AM3270/TR3271 Reference Manual

AM3270 Product Environment
A Closer Look at AM3270
086705 Tandem Computers Incorporated 3–7
subdevice protocol (one of the level 4 protocols) is invoked to provide further
processing of the request.
Level 4—the subdevice protocols—provide the file-system interface to the
subdevices.
Level 1—Bisync Driver Level 1, the bisynchronous driver, initiates physical I/O to the controller (or LIU—
6100 CSS) and handles completions of events such as interrupts and timeouts.
AM3270 formats a poll list containing the subdevices that have active READ requests.
The driver starts polling, using the list; it terminates the polling operation if a device
responds to the poll, if an error is detected, or if level 2 requests that polling be
stopped because a WRITE has arrived.
Level 2—Multipoint
Supervision
Level 2, the multipoint supervisor, provides the operations required to sequence a
communication line through the bisynchronous multipoint supervisor protocol. The
protocol accepts requests from level 3, encodes the request into driver actions, and
calls the driver. On behalf of the driver, events from the I/O process are decoded by
level 2 and the results are sent to level 3 to complete the actions that level 3 started
previously.
READ Operation
Figure 3-5 illustrates the action of AM3270 in response to a READ request. This
example assumes that the line is idle and that the subdevice has the CRT subdevice
protocol assigned to it.