remote   Controlled
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                                     19

    11.  John F. Shoch and Jon A. Hupp.  "Notes on the 'Worm'  programs -
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                                     20