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Displays
Display Image
This section describes how the 3708 controls displays when operating in protocol
conversion mode. It includes descriptions of the following subjects:
• The image that appears on the display's screen
• The types of fields that may be on the image
• Support for IBM 3270 Models 2, 3, 4, and 5
• The special cartridges available for the IBM 3151, 3161, 3162, 3163, and 3164
• The status line area of the display image
• The keyboard functions that are available to the display operator.
Data displayed on the screen of the ASCII display is stored in its ASCII form in a
device buffer in the memory of the 3708. The device buffer contains as many
locations as there are character positions on the screen. The buffer contains another
80 or 132 bytes in which the status line image is kept. The data for the screen image
portion of the buffer may be loaded from the application program at the host, or
from the keyboard of the attached display. Figure 4-3 on page 4-30 illustrates the
relationship between locations in the device buffer and locations on the display
screen.
In the example, the display image contains 24 horizontal rows with 80 character
positions in each row (Model 2). The 3708 also supports screen sizes of 32 x 80
(Model 3), 43 x 80 (Model 4), and 27 x 132 (Model 5). Refer to "Support for the
IBM 3270 Models 2, 3, 4, and 5" on page 4-33 for more information.
There is a fixed relationship between each location in the device buffer and each
character position on the display screen. Buffer addresses start from 0, for the
character position at the left of the top row, and proceed sequentially along the rows
and down the screen to the character position at the right of the bottom row. The
orders in the data stream to and from the host program use these buffer addresses to
describe the locations of data and fields on the display image. The use and format
of these orders are described in "Orders and Attributes" on page 4-10.
Each location in the buffer contains 1 byte of storage; values loaded into the buffer
are 2-digit hexadecimal codes. Write commands from the host are used to load the
screen image portion of the device buffer with the display image. The status line
area of the device buffer is managed and loaded by the 3708.
Display images may be formatted or unformatted:
• Formatted Display: A formatted display is one that has separate fields defined
by the host program. The first character position of each field contains a
control character that defines the characteristics of the field. See "Attributes
and Field Attribute Characters" on page 4-31 for a description of the control
character.
• Unformatted Display: An unformatted display is one that has no defined fields.
An operator may input data into any position on the screen.
Chapter 4. Protocol Conversion Mode
4-29
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