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Data Representation
All data is transmitted from the processor to the display
station buffer in EBCDIC or the appli'cable character set
(see Figure 2-2). The 64 EBCDIC characters include the
upper case alphanumeric characters, space, special graphic
symbols. Characters transmitted from the processor that
are "outside" the 64-character EBCDIC subset are accepted,
converted to a NULL, and displayed on the screen as a
blank.
When a character is transferred to the buffer ...
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11000001
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Figure 2-2. Data representation
Keyboard Operations
the buffer character comes onto the
screen during the next regeneration
from the buffer.
The keyboard enables the user to change, edit, or create
character displays except within fields protected from
keyboard operations by the program. As messages are being
composed or modified by keyboard operations, the changes
are inserted in the buffer and displayed on the subsequent
display fresh cycle.
When the user completes an operation and presses one
of the interrupt request keys, for example the ENTER key,
an interruption informs the program; the program may then
read the modified data fields from the display buffer.
2-2
GA34-0026
Cursor
A special symbol (that resembles an underscore), called a
cursor (Figure 2-3), is displayed beneath a character or
character position on the display screen to indicate where
the next character entered from the keyboard will be stored
(Figure 2-4). For example, when the cursor is displayed
under one character in a line of characters, that character
can be changed or deleted by keyboard action unless it is
a protected character. Also, if the cursor is displayed under
a position without a display character, a character can be
inserted in that position by keyboard action unless it is a
protected space character. However, when the cursor appears
beneath a protected character or a protected space character
position, that position cannot be modified by keyboard
action. Attempting to do so causes the cursor to be moved
one position to the right.
One, and only one, cursor is capable of being displayed
on the screen. When the display is turned on, the cursor is
automatically generated and displayed in the first location
on the screen (Figure 2-3), following a command from the
application program to unlock the keyboard. A power up
of the display station causes an interrupt to the application
program. The cursor can be repositioned by the keyboard
operator and also by the program. Following a depression
of any interrupt key, the cursor is blanked as an indication
to the operator that the keyboard is locked. A command
from the application program is required to unlock the
keyboard and allow the cursor to reappear. The cursor is
flashed during Insert Mode operation (see Insert Mode).
Figure 2-3. Cursor
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