IBM 3164 ASCII Manual page 149

Chapter 5. Programming considerations
5-62
A modified data tag (MDT) bit is used to identify a modified field during
the send operation. Fields that you modify will have a MDT bit standing of 1
(on) in the field attribute character.
The MDT bit will be set to 1 (on) under the following conditions:
1. When the host program transmits the field attribute with the MDT bit set
to 1 (on).
2. When you key- in a data character into a field.
3. When you use the Delete key/command or the Erase EOF key/command
to delete characters.
4. When you use the Del Ln key/command to delete a line.
5. When you use the Ins Ln key/command to insert a line.
The MDT bit will be set to 0 (off) (that is, the field becomes unmodified)
under the following conditions:
1. When the host program transmits the field attribute with the MDT bit set
to 0 (off).
2. When you use the Er Inp key/command.
3. When you have completed the send or read operation successfully in the
"modified field data transmission" mode. (Refer to page 5-5 for an
explanation on this mode.)
A right justified with zero or space field is a field in which all data
characters (including the space characters) in a field are right-adjusted when
~
a cursor moves out of that field; that is, there is no null character between the
~
last valid character and the next field attribute. The empty area on the left side
of the data cha'racters is automatically filled with zero or space characters
according to the specified attribute. When the cursor moves after completion
of the send operation, this function will be ignored.
A must enter field is a field in which the MDT bit must be set to 1 (on)
before you transmit any data from the page. This means that you must modify
that field specified as a 'must enter' field before performing the send
operation, The display station performs a must-enter validation test when an
operator enter action tries to initiate an inbound data transmission. The
validation test searches the page, starting at the top, for an unprotected,
must-enter field that has its MDT bit set to 0 (off).
• If such field is found, the cursor is moved to the first character position in
that field, no transmission occurs, and the 'must enter' condition is raised
so that the operator can modify the field and repeat the enter action; the
display station then repeats the must-enter validation test.
• If no such field is found, the inbound data transmission is allowed to take
place. The must-enter validation test allows transmission to take place
only when all unprotected must-enter fields in the page have their MDT
bits set to 1 (on),
A must fill field is a field which must be filled with characters other than a
null character before you can move the cursor out of the field or transmit data
from the page, The cursor can be moved into and out of that field at any time
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