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        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





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                                 19





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