A Window System


                          Mitch Bradley



                            _A_B_S_T_R_A_C_T

         This simple window system can  be  used  with  most
     ASCII  terminals.  Arbitrary rectangular regions of the
     screen may be defined as windows.  Output directed to a
     window  will  be confined to that region of the screen.
     By assigning a different window to each task, tasks may
     conveniently  share  the  screen without worrying about
     each other.  The system also manages access to the key-
     board so that multiple tasks may simultaneously request
     input without confusion.



Motivation

Sometimes it is convenient  to  divide  a  terminal  screen  into
several independent regions.  For example, an editing program may
want to display the file being edited in one area and  echo  com-
mands in another area.  In a multitasking system, different tasks
may want to display output on the same screen without interfering
with  one  another.   The  "bookkeeping" necessary to prevent the
interference can be cumbersome if each task  must  take  all  the
others  into account.  This program takes care of the bookkeeping
so that each task may write to its own part of the screen without
concern for the other tasks.  It is also possible for single task
to have more than one window,  so  that  it  can,  for  instance,
accept commands in one place and display status in another.

Design Decisions

The system is designed to use a  conventional  character-oriented
ASCII  terminal;  it  does  not require a bit-mapped display.  It
does not support sophisticated graphics, pop-up/pull-down  menus,
or  mouse  input.  Such features are useful in many applications;
however, they are currently  extremely  machine-dependent.   Many
conventional  applications  can  still  benefit  from the screen-
sharing capabilities of this simple window  system  without  bit-
mapped  graphics  capability.   Most  programs  do not have to be
changed at all in order to be used within a window.

A window does not retain a  memory  copy  of  the  characters  it
displays.   Consequently  it  is  not  possible to implement true
scrolling in an arbitrary window.  For the case where a window is



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the  full width of the screen, it is possible to scroll by delet-
ing the top line in the window and inserting a blank line at  the
bottom of the window.  The window system does this.  However, the
visual effect is unpleasant if the window is not at the bottom of
the  screen,  so scrolling is only done for full width windows at
the bottom of the screen.  For other windows, a linefeed  at  the
bottom  of the window causes the output to wrap around to the top
of the window.  This behavior is a compromise between  esthetics,
speed, and simplicity of implementation.

Windows

A window is a rectangular region of the  terminal  screen.   Each
window has a screen position, a size, and a cursor postion within
the window.

A window is created with:

window: ( line0 col0  #lines #cols -- )  ( Input stream: <name> )
    Creates a new window whose  name  is  taken  from  the  input
    stream.   The  position  and size of the window is taken from
    the stack, and the cursor position within the new  window  is
    set to 0,0 (the upper left-hand corner of the window).

<name> ( -- addr )
    When the new window <name> is later executed, an  address  is
    left  on  the stack.  This address is the address of the data
    storage area where some window  variables  is  stored.   This
    address is used to refer to the window.

Each task has a current window selected by the user variable:

my-window ( -- addr )
    addr is the address of a (per-task) user variable which  con-
    tains  the  address of a window.  The window whose address is
    stored in my-window will receive the output from the task.

Each window is described by six variables.  When  a  task  refer-
ences  one of these variable names, the address left on the stack
depends on the address stored in my-window.  In other words, only
the  variables  for  the  currently-selected  window are directly
accessible.

line0
    The topmost screen line in the window.  If line0 contains  0,
    the window starts at the physical top of the screen.

col0
    The leftmost screen column in the window.  If  col0  contains
    0, the window starts at the left-hand edge of the screen.

#lines
    The number of lines in the window.  1 <= #lines <= number  of
    lines on the screen.



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#cols
    The number of columns in the window.  1 <= #cols <= number of
    columns on the screen.

#line
    The line number of the cursor position within the window.   0
    <= #line < #lines.

#out
    The column number of the cursor position within  the  window.
    0 <= #out < #cols.

Window Functions

The primary interface between a program and the window system  is
wemit.   wemit  behaves  like  emit,  except that it displays the
character only within the selected window, wrapping around if the
program  attempts  to  write  past the edge of the window.  In my
system, emit is a "defer" word, so wemit may be installed as  the
action performed by emit.

wemit ( char -- )
    char is displayed on the  currently-selected  window  at  the
    cursor  position.  The cursor is advanced to the next charac-
    ter position.

    If the cursor was already at the end of the line, a new  line
    is  selected  before displaying the character.  If the cursor
    is already at the bottom of the window, the method of select-
    ing  a  new line depends on the position and size of the win-
    dow.  If the window is not the full width of the  screen,  or
    if  the  window  is  not at the bottom of the screen, the new
    line is selected by moving to  the  top  of  the  window  and
    clearing  that  line.  If the window is full width and at the
    bottom of the screen, the new line is selected  by  scrolling
    lines in the window up to make room for the new one.

    If the character to display is not a printable ascii  charac-
    ter,  it  is  either  interpreted  as  a control character or
    displayed with a caret, e.g. ^X.  The following control char-
    acters are interpreted:

linefeed (control J)
    Advances the cursor to column 0 of the next line, and  clears
    that line.

carriage return (control M)
    Moves the cursor to column 0 of the current line.

backspace (control H)
    Moves the cursor backwards one character position, unless  it
    was  already at column 0 in the window, in which case nothing
    happens.




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tab (control I)
    Inserts enough spaces to move the cursor to the next tab stop
    within  the  window.  The variable tabstops contains a number
    (initially  8)  which  is  the  distance  between  successive
    tabstops.

formfeed (control L)
    Erases the window and  positions  the  cursor  at  the  upper
    left-hand corner.

The window system also provides some functions for cursor control
within  the  window, such as are commonly used by screen-oriented
editors, etc.

right ( -- )
    The cursor is moved one position to the right, unless  it  is
    already at the right edge of the window.

left ( -- )
    The cursor is moved one position to the left,  unless  it  is
    already at the left edge of the window.

+chars ( n -- )
    The cursor is moved n positions to the right if  n  is  posi-
    tive, or -n positions to the left if n is negative.  The cur-
    sor will not move past either edge of  the  window;  it  will
    stop at the edge if n is too large.  +chars is generally fas-
    ter than repeated calls to left or right for large  n  (where
    "large"  is  dependent on the terminal and the baud rate).  A
    general guideline is:  If you know you only want to move 1 or
    2 positions, use left or right, otherwise use +chars.

kill-line ( -- )
    The line containing the cursor within the current  window  is
    erased  (filled  with blanks) from the cursor position to the
    right edge of  the  window.   The  cursor  position  is  left
    unchanged.

erase-screen ( -- )
    The entire window is erased, and the cursor is  left  at  the
    upper left-hand corner of the window (line 0, column 0).

insert-char ( char -- )
    char is displayed on the screen at the cursor  position,  and
    the  rest  of  the  line  within  the window is moved over to
    accomodate it.  The character  that  was  previously  at  the
    rightmost  edge  of  the screen is lost.  The cursor is moved
    one position to the right, so that repeated calls to  insert-
    char will insert characters from left to right.

delete-char ( -- )
    The character at the cursor position is deleted from the win-
    dow, and the rest of the line is moved to the left to fill in
    the deleted position.  A  blank  is  placed  in  the  vacated



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    position  at  the right edge of the window.  The cursor posi-
    tion is left unchanged.

border ( -- )
    A border is drawn around the  window.   The  characters  that
    make  up the border appear in the lines and columns just out-
    side the extent of the window.  If an edge of the  window  is
    at  the  edge of the screen, the border is omitted along that
    edge.

Input

In a multitasking environment, if several tasks each  want  input
from  the same keyboard, care must be taken that only one task at
a time actually gets the keys typed.  The window  system  manages
this  by assigning the keyboard input stream to one of the tasks.
All keys typed will go to that task, unless the user  types  con-
trol  X.   When  control  X is typed, the keyboard is assigned to
another task that is requesting input.   Repetitive  control  X's
will cycle through all the tasks that want input.

Terminal Interface

To be used with a particular terminal, a number of functions must
be defined appropriately for the terminal.

(#lines ( -- n )
    n is the number of lines on the terminal's screen.

(#columns ( -- n )
    n is the number of columns on the terminal's screen.

(left ( -- )
    Moves the terminal cursor one position to the left.   If  the
    cursor  is  already  in  the leftmost column, the behavior is
    undefined.

(right ( -- )
    Moves the terminal cursor one position to the right.  If  the
    cursor  is  already  in the rightmost column, the behavior is
    undefined.

(up ( -- )
    Moves the terminal cursor up one  line.   If  the  cursor  is
    already on the topmost line, the behavior is undefined.

(down ( -- )
    Moves the terminal cursor down one line.  If  the  cursor  is
    already on the bottom line, the behavior is undefined.

(at ( line column -- )
    Moves the cursor to the indicated  line  and  column  on  the
    screen.  If line is outside the range 0 <= line < (#lines, or
    if column is outside the range 0 <= column <  (#columns,  the



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    behavior is undefined.

(insert-char ( char -- )
    char is displayed on the screen at the cursor position.   The
    rest  of the line is moved over to the right to make room for
    the inserted character.  The righmost character on  the  line
    is lost.

(delete-char ( -- )
    The character at the cursor  position  is  deleted  from  the
    screen, and the rest of the line is moved over to fill in the
    vacated position.

(kill-line ( -- )
    The current line is erased from the cursor  position  to  the
    edge of the screen.  The cursor position is left unchanged.

(kill-screen ( -- )
    The screen is erased from the cursor position to the  end  of
    the screen.  The cursor position is left unchanged.

(insert-line ( -- )
    The current line and all subsequent lines are moved  down  to
    make  room  for  a  blank line that is inserted at the cursor
    position.  The last line on the screen is lost.

(delete-line ( -- )
    The current line is deleted from the screen,  and  subsequent
    lines  are  moved  up to fill in the space.  The last line on
    the screen is filled with blanks.

(erase-screen ( -- )
    The entire screen is erased and the cursor  is  left  in  the
    upper right-hand corner.

beep ( -- )
    The bell (or whatever noisemaker the terminal has) is rung.

dark ( -- )
    Subsequent characters will be displayed in some sort of stan-
    dout  mode,  if the terminal has one.  The preferred standout
    mode is inverse video.

light ( -- )
    Subsequent characters will be displayed in  the  normal  (not
    standout) mode.

Source Code

The code for the window system is written in Forth 83.   It  will
be published later.






                         March 20, 1986