Compare this file to the similar file:
Show the results in this format:

.pa 1
.he 'VT (IV)'2/11/73'VT (IV)'
.ti 0
NAME		vt -- 11/20 (vt01) interface
.ti 0
.in 8
The file vt0___ provides
the interface to a PDP 11/20 which
runs both a VT01A-controlled Tektronix 611
storage display,
and a Federal Screw Works (Vocal Interface Division)
voice synthesizer.
The inter-computer interface is
a pair of DR-11C word interfaces.

Although the display has essentially only two commands,
namely "erase screen" and "display point", the 11/20 program
will draw points, lines, and arcs, and print
text on the screen.
The 11/20 can also type information
on the attached 33 TTY and generate utterances via the
voice synthesizer.

This special file operates in two basic modes, selected
by bit 2 (octal 04) on the 11/20's console switches.
If this bit is on at the opening of the file,
all bytes written on the file are interpreted as ASCII characters
and written on the screen.
The screen has 33 lines (1/2 a standard page).
The file simulates a 37 TTY: the control characters
NL, CR, BS, and TAB are interpreted correctly.
It also interprets the usual escape sequences
for forward and reverse half-line motion and for
full-line reverse.
Greek is not available yet.
Normally, when the screen is full (i.e. the 34th line is started)
the screen is erased before starting a new page.
To allow perusal of the displayed text, it is usual to
assert bit 0 of the console switches (octal 01).
As explained below, this causes the program to
pause before erasing until one of the attached pushbuttons
is depressed.

If bit 2 of the switches is down, the display is in graphic
In this case bytes written on the file
are interpreted as display and vocal
Each command consists of a single byte usually followed
by parameter bytes.
Often the parameter bytes represent
points in the plotting area.
Each point coordinate consists of 2 bytes
interpreted as a 2's complement 16-bit number.
The plotting area itself measures (+_03777)X(+_03777)
(numbers in octal);
that is, 12 bits of precision.
Attempts to plot
points outside the screen limits are ignored.

The graphic and sonic commands are:

.in +5
.ti -5
order (1); 1 parameter byte
The parameter indicates a subcommand, possibly followed
by subparameter bytes, as follows:
.in +5

.ti -5
erase (1)
The screen is erased.
This action may be delayed, as explained below,
until a pushbutton is depressed.

.ti -5
label (2); several subparameter bytes
The following bytes up to a null character
are taken as a label and typed on the console TTY.
One of the console switches gives labels
a special interpretation, as explained below.

.ti -5
display label (3); several subparameter bytes
The following bytes up to a null byte are printed
as ASCII text on the screen.
The origin of the text is the last previous point
plotted; or the upper left hand of the screen if there were none.

.in -5
.ti -5
point (2); 4 parameter bytes
The 4 parameter bytes are taken as a pair of coordinates
representing a point to be plotted.

.ti -5
line (3); 8 parameter bytes
The parameter bytes are taken as 2 pairs of coordinates
representing the ends of a line segment which is plotted.
Only the portion lying within the screen is displayed.

.ti -5
frame (4); 1 parameter byte
The parameter byte is taken as a number of sixtieths of a second;
an externally-available lead is asserted for that time.
Typically the lead is connected to
an automatic camera
which advances its film and opens the
shutter for the specified time.

.ti -5
circle (5); 6 parameter bytes
The parameter bytes are taken as a coordinate pair representing
the origin, and a word representing the radius of a circle.
That portion of the circle which lies within the screen is plotted.

.ti -5
arc (6); 12 parameter bytes
The first 4 parameter bytes are taken to be a coordinate-pair
representing the center of a circle.
The next 4 represent a coordinate-pair specifying a point
on this circle.
The last 4 should represent another point on the
An arc is drawn counter-clockwise from the first circle point
to the second.
If the two points are the same, the whole circle is drawn.
For the second point, only the smaller in magnitude
of its two coordinates is significant;
the other is used only to find the quadrant of the end of the arc.
In any event only points within the screen limits are

.ti -5
dot-line (7); at least 6 parameter bytes
The first 4 parameter bytes are taken
as a coordinate-pair representing the origin
of a dot-line.
The next byte is taken as a signed
The next byte is an unsigned word-count,
with "0" meaning "256".
The indicated
number of words is picked up.
For each bit in each word a point is plotted
which is visible if the bit is "1", invisible
if not.
High-order bits are plotted first.
Each successive point (or non-point) is offset
rightward by the given x-increment.

.ti -5
speak(8); several parameter bytes
The following bytes up to a null byte are taken
to represent phonemes which are fed to
the voice synthesizer.
vsp(VII) gives the encoding.

.in -5
The 3 low-order console switches of the 11/20
modify the operation of the display as follows.

Bit 2 (octal 04) is examined at the time the display
file is opened (more precisely, when the first byte
is written after an open); as indicated,
when on__ it selects character mode, otherwise graphic

Bit 1 (octal 02)
determines whether TTY labels are to be interpreted.
Unless this bit is on__, labels are ignored.
(except to terminate skip mode, see below).

Bit 0 (octal 01) determines
whether the display will pause before erasing
the screen;
if off___ there will be no pause.
If bit 0 is on__, the erase will occur
and displaying will resume only when one of the
16 pushbuttons is depressed.

There is a box with 16 pushbuttons connected
to the 11/20.
Their state is at all times available in the 11/45
by executing the csw___ system call (II).
They are used by the 11/20 when it is pausing
before an erase.
14 of the buttons merely serve to allow the display
to continue.
If, however, button 7
is pushed,
the display will ignore commands up to the
next erase command, then ring the TTY console's bell,
thereby skipping an entire picture.

If button 8 is depressed,
the display will ignore commands up to the next TTY label
(whether or not its typing is suppressed) before
resuming the displays.
Thus a sequence of frames may be skipped.
.in 16
.ti 0
FILES		/dev/vt0
.ti 0
SEE ALSO	csw(II), vsp(VII)
.ti 0
users using vt0 simultaneously
can interfere with each other,
e.g. plot phonemes or speak display coordinates.