FEPI programming commands

EXEC CICS® FEPI commands provide several ways of developing CICS FEPI applications. The commands are at three logical levels:

High-level:
a straightforward interface for normal 3270 applications
Data stream-level:
for use with IMS™ SLU P applications and more complicated 3270 applications
Specialized-level:
for access to complex VTAM® communication functions and events, designed for use by vendors and experienced CICS FEPI application developers.

High-level FEPI commands

The high-level front-end programming interface consists of two interfaces for everyday use: key stroke and screen-image, collectively known as formatted data. They allow programmers to build their own CICS FEPI applications in a straightforward manner. However, the programmer must understand data communication and protocols.

See FEPI key stroke and screen-image applications for details.

The key stroke interface

The key stroke interface allows programmers writing in any of the CICS-supported languages, to specify the keys that an operator might press while using an existing application. The key strokes are specified using easily coded mnemonics; no hexadecimal values are required.

The screen-image interface

The screen-image interface allows programmers writing in any language supported by CICS, to define the contents of a 3270 screen, using a data structure appropriate to the programming language. It uses a buffer with one byte for each screen position (for example, 1920 bytes for a 24 × 80 character screen). This buffer can be defined in any way that suits the application program and the programming language. It is passed as a complete screen buffer to the back-end application.

In both cases, key stroke and screen-image, the data received from the back-end application is presented as a screen image.

Data-stream-level commands

For many applications, the key stroke and screen-image interfaces should be quite adequate. However, where they are not, FEPI data-stream-level commands give an application complete control of the 3270 data stream. These commands are also needed for SLU P applications, which can use only this interface. FEPI does not buffer or interpret the data stream; it is presented as it arrives from the back-end application, and the front-end application must be prepared to handle whatever is presented. Similarly, data sent by the front-end application is transmitted without verification.

A detailed knowledge of data communication and protocols and of data stream format is required.

See FEPI data stream applications for details.

Specialized-level commands

These are some of the specialized functions that can be accessed through FEPI:

STSN for SLU P applications:
Set and test sequence number (STSN) is a communication protocol used to check and control transmissions. FEPI normally handles all necessary STSN processing automatically. However, FEPI also provides access to STSN information for those applications that need to control sequence number data.
Application access to definite responses:
When a flow is received, the receiving LU can choose what response to return to the sending LU. FEPI normally handles this automatically, but also provides facilities for applications to determine this flow.
Other VTAM facilities:
Some applications use a VTAM facility known as CLSDST(PASS); this can be used in more sophisticated CICS FEPI application programming.

See Specialized FEPI functions for details.

List of commands

All the logical levels use more or less the same set of commands, though the options used may vary. The EXEC CICS FEPI application programming commands are:

ALLOCATE
Establishes communication with a back-end application
FREE
Frees communication with a back-end application
SEND
Sends data from a CICS FEPI application to a back-end application
RECEIVE
Receives data into a CICS FEPI application from a back-end application
CONVERSE
Sends data to and receives data from a back-end application
ISSUE
Sends control data to a back-end application
EXTRACT
Gets field data and attributes, set-and-test-sequence-number (STSN) data, or conversation status
START
Schedules a CICS transaction to handle inbound data.
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