Read Operation; Read Buffer - IBM 3708 Description

Read Operation
Read Buffer
The process of sending data from the display to the host is called a read operation.
A read operation can be initiated by:
• The host application sending an explicit read command
• An operator action (for example, pressing the ENTER key).
Note: Because the information that the host reads may be incomplete, it is
recommended that the host application not send Read commands.
The 3708 supports three read commands:
• Read Buffer
• Read Modified
• Read Modified All.
These commands allow the host program to be informed of changes in the device's
buffer that resulted from keyboard input. When sent by the host program, these
commands cause a specific data stream to be sent in response. Also, the Read
Modified response may be sent by the 3708 without having received the command
from the host. This would be done when the terminal operator presses the keys to
perform a function that requires a host response, such as ENTER or a PF key.
The 3708 transfers all data in the device buffer to the host when it receives the Read
Buffer command. The 3708 transfers data from location 0 to the end of buffer.
The response consists of a 3-byte read heading that contains the AID byte and a
2-byte cursor address. This is followed by the contents of the entire buffer, including
null characters. At a location occupied by a field attribute byte, the 3708 inserts a
Start Field order to identify the beginning of a field. (See "Start Field (SF)" on
page 4-11 for the format of the Start Field order.)
The AID value is set when the terminal operator has performed an operation that
requires host program intervention, such as pressing an Enter or PF key. Table 4-1
contains a list of the AID functions and their hex values.
Chapter 4. Protocol Conversion Mode
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