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example, the letter G is specified when a data byte
is coded C7 (hexadecimal).
When an Enter Character Mode order is detected,
the 2840 control section prepares the character
generator for operation. Then, each pair of char-
acter data bytes read from the buffer following the
order is transferred from the buffer to the data
register associated with the 2250 for which the data
is being provided. The first of these two bytes is
then transferred to the character generator, which,
in turn, transfers a series of bytes to the 2250.
These bytes contain the stroke codes (X, Y character
grid coordinates) required by the 2250 to form the
character specified by the data byte. When more
than one 2250 is in Character mode, the character
generator provides stroke bytes to each 2250 as
required. When the proper number of stroke codes
are generated for the character selected by the first
data byte in a data register, the second data byte is
transferred to the character generator, initiating the
generation of a new series of stroke bytes.
The Space code causes the generation of a signal
that initiates one character space operation by the
2250 main deflection section. Each of the other 62
legitimate character codes causes a series of coded
stroke bytes to be generated.
New line characters can appear anywhere in the
string of character data except in the position
immediately preceding or following an order; null
characters can appear anywhere in the string.
Be-
cause the null character requires a data byte, it can
be used for reserving, initializing, or filling out a
string of character bytes to satisfy a control boundary
requirement.
Characters selected at an alphameric keyboard
can be entered into a portion of the buffer area in
which the block of data is preceded by an unprotected
Character Mode order. When received by the 2840,
the character code is inserted into the buffer location
to which the cursor is assigned, replacing the code
currently in the location, and the cursor is auto-
matically assigned to the next sequential location.
A
cursor inserted into the buffer by the channel pro-
gram identifies the specific buffer byte location into
which a keyboard character can be entered. The
2840 interrogates the 2250 to determine whether an
alphameric keyboard key is depressed. If any key
other than END or CANCEL is depressed, the 2250
responds with the code of the depressed key.
The assignment of a cursor to a buffer location
does not disturb the data in that location. When a
buffer location contains a cursor, and a character is
read for display regeneration, the character in that
location is handled in a normal manner by the 2840.
When the 2840 receives the code of the ADVANCE,
BACKSPACE, or JUMP key in response to the
interrogation, the cursor is moved accordingly, but
data in the location is not changed. When a message
is completed, the user can signal the channel pro-
gram by pressing the END key. The program can
then retrieve all or part of the message from the
2840 buffer and store it in CPU main storage, either
in place of the message that was changed or modified,
or elsewhere in main storage.
The character represented by the first data byte
following an Enter Character Mode order is drawn
on the CRT display area at the coordinate at which
the beam was last positioned. Therefore, the beam
is normally moved to a desired starting location by
a blanked Graphic mode vector or point before enter-
ing Character mode. Automatic spacing circuitry
in the 2250 then controls the spacing between succes-
sive characters and between lines. Spacing can be
accomplished through alphameric keyboard operations
such as Space and Jump, or it may be accomplished
through programmed Graphic mode operations.
Control Mode Orders
Control mode orders are used to maintain and/or to
change the status of regeneration for a 2250.
2-Byte No-Op (GNOP2)
2A
80
0
7
8
15
The SM-MC byte pair constitutes the complete order.
This order causes the 2840 to reset either Character
mode or Graphic mode, if existent, and to perform
no operation with subsequent data bytes from the
buffer area until the next SM code is found at an even
address.
4-Byte No-Op (GNOP4)
N, N+l
N+2, N+3
0
2A
I
co
Available to Programmer
-1
7
8
15
This SM-MC byte pair requires any two additional
bytes to complete the four-byte order. This order
causes the 2840 to reset the last mode and to per-
form no operation (both with the last two bytes of the
order and with subsequent data bytes from the buffer
area) until the next SM code is encountered in an
even location. This order can be used as a No
2840 Operations with Attached 2250's
21
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