Notes

included is a discussion of the use of multiple file viewing
display areas in NLS to provide cross file editing capabilities.
A description of our display and terminal input equipment will
be issued as a separate document.

NOTE: RFC 190 includes a functional description of the
implementation of the interface to our displays and is a
description of the way this interface was extended to include
"Processor-displays" (an IMLAC PDS-1, in this case) to our
system, thus enabling one to use Display NLS over any of our
teletype lines (including the network).

A "processor dsplay" is a display with Processing power which
can be controlled by character strings.

Description of the "conceptual display" implemented at ARC

The allocatable output unit for our display terminals (which
include our local terminals and all remote processor-displays) is

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a rectangular "display area". A program treats this display area
much like it would a file which it has opened with write access.

When requesting the allocation of a display area, a program
specifies its attributes, including where it is to be on the
screen. The program is returned an identifier which it
subsequently uses to manipulate images within the display area
and the display area itself. Each string which the program
writes into the display area is also given an identifier, which
can subsequently be used to move, delete, replace, or change the
characteristics of that string.

The currently implemented characteristics are character size,
horizontal spacing between characters, and font of the
characters (e.g. blinking, italics, intensity, etc.).

The position of items in the display area are given relative
to the 0,0, which is the lower left corner of the display
area. The horizontal coordinate increases to the right and the
vertical coordinate increases toward the top.

In addition to above described manipulation of strings within
display areas a program can suppress the display of individual
strings within display areas or suppress whole display areas.

Also, a program can switch the terminal's state from teletype
simulation to display mode and vis versa.

When in display mode, the teletype simulation display area is
suppressed and the coordinates of the cursor are input with
each character. When in teletype simulation mode, all user
owned display areas are suppressed and the coordinates of the
cursor are not input with each character.

At TENEX startup time, display areas are allocated for a teletype
simulation and a cursor for each local display terminal. Programs
can change the string being displayed as the cursor to give the
human feedback as to the programs state.

Within NLS:

The NLS subsystem deals only with the cursor and the display
areas it has requested from the system for output to the user.
The display area formatters assumes that the display has 64K
by 64K addressable points (with 0,0 at upper left), several
different character sizes and fonts, and 7-bit ASCII.

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The display area formatters use format parameters during the
format process and post-processors to convert the vertual
format to one that is acceptable to the device for which the
formatting was being done (a display area on the screen, a
page for a printer, a microfilm device, or a teletype).

NLS allows the user to specify arguments to commands by
selecting items from the current display image. This is
accomplished through the use of a data structure, which
describes the current display image, to map the cursor
coordinates, which are input with each character, into the
proper selection.

Multiple text display areas in NLS

When the user's device is a display, NLS allows him to subdivide
the file-viewing display area (the one in which he views his
file) and view (and edit across) several different files at once.
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