AM3270/TR3271 Reference Manual

Using Multiple-Level Access Methods
A Closer Look at TR3271
5–12 086705 Tandem Computers Incorporated
Level 4—the subdevice protocols—provide the file system interface to the
subdevice.
Note Levels 3 and 4 describe Tandem’s implementation of these protocols, rather than describing an
International Standard.
I/O Interface Procedure
(IOP)
The I/O interface procedure (IOP) provides the communications path between
GUARDIAN 90 and TR3271. Requests to TR3271 are performed through the IOP.
Typical requests are:
file-system requests such as READ
line requests
interline handler requests such as PTP WRITE
interrupt notifications
timeout notifications
message system completion notifications such as LACK, LDONE, or LCAN.
The IOP provides first-level routing of these incoming items. Most requests and all
message-system notifications are passed to level 3. From there, they may be passed to
the appropriate level 4 protocol on a per subdevice basis. Interrupts are always passed
to the driver (level 1). Timeout notifications are passed to the level(s) that requested
them.
As discussed previously, the driver (level 1) normally is called by the IOP or by level 2;
level 2 normally is called by level 3 or level 1. A level must often perform a delayed-
completion action; for example, an action that will be completed by the occurrence of
an interrupt.
Before the interrupt occurs, the level returns to its caller to report that the request has
been initiated but not completed. When the completion interrupt does eventually
occur, the IOP calls the driver (level 1). The driver interprets and determines that a
completion occurred and must be reported to level 2. The driver returns a code to the
IOP, which causes the IOP to call level 2 and pass the completion information.
This method of communicating in the reverse-of-normal direction and transferring
information on behalf of another level eliminates potential problems caused by
recursive calling of procedures.
The IOP also notifies level 3 if ownership changes are detected.
Level 1 Protocol Level 1, the bisynchronous driver, initiates physical I/O to the controller. It also
handles completions such as interrupts and timeouts which are received through the
I/O interface procedure.